This tutorial builds on the following tutorials:

• RAIL Tutorial 1: Introduction, RAIL components and the Empty Example
• RAIL Tutorial 2: Transmitting a Packet
• RAIL Tutorial 3: Event Handling and Automatic State Transitions

Please read them first if you haven't.

You can find other tutorials on the table of contents site.

On the previous tutorial, we had both transmit and receive working, but we couldn't see what we received. This tutorial will solve that problem.

We're also adding calibrations, which is recommended for all projects.

You can find a SimpleTRX example in Simplicity Studio, which has a similar purpose. However, the result of this tutorial will be somewhat different. Mainly, this tutorial is not very pedantic on checking errors returned by all functions, to make it easier to understand. However, we highly recommend checking all possible errors, therefore using the SimpleTRX example in Studio as a reference.

Enabling Calibrations

First, let's load PA factory calibrations: Open the configuration for the component RAIL Utility, Power Amplifier, and turn on the Enable PA Calibration switch.

All other calibrations are enabled by default in RAIL Utility, Initialization, but require some application code to perform them. First, we need to add the related event during initialization to enable it (in app_init.c):

RAIL_Handle_t app_init(void)
{
  //...
  RAIL_ConfigEvents(rail_handle, RAIL_EVENTS_ALL,
                     RAIL_EVENTS_TX_COMPLETION | RAIL_EVENTS_RX_COMPLETION |
                     RAIL_EVENT_CAL_NEEDED);
  //...
}

Next, we need to perform calibrations when RAIL requests them (in app_process.c):

void sl_rail_app_on_event(RAIL_Handle_t rail_handle, RAIL_Events_t events)
{
  //...
  if ( events & RAIL_EVENT_CAL_NEEDED ) {
    RAIL_Calibrate(rail_handle, NULL, RAIL_CAL_ALL_PENDING);
  }
}

This is all that's needed from an application to perform all calibrations available on EFR32, to have the maximum available performance. Calibrations are detailed further in another tutorial.

Preparing for printing

In the previous tutorials, we already enabled LEDs and buttons, but we'll need a more user friendly output: We need a serial console. To enable that, install the following components:

• IO Stream: USART on vcom (the text to serial driver)
• Tiny printf (the printf implementation we recommend) - note that you should use #include "printf.h" to use it, not stdio.h!
• IO Stream: Retarget STDIO (to connect the two)

Finally, we need to enable Virtual COM UART on the Board Control component:

This pulls up a GPIO, which will enable the USB-UART bridge on the WSTK.

Receive FIFO

We already configured Tx FIFO multiple ways. RAIL also implements an Rx FIFO. By default, it's 512B, allocated by the RAIL library. We will use that, but it is configurable. See RAILCb_SetupRxFifo() and RAIL_SetRxFifo() in the API documentation for details.

By default, you cannot use the whole 512B, as RAIL adds a few bytes to each packet to store timestamp, RSSI, and similar information in the FIFO (sometimes we call this "appended info").

We're working on what we call packet mode. It's the simpler, and recommended way, but it's not possible to handle packets bigger than 512B with this method, in which case FIFO mode should be used. We return to that topic in a later tutorial.

The Packet Handle

The FIFO is accessible with RAIL_RxPacketHandle_t variables or packet handles. Obviously, we don't have the handle of the packet we just received, but we have "sentinel" handles:

• RAIL_RX_PACKET_HANDLE_NEWEST
• RAIL_RX_PACKET_HANDLE_OLDEST
• RAIL_RX_PACKET_HANDLE_OLDEST_COMPLETE
• RAIL_RX_PACKET_HANDLE_INVALID

Note that RAIL_RX_PACKET_HANDLE_OLDEST and RAIL_RX_PACKET_HANDLE_NEWEST will always return with packet information. For example, OLDEST will return with one of the following:

1. the oldest packet received, if available
2. the packet the radio is receiving at the moment, if available and if no fully received packet is available
3. the placeholder which will be used for the next packet the radio will receive, if not fully or partially received packet is available

If you don't care about upcoming or ongoing packets, you can use RAIL_RX_PACKET_HANDLE_OLDEST_COMPLETE: It will only return with fully received packets.

When to Read the FIFO

Let's say we want to download the packet when it's fully received (although it's possible to access it during reception). We must let RAIL know this in the event RAIL_EVENT_RX_PACKET_RECEIVED. If we don't, RAIL automatically frees the memory allocated to the packet we just received.

We have two options:

• Download the packet from the event handler
• Hold the packet in the FIFO and download later

While the first one is simpler, it is desirable in most cases to keep large memory copy operations outside of interrupt handlers, therefore the attached example demonstrates the second method.

The Event Handler

In the event handler, we want to instruct RAIL to hold all successfully received frames:

void sl_rail_app_on_event(RAIL_Handle_t rail_handle, RAIL_Events_t events)
{
  //...
  if ( events & RAIL_EVENTS_RX_COMPLETION ) {
    if ( events & RAIL_EVENT_RX_PACKET_RECEIVED ) {
      RAIL_HoldRxPacket(rail_handle);
      sl_led_toggle(&sl_led_led0);
    } else {
      sl_led_toggle(&sl_led_led1); //all other events in RAIL_EVENTS_RX_COMPLETION are errors
    }
  }
}

The API RAIL_HoldRxPacket will also return a packet handler, but we ignore that since we're going to use sentinel handles. Keep in mind that you must release all packets that were held, otherwise, you will lose part of your RX fifo, essentially leaking memory.

Download and Print

First, let's create a buffer we can use to download the packet:

static uint8_t rx_buffer[256];

In app_process_action(), if there's something in the handle, let's download and print it:

void app_process_action(RAIL_Handle_t rail_handle)
{
  //...
  RAIL_RxPacketHandle_t packet_handle;
  RAIL_RxPacketInfo_t packet_info;
  packet_handle = RAIL_GetRxPacketInfo(rail_handle, RAIL_RX_PACKET_HANDLE_OLDEST_COMPLETE, &packet_info);
  if ( packet_handle != RAIL_RX_PACKET_HANDLE_INVALID ){
    RAIL_RxPacketDetails_t packet_details;
    RAIL_CopyRxPacket(rx_buffer, &packet_info);
    RAIL_GetRxPacketDetails(rail_handle, packet_handle, &packet_details);
    RAIL_ReleaseRxPacket(rail_handle, packet_handle);

    printf("RX");
    for(int i=0; i < packet_info.packetBytes; i++){
      printf(" 0x%02X", rx_buffer[i]);
    }
    printf("; RSSI=%d dBm\n", packet_details.rssi);

  }
}

Let's go through this, line by line:

First, RAIL_GetRxPacketInfo() will return with a valid packet_handle if we have a completely received frame. In that case, it also returns a usable packet_info which has the length of the received packet, and pointers to download it. Note that the length will include the full frame, and is always valid, whichever length configuration is used (fixed or variable).

Since the receive buffer is a ring buffer, the packet in the buffer might not be in a single sequential buffer, copying from that using memcpy() is a bit complicated: RAIL_CopyRxPacket() inline function implements that.

The API RAIL_GetRxPacketDetails() will return some useful information of the packet, such as RSSI or timestamps.

Finally, we release the packet, using RAIL_ReleaseRxPacket().

Note that this method is safe, even if we receive a new packet while downloading one: That packet will be downloaded and printed when app_process_action() is called the next time.

Download in the Event Handler

Downloading the packet in the event handler is essentially the same as what we did in the main loop above, except we don't need to worry about releasing:

if (events & RAIL_EVENT_RX_PACKET_RECEIVED) {
  RAIL_RxPacketInfo_t packet_info;
  RAIL_GetRxPacketInfo(rail_handle, RAIL_RX_PACKET_HANDLE_NEWEST, &packet_info);
  RAIL_CopyRxPacket(rx_buffer, &packet_info);
  RAIL_GetRxPacketDetails(rail_handle, packet_handle, &packet_details);
}

Note that packet_details should be a global variable as well in this case.

Conclusion

With this, you can use RAIL for basic transmit and receive, which concludes the getting started series. However, RAIL can do much more - you can continue with the basic tutorials from the table of contents site.

API Introduced in this Tutorial

Functions

• RAIL_Calibrate()
• RAILCb_SetupRxFifo()
• RAIL_SetRxFifo()
• RAIL_HoldRxPacket()
• RAIL_ReleaseRxPacket()
• RAIL_GetRxPacketInfo()
• RAIL_GetRxPacketDetails()
• RAIL_CopyRxPacket()

Types and enums

• RAIL_RxPacketHandle_t
• RAIL_RxPacketInfo_t
• RAIL_RxPacketDetails_t
• RAIL_EVENT_CAL_NEEDED
• RAIL_RX_PACKET_HANDLE_NEWEST
• RAIL_RX_PACKET_HANDLE_OLDEST
• RAIL_RX_PACKET_HANDLE_OLDEST_COMPLETE
• RAIL_RX_PACKET_HANDLE_INVALID

This tutorial builds on the following tutorials:

• RAIL Tutorial 1: Introduction, RAIL components and the Empty Example
• RAIL Tutorial 2: Transmitting a Packet

Please read them first if you haven't.

You can find other tutorials on the Table of Contents site.

In the previous tutorials, we set up RAIL to do various tasks through API commands. However, RAIL can inform the application on various events almost real time (basically, wiring interrupts through RAIL). So let's improve what we had in the previous tutorial.

The Event Handler Function

You might have already seen this function in app_process.c:

void sl_rail_app_on_event(RAIL_Handle_t rail_handle, RAIL_Events_t events)

This is the event handler function, which is called by the RAIL library, and routed through the RAIL Utility, Initialization component.

The events variable is a bit field, holding all possible events. Be careful, this bit field does not fit into a 32-bit variable, which is the size of the int on our MCUs.

The event handler should be prepared for multiple events in a single call. There's no "clearFlag" mechanism, each event will be signaled once, and the application is responsible for handling it.

Keep in mind that almost all events are generated from interrupts, so the event handler is usually running from interrupt context (i.e., interrupts are disabled). Therefore, the event handler should run fast; if you have to do something that takes more time, set a flag, and do it from the main loop.

You should be also careful with RAIL interrupt safety. Calling state changing APIs (RAIL_Start*, RAIL_Stop* or RAIL_Idle) from the interrupt handler is allowed, but we recommend to read the interrupt safety article before doing so.

Configuring Events

The events enabled by default are configured on the RAIL Utility, Initialization component  (under 'Platform/Radio', press 'Configure'):

However, using the RAIL API directly is actually simpler than using the event configuration capabilities of this component. The API for this is RAIL_ConfigEvents(). You can keep the event setup enabled, it doesn't really matter.

RAIL_ConfigEvents() has a mask and an events parameter, making it simple to configure multiple events at the same time; e.g., to enable or disable a single event, leaving the others untouched:

//disable RAIL_EVENT_TX_PACKET_SENT
RAIL_ConfigEvents(railHandle, RAIL_EVENT_TX_PACKET_SENT, 0);

//enable RAIL_EVENT_TX_PACKET_SENT
RAIL_ConfigEvents(railHandle, RAIL_EVENT_TX_PACKET_SENT, RAIL_EVENT_TX_PACKET_SENT);

RAIL also defines a few event groups (defined in rail_types.h, documented in events), the most useful are:

• RAIL_EVENTS_ALL includes all the events
• RAIL_EVENTS_NONE includes no events (equivalent to 0)
• RAIL_EVENTS_TX_COMPLETION includes all possible events after a successful RAIL_StartTx
• RAIL_EVENTS_RX_COMPLETION includes all possible events that can result in state transition from Rx mode

This can be used, for example, to disable or enable all events:

//disable all events
RAIL_ConfigEvents(railHandle, RAIL_EVENTS_ALL, 0);

//enable all events
RAIL_ConfigEvents(railHandle, RAIL_EVENTS_ALL, RAIL_EVENTS_ALL);

If you enable/disable events in group, it's a good practice to create similar group of events in your application.

RAIL has no API to flush pending events, but disabling and enabling them will have that result:

//flush RAIL_EVENT_TX_PACKET_SENT
RAIL_ConfigEvents(railHandle, RAIL_EVENT_TX_PACKET_SENT, 0);
RAIL_ConfigEvents(railHandle, RAIL_EVENT_TX_PACKET_SENT, RAIL_EVENT_TX_PACKET_SENT);

However, you only need to do this in rare cases. Enabling events is not like unmasking interrupts: Enabling them will not trigger the event handler on past events.

Tx Error Handling

In the last tutorial, we sent out a frame, but we didn't check the results because it would have needed the event handler. Let's turn on led0 on success and led1 on any error (we'll be turning led1 on when transmitting and clearing led1 if no Tx error occurs).

First, for the LEDs, enable the Simple LED component with led0 and led1.

In app_process.c (to get access to the led instances, now declared in autogen/sl_simple_led_instances.h):

#include "sl_simple_led_instances.h"

For the actual radio related changes, we'll need to enable the success and error events. We can do that during init since we don't really need to disable anything at runtime for such simple applications:

RAIL_Handle_t app_init(void)
{
  //...
  RAIL_ConfigEvents(rail_handle, RAIL_EVENTS_ALL, RAIL_EVENTS_TX_COMPLETION);
}

Next, let's check for errors returned by the StartTx call:

RAIL_Status_t status = RAIL_StartTx(rail_handle, 0, RAIL_TX_OPTIONS_DEFAULT, NULL);
if ( status != RAIL_STATUS_NO_ERROR ) {
  sl_led_toggle(&sl_led_led1);
}

Finally, let's check the events for success and errors:

void sl_rail_app_on_event(RAIL_Handle_t rail_handle, RAIL_Events_t events)
{
  (void)rail_handle;
  if ( events & RAIL_EVENTS_TX_COMPLETION ) {
    if ( events & RAIL_EVENT_TX_PACKET_SENT ) {
      sl_led_toggle(&sl_led_led0);
    } else {
      sl_led_toggle(&sl_led_led1); //all other events in RAIL_EVENTS_TX_COMPLETION are errors
    }
  }
}

With this, you should see led0 toggling on every transmitted packet.

Receiving Packets

Let's add receive capabilities to this application! Again, use led0 to report success and led1 for errors.

First, we need to modify the init code.

In app_init.c (to get access to the led instances):

#include "sl_simple_led_instances.h"

We need RX success/error events enabled, and we need to start the radio in RX mode:

RAIL_Handle_t app_init(void)
{
  //...

  RAIL_ConfigEvents(rail_handle, RAIL_EVENTS_ALL,
                     RAIL_EVENTS_TX_COMPLETION | RAIL_EVENTS_RX_COMPLETION);
  
  RAIL_Status_t status = RAIL_StartRx(rail_handle, 0, NULL);
  if ( status != RAIL_STATUS_NO_ERROR ){
    sl_led_toggle(&sl_led_led1);
  }
}

The API RAIL_StartRx() is similar to Tx: the only important option is the second one, which is the channel.

Finally, let's check the events for success and errors (in app_process.c):

void sl_rail_app_on_event(RAIL_Handle_t rail_handle, RAIL_Events_t events)
{
  //...
  if ( events & RAIL_EVENTS_RX_COMPLETION ) {
    if ( events & RAIL_EVENT_RX_PACKET_RECEIVED ) {
      sl_led_toggle(&sl_led_led0);
    } else {
      sl_led_toggle(&sl_led_led1); //all other events in RAIL_EVENTS_RX_COMPLETION are errors
    }
  }
}

Auto state transitions

By default, after receiving or transmitting a packet, RAIL will switch to idle mode (i.e., turn off the radio). Since we want it to keep receiving future packets, we can use RAIL's auto state transition feature. This can be configured on the RAIL Utility, Initialization component, let's set all dropdowns to RX:

We're basically saying that after the packets are received/transmitted, we want to return to Rx. These configurations can be also changed in run-time, using the RAIL_SetRxTransitions() and RAIL_SetTxTransitions() APIs.

If you compile and flash the program after this modification, you should see that now both WSTK toggles led0 on every button press.

Testing and Conclusion

If you install this example to two boards (make sure you attach the antennas if you use sub-GHz kits on sub-GHz config), you will see that on the press of btn0 on either device, led0 on both devices toggle. However, we can't see what we're receiving, since we never actually download the messages.

We're going to fix this Next time

API Introduced in this Tutorial

Functions

• RAIL_ConfigEvents()
• RAIL_StartRx()
• RAIL_SetRxTransitions()
• RAIL_SetTxTransitions()

Types and enums

• RAIL_Events_t
• RAIL_EVENTS_ALL
• RAIL_EVENTS_TX_COMPLETION
• RAIL_EVENTS_RX_COMPLETION
• RAIL_EVENTS_NONE
• RAIL_EVENT_TX_PACKET_SENT
• RAIL_EVENT_RX_PACKET_RECEIVED

This tutorial builds on the following tutorial/s:

• RAIL Tutorial 1: Introduction, RAIL components and the Empty Example

Please read them first if you haven't.

You can find other tutorials on the Table of Contents site.

In this tutorial, we're going to modify the project Flex (RAIL) - Empty Example to transmit a packet. We're going to use the default, fixed length 16Byte payload configuration first.

This tutorial references some more advanced articles - we don't recommend reading them yet if you just started learning RAIL. They are only linked to highlight the connection.

Code snippets in this example are for illustration only. Refer to the attached files for reference code.

Buffer Handling

Setting Up the Packet Buffer

RAIL requires a buffer (usually called FIFO) to be configured for Tx. First, we have to allocate some memory for that buffer:

#define BUFFER_LENGTH 256
static uint8_t tx_buffer[BUFFER_LENGTH];

Note that you can't use any arbitrary size; EFR32 creates a FIFO ring buffer on this memory area, and it can only handle buffer sizes of power of 2 between 64 and 4096 (i.e., 64, 128, 256, 512, 1024, 2048, or 4096).

The tx buffer can hold more than one packets, which can be useful if you need to send out a lot of messages quickly.

To load the payload of the packet to the buffer, we have two options: write it via RAIL APIs or write to the memory directly.

Writing to the Buffer Directly

In app_process.c:

#define PAYLOAD_LENGTH 16
#define BUFFER_LENGTH 256
static const uint8_t payload[PAYLOAD_LENGTH] =
    {0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07,
     0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0F};

static uint8_t tx_buffer[BUFFER_LENGTH];

void load_payload_directly(RAIL_Handle_t rail_handle) {
  memcpy(txBuffer, payload, PAYLOAD_LENGTH);
  RAIL_SetTxFifo(rail_handle, tx_buffer, PAYLOAD_LENGTH, BUFFER_LENGTH);
}

The memcpy() is just a standard C instruction to write to buffers, and we use RAIL_SetTxFifo() to pass that buffer to RAIL. We also tell RAIL how long the buffer is, and how much data it has already in it.

Writing to the Buffer Indirectly

(See the attached app_init.c and app_process.c files for reference code)

In app_init.c:

#define BUFFER_LENGTH 256
static uint8_t tx_buffer[BUFFER_LENGTH];

RAIL_Handle_t app_init(void)
{
  // Get RAIL handle, used later by the application
  RAIL_Handle_t rail_handle = sl_rail_util_get_handle();

  RAIL_SetTxFifo(rail_handle, tx_buffer, 0, BUFFER_LENGTH);
  return rail_handle;
}

In app_process.c:

#define PAYLOAD_LENGTH 16
static const uint8_t payload[PAYLOAD_LENGTH] =
    {0x00, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07,
     0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0F};

static void loadPayloadIndirectly(RAIL_Handle_t rail_handle) {
  RAIL_WriteTxFifo(rail_handle, payload, PAYLOAD_LENGTH, false);
}

In this case, we pass the buffer to RAIL when it's still empty, and we write to that buffer using a RAIL API. Note that using memcpy() instead of WriteTxFifo would not work: while it would write the memory itself, it wouldn't change the read and write pointers of the FIFO, handled by RAIL, so RAIL would still think the buffer is empty.

Direct or Indirect

There are four main factors that could decide which method to use to write the FIFO:

• With indirect mode, the buffer can be used for partial packets. It is  simpler to use for multiple packets.
• Calling RAIL_SetTxFifo() does not move any memory, just sets a few register, while calling RAIL_WriteTxFifo() does need some time to copy the payload to the buffer.
• Calling RAIL_SetTxFifo() is not allowed during transmission.
• Indirect mode very clearly separates init and process code.

For simpler applications, RAIL_SetTxFifo() can be simpler. If you want to send the same packet over and over (or only change a few bytes in it), it's much better, since you don't have to write the whole packet again.

On the other hand, the separation in RAIL examples between app_init and app_process works very nicely with the indirect method, and in most real cases, you need to move memory anyway. If you want to send longer packets than your buffer, you must use RAIL_WriteTxFifo(), as you can call it during transmit. It's also useful if you want to send out a lot of packets: Use RAIL_WriteTxFifo() to load each message one after the other, then send them out quickly without any buffer operation.

The attached code uses indirect mode mainly for the clear separation.

FIFO Reset

The last parameter of RAIL_WriteTxFifo can reset the FIFO, which means it will invalidate the data already in there. This is also possible with RAIL_ResetFifo. We generally recommend not to use it, a well-written code shouldn't need it (as it should only store in the FIFO what should be sent out), and it might mask bugs. Resetting the FIFO also takes extra time.

Transmitting the Packet

Starting the transmission is very simple:

RAIL_StartTx(rail_handle, 0, RAIL_TX_OPTIONS_DEFAULT, NULL);

This instructs RAIL to start transmitting the data stored in the FIFO on channel 0 (second parameter). The third parameter can change various options, like using the second configured sync word (see RAIL API reference). The last parameter is only required for DMP (dynamic multiprotocol) mode.

Changing the Packet Length

You can change the configured fixed length on the Radio Configurator, but that's obviously not possible at runtime, which is often needed. Note that the amount of data loaded into the FIFO does not matter as long as it's equal to or greater than the length of the frame.

Changing Packet Length with a Fixed Length Configuration

With the API RAIL_SetFixedLength(), it's possible to change the pre-configured length (stored in rail_config.c) at runtime. Note that this changes the length of the packets on the Rx side as well. If you want to return to the pre-configured value, you can use RAIL_SetFixedLength(railHandle, RAIL_SETFIXEDLENGTH_INVALID).

Using Variable Length Configurations

A lot of protocols store the length of the packet in the header of the frame. For example, in IEEE 802.15.4, the first byte stores the length (and the maximum is 127). To set this up, use the following settings in the Radio Configurator:

• Set Frame Length Algorithm to VARIABLE_LENGTH
• This automatically enables the Header
• Set the length of the header to 1
• Set variable length bit size to 8
• Set the maximum length to 127

For more information on this setup, and on more advanced variable length configs, see AN1253. Note that the above configuration is not fully IEEE 802.15.4 compatible to make it simpler.

Length decoding works the same way for both rx and tx. This means that during Tx, we have to make sure that the committed length in the header matches the number of bytes you load into the FIFO. It also means that if we set the length field to more than 127, we will get a transmit error.

This would be a valid 16B frame, both for the above described variable length and 16 Byte fixed length mode, so this is used in the attached sample code:

static const uint8_t payload[PAYLOAD_LENGTH] =
    {PAYLOAD_LENGTH-1, 0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07,
     0x08, 0x09, 0x0a, 0x0b, 0x0c, 0x0d, 0x0e, 0x0F};

We use PAYLOAD_LENGTH-1 in the length field, since the 1B length field itself shouldn't be counted.

The API RAIL_SetFixedLength() is available in variable length mode, and it changes the radio to fixed length operation (again, both for rx and tx). Calling RAIL_SetFixedLength(railHandle, RAIL_SETFIXEDLENGTH_INVALID) will restore it to variable length mode.

Setting up the example

Button handling

The attached example sets the tx fifo on init, loads, and sends the packet on any button press. This is implemented using the Simple Button component on btn0, and implementing its callback:

In app_process.c (to get access to the button instance, now declared in autogen/sl_simple_button_instances.h):

#include "sl_simple_button_instances.h"

Also add the button callback code (the callback is already declared in sl_button.h):

void sl_button_on_change(const sl_button_t* handle){
  if ( sl_button_get_state(handle) == SL_SIMPLE_BUTTON_PRESSED ){
      send_packet = true;
  }
}

By default, the Simple Button is configured for PF6, which matches the WSTK PB0 button

Since this callback is in interrupt context, we avoid using RAIL API directly. It is safe in almost all cases, but calling RAIL_StartTx() from interrupt context should be done carefully. Instead, we set the volatile send_packet variable to true, and call RAIL APIs from app_process_action() only:

void app_process_action(RAIL_Handle_t rail_handle)
{
  if ( send_packet ){
    send_packet = false;
    RAIL_WriteTxFifo(rail_handle, payload, PAYLOAD_LENGTH, false);
    RAIL_StartTx(rail_handle, 0, RAIL_TX_OPTIONS_DEFAULT, NULL);
  }
}

To simply set this up:

1. Create an empty RAIL example
2. Enable Simple Button component on btn0
3. Overwrite the attached c files in the root folder of the project

Conclusion

We didn't care about possible errors generated by RAIL: we do that next time, which also provides hints for receiving packets.

API Introduced in this Tutorial

Functions

• RAIL_SetTxFifo()
• RAIL_WriteTxFifo()
• RAIL_StartTx()
• RAIL_SetFixedLength()

Types and enums

• RAIL_TX_OPTIONS_DEFAULT
• RAIL_SETFIXEDLENGTH_INVALID

What is RAIL?

RAIL is short for Radio Abstraction Interface Layer, and is the most direct interface available for EFR32 radios. You can think of it as a radio driver. While this sounds like a restriction, this actually makes software development for proprietary wireless much simpler:

• EFR32 has a very flexible radio, which also makes it quite complex. RAIL (and the radio configurator) provides an easy-to-use interface of EFR32's features.
• The various generations of EFR32s are slightly different, but the RAIL API is the same (except maybe the chip specific updates), which makes hardware updates almost seamless.
• We plan to add RAIL support to all of our future wireless MCUs.

See this KBA for more details.

RAIL application development is currently possible in the Flex SDK in Simplicity Studio. To progress further with this tutorial, please remember to open the RAIL API documentation.

When to Use RAIL?

If you have an existing protocol, and you must be compatible with it, you'd probably have to use RAIL (as all of our stacks use a standardized frame format).

If you want to use sub-GHz communication, you have a few options, including RAIL. For simple point to point communication, RAIL might be the simplest solution. However, as soon as you need security or addressing, it might be better choose a protocol stack, such as Connect.

RAIL also has the benefit that it adds the least amount of delays to all radio events: In fact, some event notification will happen in interrupt context.

What is covered by RAIL?

RAIL only supports what the radio hardware supports. For example, auto ACK and address filter is available, as the hardware provides support for it. On the other hand, while security is important for wireless communication, it's not really a radio task, hence RAIL does not support it. The crypto engine is an independent peripheral that can be used to encrypt payloads before giving them to RAIL and the radio hardware.

Supported Energy Modes

EM1p or higher is required for the radio to be operational (i.e., Transmitting, receiving, or waiting for packets, RAIL timer running from HF clock). On devices without EM1p support, EM1 is required for the radio. See AN1244: EFR32 Migration Guide for Proprietary Applications for more details on EM1p.

EM2 or higher is required for RAIL scheduling or timers (Running from LF clock, which needs configuration, a topic in Tutorial 5).

EM3 or higher is required for RAIL to work without re-initialization.

Writing Code from Scratch

We do not recommend writing code from scratch because it is much simpler to have something existing that sets up the include paths and linker settings correctly. The best way to start is the example Flex (RAIL) - Empty Example.

RAIL Components

The empty application includes the following RAIL related components (under 'Platform/Radio'):

• RAIL Library, Single Protocol, which is the library itself
• RAIL Utility, Power Amplifier, which configures and initializes the power amplifier
• RAIL Utility, Initialization, which initializes RAIL and loads an initial configuration

There are a few components that you might want to install

• RAIL Utility Packet Trace Information, which enables PTI, the hardware that feeds data to Network Analyzer
• Radio Utility, Front End Module, which can be used to drive a FEM (or external PA/LNA), if you HW uses one

If you work on a starter kit, you don't need to change the configuration of most of these components (for custom hardware, there will be a new article coming soon), except the Initialization component. However, the initialization component also includes a good starting point, that we don't need to modify for simple applications.

The Radio Configurator

The Radio Configurator is accessible by configuring the component Advanced Configurators/Radio Configurator. The usage of the radio configurator is out of scope for this tutorial series. For more details, see AN1253: EFR32 Radio Configurator Guide.

The project structure

Projects always include the following parts:

• autogen folder: Only the autogen folder includes generated code. It includes the PHY configuration (rail_config.c), init code, the linker script, and other generated code used by components, like the command descriptors for the CLI interface
• config folder: Component configuration headers are placed into this folder. These can be edited with the Component Editor that can be opened on the Project Configurator via the Configure button, but directly editing the header file is also possible.
• gecko_sdk folder (with version number): Source and binary files added by components
• files in the root folder: Only the application specific files should be in the root folder, including source files, the project configurator (.slcp file) and the Pin tool (.pintool file)

Note that all files related to the project should be visible in the project explorer, including header and library files.

All projects include main.c, which is not recommended to modify. Instead, add the initialization code to app_init.c, and implement the main loop in app_process.c. This way, the System components can initialize components, and call the "process" function of the components that requires this. Additionally, enabling an RTOS will convert app_process's main loop into an RTOS task.

Conclusion

This project is ready to send out frames. We're going to do that in the next part. You can also find other tutorials from the Table of Content site.

RFSense is a low power feature of the EFR32 Wireless MCU family. It can "wake up" an MCU from its EM2 or even EM4 power modes. Practically, it is an ultra low power interrupt source, running on ULFRCO clock.

The RFSense is a wide band circuit, it can detect energy in the 100MHz - 5 GHz frequency range, filtered only by the matching network of the RF front end. This is an advantage, as no need for separate PCB components. But it’s also a drawback: it is sensitive to any kind of interferer signal as well.

EFR32xG22 has an updated RFSense module, which improves the performance compared to EFR32 Series 1 in multiple ways:

• RFSense works below 0C degree
• RFSense works if voltage is scaled down
• Introduces Selective mode

Legacy mode

In legacy mode, EFR32xG22 RFSense is fully compatible with the one in Series 1. This means that if the RFSense module detected energy for a configured time, it generates an interrupt.

Selective mode

Selective mode mitigates the unfiltered nature of RFSense. Instead of simply detecting energy for a given time period, it detects "a pattern of energy", which is essentially an OOK packet. The packet is Manchester coded, uses fixed 1kbps bitrate, 1B preamble and 1-4B sync word (no payload added). This packet can be transmitted by any OOK capable device, including all EFR32 wireless MCUs (Series 1 and Series 2).

Selective mode offers 2 configuration options to select from: "optimized for sensitivity" or "optimized for noisy environment".

The Wakeup Packet

The wakeup packet is a fixed-configuration OOK packet with the following settings:

• Starts with a 1 B preamble (always 0x55).
• Followed by a 1-4 B sync word.
• No payload required.
• Both the preamble and sync word are transmitted LSB first.
• 1 kbps bitrate (before coding).
• Recommended carrier is 2.45 GHz.

Selective Mode Transmit Example Without Using API

Assume you select 0xb16e as your sync word, and you want to transmit it with only a signal generator (or a simple radio with MSB-first byte handling and no Manchester coder).

First, flip the endianness of both preamble and sync word: 0x55 becomes 0xaa and 0xb16e becomes 0x768d.

The full packet is then 0xaa768d, which after Manchester coding becomes 0x99996a6995a6.

Configuring this encoded packet and transmitting on 2.45 GHz with high enough TX power should wake up a device configured for selective RF Sense with the 0xb16e sync word

For more details on selective mode, see AN1244: EFR32 migration guide for Proprietary applications

The function RAIL_Idle() takes a mode argument, with which you select one of the four available methods of idling the radio to use. Though the API documentation briefly describes each mode, it might be difficult to understand their differences without seeing examples and usecases. This document aims to help you more clearly understand each mode.

General recommendations

In most cases, RAIL_IDLE is the recommended mode to use with this API. RAIL_IDLE_ABORT is helpful when you want to also abort an ongoing ("active") tx or rx operation.

On the other hand, RAIL_IDLE_FORCE_SHUTDOWN is not recommended for use, and RAIL_IDLE_FORCE_SHUTDOWN_CLEAR_FLAGS should be handled with care.

RAIL_IDLE

The mode RAIL_IDLE turns off the radio after the current operation is finished. It also cancels any transmit or receive scheduled in the future. Current operations that won't be aborted include:

• active transmit, which starts when the first bit (usually preamble) is transmitted and ends when the last bit (usually CRC) is transmitted.
• active receive, which starts at sync word detect, and ends when the last bit is received (which depends on the decoded length of the packet).

RAIL_IDLE_ABORT

The mode RAIL_IDLE_ABORT works the same as RAIL_IDLE, but it will also abort active operations. However, RAIL_IDLE_ABORT will always wait for a stable state before turning off the radio, e.g. if the radio was preparing to enter rx mode and is waiting for the PLL to lock, RAIL will wait until the rx state is reached before idling the radio.

RAIL_IDLE_FORCE_SHUTDOWN

Unlike RAIL_IDLE_ABORT (which waits for a stable radio state), RAIL_IDLE_FORCE_SHUTDOWN immediately forces the radio off. As this is an abrupt transition, it may corrupt data in the receive or transmit buffers. This buffer corruption can only be resolved by completely clearing the FIFO - which loses data, and can also consume additional time.

In our experience, it's almost always slower than RAIL_IDLE_ABORT, so the costs typically outweigh the benefit of using RAIL_IDLE_FORCE_SHUTDOWN (except when recommended by our support team for diagnostic purposes).

RAIL_IDLE_FORCE_SHUTDOWN_CLEAR_FLAGS

The mode RAIL_IDLE_FORCE_SHUTDOWN_CLEAR_FLAGS works the same as RAIL_IDLE_FORCE_SHUTDOWN, but it also clears any pending events. For more details on scenarios where this can be useful, see the article on RAIL interrupt safety.

In embedded software development, some of the most complicated debug challenges are caused by calling non-reentrant functions in interrupt context. Hence, it's in the developer's best interest to carefully design their application to avoid these scenarios.

To do so, however, requires sufficiently detailed knowledge of the interrupts and API functions - which are not accessible in a closed-source product like RAIL. This document aims to provide the information required to develop interrupt- and thread-safe applications in RAIL.

Thread Safety

In an application without a task scheduler, only an interrupt request can interrupt the main program. If you use a preemptive scheduler, like the scheduler available in most embedded OSes (including Micrium OS), higher priority tasks can interrupt lower priority tasks as well. Regardless, when looking at the RAIL APIs, the same concerns are present in either case:

• Is it safe to interrupt this API?
• Is it safe to call this API from a thread/interrupt which interrupted something?

The Event Handler

RAIL uses an event handler, which is set up by RAIL_Init(). In our examples, it's usually called RAILCb_Generic(). This function is called by the RAIL library, and it's almost always called from an interrupt handler. This means the event handler should be used with care:

• It should be kept in mind that interrupts are disabled when the event handler is running, so the function must not take long to return
• More importantly, the function might be interrupting the main loop (or some other task)

Note that the first point above might not be completely true if interrupt priorities are used, in which case only interrupts at the same and lower priorities are disabled. However, the event handler will never be interrupted by another event handler as all RAIL interrupts must be used at the same priority.

General Rules for the RAIL API

First, let's collect the general rules of the API, and we'll detail exceptions in later points:

1. Calling any RAIL API from the main thread (or a single OS thread) is safe.
2. Calling any API from multiple threads is unsafe, except for DMP.
3. Calling most APIs from an interrupt handler is safe (see exceptions below).

Dynamic Multiprotocol (DMP)

In general, if you have a multi-threaded application, you should use RAIL from a single thread. The exception to this guidance is DMP, where in most cases each protocol runs in its own thread. In this scenario, using RAIL from each thread is safe, as each protocol has its own railHandle. So, a more generalized wording of rule 2 is:

Calling any API from multiple threads is only safe if each thread has a dedicated railHandle, and each thread only accesses RAIL with its own handle.

The few APIs that don't use railHandle - like RAIL_GetTime(), RAIL_Sleep(), or RAIL_Wake() - can be called from any thread.

Interrupt Safety in general

In general, calling an API which changes the radio state (i.e. between rx, idle and tx) can be risky. The simplest way to write interrupt safe application is to not call state changing APIs from any interrupt handler, including the RAIL event handler. This can be achieved by setting a flag or changing a state variable in the event handler instead of calling an API directly:

typedef enum  {
  S_IDLE,
  S_START_RX,
  S_START_TX,
} state_t;

volatile state_t state;
volatile RAIL_Time_t lastEvent;

int main(){
  //init code

  state = S_START_TX;
  while(1){
    switch(state){
      case S_START_TX:
        RAIL_StartTx(railHandle, 0, RAIL_TX_OPTIONS_DEFAULT, NULL);
        state = S_IDLE;
        break;
      case S_START_RX:
        RAIL_StartRx(railHandle, 0, NULL);
        state = S_IDLE;
        break;
      default:
        break;
    }
  }
  return 0;
}

void RAILCb_Generic(RAIL_Handle_t railHandle, RAIL_Events_t events)
{
  lastEvent = RAIL_GetTime();
  if ( events & RAIL_EVENTS_TX_COMPLETION ){
    state = S_START_RX;
    RAIL_SetTxPower(railHandle, 200);
  }
}

Note that some RAIL API was called from the event handler, but none of those were state changing APIs.

Interrupt safety with state changing APIs

In some (usually time critical) cases however, it's not possible to avoid calling state changing APIs from the event handler (or other interrupt handler). State changing APIs are not always risky: Some APIs might be safe, as long as they don't interrupt another specific API.

Hence, in the following list, we identify the risky API after first specifying which initially-running (i.e., "interrupted") API makes it risky (and how). We've included in this list some interrupt combinations that might be "safe", but the end result is not predictable - i.e. the radio might be in rx or in idle, depending on which API is called first.

• Interrupting RAIL_Start<something>() with another RAIL_Start<something>() is risky, especially if they would start on different channels.
• Interrupting RAIL_Idle(handle, <something>, true) with any RAIL_Start<something>() is risky.
• Interrupting RAIL_Idle(handle, <something>, false) with any RAIL_Start<something>() is safe, but the end result is not predictable (i.e. the radio will either be in Idle, or start the requested operation).
• Interrupting RAIL_Start<something>() with RAIL_Idle() is safe but the end result is not predictable, and might cause strange events (see the next section for details).
• Interrupting RAIL_StopTxStream() with any RAIL_Start<something>() is very risky (the radio might remain in test configuration and start transmitting/receiving).
• Interrupting RAIL_StopTx() is safe. Interrupting RAIL_StopTx() with RAIL_Start<something>() is safe but the end result is not predictable (i.e. the radio will either be in Idle, or start the requested operation).
• Interrupting anything with RAIL_StopTx() is safe (see next section for important clarification). Interrupting RAIL_StartTx() with RAIL_StopTx(handle, RAIL_STOP_MODE_ACTIVE) is safe, but not predictable.
• Interrupting anything with RAIL_StopTxStream() is safe. Interrupting RAIL_StartTxStream() with RAIL_StopTxStream() is safe but not predictable.

RAIL_Idle in the Event Handler

Calling RAIL_Idle() or RAIL_StopTx(handle, RAIL_STOP_MODE_ACTIVE) from the event handler might cause strange results. For example, let's say you're receiving on a channel and want to detect preambles using the event RAIL_EVENT_RX_PREAMBLE_DETECT and RAIL_EVENT_RX_PREAMBLE_LOST. The following scenario may unfold:

• Preamble lost interrupt is received, so (at least) other radio interrupts are temporarily disabled.
• You enter the event handler with RAIL_EVENT_RX_PREAMBLE_LOST.
• At this point, the radio detects a preamble. The interrupt is logged, but the handler cannot run since the interrupts are masked.
• Still in the event handler, you decide to turn off the radio with RAIL_Idle(railHandle, RAIL_IDLE_ABORT, true).
• The radio turning off will generate a preamble lost interrupt.
• The radio is now off, and you return from the event handler.
• Interrupts are enabled again, so the pending preamble detect interrupt handler starts running.
• You enter the event handler with RAIL_EVENT_RX_PREAMBLE_DETECT and RAIL_EVENT_RX_PREAMBLE_LOST both set at the same time

So you end up with a preamble detect event, even though the radio is off. This is usually harmless, since you always have the _LOST or _ABORTED event as well - but this demonstrates why your design must carefully consider in what order to handle events.

The easiest way to avoid this conflicted outcome is to disable the events that might cause problems when turning off the radio.

Another way to avoid this issue is to use RAIL_Idle(handle, RAIL_IDLE_FORCE_SHUTDOWN_CLEAR_FLAGS, true), which will clear the pending interrupts. However, using RAIL_IDLE_FORCE_SHUTDOWN_CLEAR_FLAGS has other drawbacks. It does force the radio state machine to idle state, and it might corrupt the transmit or receive FIFOs - in which case it must clear them, losing all data that might already be in there. It could also take more time to finish running than RAIL_IDLE_ABORT.

Critical Blocks

One usual way to avoid internal safety issues is to create critical (a.k.a. atomic) blocks, in which interrupts are disabled, in the main thread to make sure some code segment is never interrupted. However, this can create other problems, so it should be used carefully. There's no general rule to avoid this kind of "collateral damage", but here's an example that should be avoided:

RSSI averaging is running, and just before it finishes, we interrupt it with RAIL_StartTx() which is called from a critical block. The following race condition could happen:

• We enter the critical block, interrupts are disabled.
• RSSI averaging done interrupt is received, but the interrupt handler won't start since interrupts are masked.
• StartTx turns off the radio, prepares it for transmit, then starts transmitting.
• We leave the critical block, interrupts are enabled again.
• RSSI averaging done interrupt handler runs at this point which will turn off the radio, aborting the current transmit.

One way to avoid the problem above is to clear interrupts in the critical block. This can be done by using RAIL_Idle(handle, RAIL_IDLE_FORCE_SHUTDOWN_CLEAR_FLAGS, true) at the beginning of the critical block, but the drawbacks of doing so (mentioned above) should be kept in mind. In general, it's better to avoid risky interrupts without using critical blocks in the main thread.

Using FORCE_SHUTDOWN

In the two sections above we mentioned two usecases where RAIL_IDLE_FORCE_SHUTDOWN_CLEAR_FLAGS can be useful. In general however, RAIL_IDLE or RAIL_IDLE_ABORT is a sufficient and preferred way to stop transmitting/receiving - therefore the FORCE_SHUTDOWN modes should be only used when they are really needed (as in the specific scenarios described here). For more details, see the article on the idle modes.

This tutorial builds upon the material covered in previous offerings:

• RAIL Tutorial 1: Introduction and init
• RAIL Tutorial 2: Transmitting a Packet
• RAIL Tutorial 3: Event Handling
• RAIL Tutorial 4: Combining Transmit and Receive
• (*) Introduction to Multi-PHY and Multiprotocol

(*) Make sure to read this one, as DMP is often misunderstood and used in applications that could be better served by multi-PHY. We recommend that you (at least) browse through the other tutorials before reading this (DMP) tutorial, since DMP affects most APIs in one way or another.

You can find additional RAIL tutorials on the table of contents page.

Note that this tutorial will not provide you with all of the knowledge required for DMP development. Rather, it will summarize what's already documented, and highlight differences between RAIL single protocol and multiprotocol operations.

Some chapters below are not important if you need RAIL-Bluetooth DMP, but even in that case most chapters are still relevant because they describe RAIL development in a DMP environment.

Note that this tutorial is slightly outdated in Studio v5, since the we now have components, not plugins, and the way we initialize RAIL in the examples are performed by the RAIL Utility, Initialization component, and not by the application. However, these are minor details and the wast majority of this article is accurate.

What is Dynamic Multiprotocol

Dynamic multiprotocol time-slices the radio and rapidly changes configurations to enable different wireless protocols to simultaneously operate reliably in the same application.

A typical usecase involves a RAIL-based proprietary protocol and the Silicon Labs Bluetooth stack running on the same chip at the same time. Since Bluetooth communication is periodic and therefore very predictable, other protocols can use the radio between Bluetooth communication bursts. However, it's also helpful to use DMP when all involved protocols are proprietary RAIL-based, to benefit from the code clarity provided by the separate RAIL instances.

From a firmware perspective, RAIL-DMP is similar to an object-oriented programming (OOP) class. It provides the radio driver, and multiple instances can be "created". Each instance can use the driver in the same way, and do so almost independently.

However, from a hardware perspective the chip has only a single instance of the radio. As such, RAIL-DMP is similar to multitasking: the same resource must be shared among multiple "tasks" demanding its time - this sharing is managed by the radio scheduler.

The Radio Scheduler

The radio scheduler's job is to give (or revoke) radio hardware access to (or from) the protocols running on it. To accomplish this it implements cooperative, preemptive multitasking.

• It's cooperative: Protocols should tell the scheduler when and for how long they need the radio, and protocols should yield the radio when not using it anymore
• It's preemptive: Protocols should define priority to each radio task, and the scheduler will abort tasks if a higher priority radio task must run

Note that the radio scheduler schedules only radio tasks. Although DMP is often used with an RTOS (like Micrium OS), RTOS tasks and Radio tasks - despite the common name - are very different terms. To make this less confusing, our documentation sometimes uses radio operation instead of radio task.

For the radio scheduler to calculate the time required for a given radio operation, it must know how much time it takes to change radio access from one protocol to another. Therefore, in DMP, a protocol change always takes 450us on EFR32xG1x, which accommodates the worst-case protocol change time. This makes DMP protocol change much slower than Multi-PHY PHY change.

Protocol change time in RAIL 2.5.x (Flex 2.4.x) and earlier was 750us (and it might change further in the future).

Setting up a project for RAIL-RAIL DMP

If you need RAIL+Bluetooth DMP, follow AN1134. This chapter describes how to set up DMP in a RAIL-only project.

To use DMP, you should first switch to the multiprotocol RAIL library, under plugins. Since DMP has a bigger memory footprint (both RAM and code), it's provided in a library which is not enabled in single protocol examples.

Next, you'll need PHY configurations for your protocols. You can either use Protocol Based Multi-PHY (see the Multi-PHY usecases and examples tutorial for details), or you can use embedded protocols, e.g. IEEE 802.15.4 2.4GHz 250kbps by calling RAIL_IEEE802154_Config2p4GHzRadio().

Micrium OS

Micrium OS is not technically required for RAIL DMP, although it's highly recommended to clearly separate protocols (and for RAIL+Bluetooth DMP, we actually only support Micrium-enabled projects.) Hence, Micrium OS will not be discussed in this article - it doesn't impact how you use RAIL APIs in RAIL-only DMP.

RAIL initialization

Let's see how one RAIL protocol should be initialized in DMP:

static void RAILCb_Generic(RAIL_Handle_t railHandle, RAIL_Events_t events);
static RAIL_Handle_t railHandle;
static RAILSched_Config_t railSchedState;
static RAIL_Config_t railCfg = {
  .eventsCallback = &RAILCb_Generic,
  .scheduler = &railSchedState,
  .protocol = NULL,
};

void initRadio1()
{
  railHandle = RAIL_Init(&railCfg, NULL);
}

static void RAILCb_Generic(RAIL_Handle_t railHandle, RAIL_Events_t events)
{
}

Let's compare it to the very first RAIL tutorial:

• railSchedState is a new static variable required for RAIL. This keeps the state of the protocol when it's not active (e.g. configured Rx options)
• railCfg.protocol is set to NULL. This is also a DMP specific field, but it's only used by the Silicon Labs Bluetooth stack.
• RAILCb_Generic is now static: Since all protocols will have their own callback functions, we should either make the function names unique, or put them in different C files as static. Although technically it's possible to use a common callback function for all protocols, it's not recommended as the readability of the code is much better with separate event handlers.
• halInit() is no longer called from initRadio1() - since we have multiple protocols to support, it makes more sense to set up RAIL requirements elsewhere in a protocol-independent init function

This code snippet initializes a single protocol. However, all remaining protocols can be initialized in the same way, noting that railHandle, railSchedState and RAILCb_Generic should be different for all protocols (probably by placing in separate C files).

From this point you can use RAIL almost the same way as in single protocol mode. Since almost all APIs have a railHandle argument, RAIL will know which protocol you want to access. However, you should be careful with:

• protocol independent APIs, i.e. the ones that don't have a railHandle argument
• APIs that affect the state of the radio scheduler, i.e. schedule or stop a radio task

Protocol independent APIs

There are a few APIs that always work the same way, regardless of which protocol they were called from:

• RAIL_SetTime()/RAIL_GetTime() - the timebase (which is also used for timestamping) is common for all protocols. It is not recommended to call RAIL_SetTime() in a DMP application (and calling it in a single protocol application can still be dangerous)
• RAIL_ConfigMultiTimer() - MultiTimer mode is a requirement in DMP, so it's enabled by default and cannot be disabled
• RAIL_Sleep()/RAIL_Wake() - Sleep management should be handled protocol-independently (e.g. in the idle task of the RTOS)

There are some other APIs that don't require railHandle, but they usually need a packet handler or timer handler, and so are still tied to a protocol.

Scheduling a finite radio task

The following APIs will trigger the radio scheduler to schedule a finite length new radio task:

• RAIL_ScheduleRx()
• RAIL_StartTx()
• RAIL_StartScheduledTx()
• RAIL_StartCcaCsmaTx()
• RAIL_StartCcaLbtTx()
• RAIL_StartAverageRssi()

(Note that RAIL_StartRx() and RAIL_StartTxStream() is not listed, as these as these are not finite length tasks.)

All the above APIs have the optional argument RAIL_SchedulerInfo_t. In single protocol mode, this argument is ignored, and can be NULL. In multiprotocol mode, this is the base of the scheduling, with its 3 member:

• priority - 0 to 255, 0 being the highest. Equal priority does not preempt
• slipTime - maximum time the task can be delayed due to other protocols, in us
• transactionTime - the time the task takes, in us

When the radio hardware is available, none of these scheduler info arguments are used. However, if two tasks are scheduled at the same time, the radio scheduler will try to stagger the operations using slipTime in order to satisfy both tasks. If that fails, the lower priority task will not get access to the radio.

It is also possible that an application schedules a high priority radio task when a conflicting lower priority task has already started. In that case, the lower priority task will be aborted (possibly in the middle of frame transmission).

If RAIL_SchedulerInfo_t is NULL, the scheduler will assume all parameters to be 0, i.e. highest priority, with no slipTime - it is highly recommended to not use this feature, and it's only mentioned here to simplify debugging.

Each protocol can schedule at most a single finite task.

For more details on the radio scheduler, see UG305 chapter 3 and the Multiprotocol description in the RAIL API doc.

Yielding

After each radio task the application must yield the radio. That can be achieved by calling RAIL_YieldRadio() or RAIL_Idle() - the latter also turns off the radio, which should have already occurred for finite tasks, so it's recommended to use RAIL_YieldRadio(). Manual yielding is required because the scheduler doesn't know about any automatic follow-up tasks (like ACKs) - you might want to yield immediately after receiving a packet, or you may instead want to wait for the auto ACK to go out.

Since yielding the radio will also call the radio scheduler to start/schedule the next radio task, you must first capture everything you need that might be overwritten by another protocol after yield, even in interrupt context. For example, timestamps of a transmitted packet must be accessed before yielding.

A Tx operation scheduled in the future can be cancelled using RAIL_Idle() or RAIL_StopTx(), where the latter doesn't yield the radio. An Rx operation scheduled in the future can only be cancelled using RAIL_Idle().

Background Rx

RAIL_StartRx() receives special handling in the radio scheduler, as it schedules an infinite task - namely, background Rx. The main difference when compared to finite tasks is that an infinite task sets the "default" state of the radio. It can be aborted by higher priority finite tasks, but it will be resumed automatically after the higher priority task is finished. For background Rx, this practically means that the radio will automatically return to Rx after Tx or packet reception, and you don't have to use auto state transitions like you do in single protocol mode. (Currently, the only infinite task supported by RAIL is background Rx).

Keep in mind that RAIL does not store the channel of background Rx: if you interrupt background Rx with a finite task on a different channel, the channel of the background Rx will change as well. E.g. if you're receiving in channel 0, then transmit on channel 1, the radio will "return" to receiving on channel 1. To change the background Rx channel back to 0, simply call RAIL_StartRx() again after you yield the radio.

RAIL_StartRx() has the same RAIL_SchedulerInfo_t configuration as the finite tasks, but depends only on the priority member - which, in general, should be the lowest priority used by the protocol.

Background Rx can be aborted using RAIL_Idle(), which also yields the radio. In DMP mode, it's recommended to only use RAIL_Idle() to stop background Rx.

It is also possible to change the priority of the background RX using RAIL_SetTaskPriority() - this is useful e.g. to increase the priority during packet reception, or after a particular packet.

TxStream

RAIL_StartTxStream() is called a debug task: A debug task must have the highest priority, because it can't be aborted. Otherwise, it works similarly to infinite tasks.

This API is different from all others as it doesn't use RAIL_SchedulerInfo_t. Since a stream can't really be aborted, the radio scheduler will handle this with the highest priority and no slip time.

Stream can be stopped with RAIL_StopTxStream(), RAIL_Idle() or RAIL_YieldRadio(), although it is recommended to use RAIL_StopTxStream() for clarity.

Auto state transitions

You might wonder, "what's the point of auto state transitions in DMP, if background Rx changes the default state to Rx?" The added value of auto state transitions is that the resulting state will inherit the original state's priority.

For example, you probably want the ACK reception on the same priority as the transmission itself, so you can set RAIL_SetTxTransition() to set up receive after the transmission for the ACK, and configure a timeout for it using RAIL_SetStateTiming().

If you have automatic state transition set up, you should only yield the radio after the whole operation (e.g. ACK reception or timeout) is finished. It's highly recommended to have automatic timeouts to prevent a high priority infinite task, but if you haven't set up timeouts, you can cancel the receive and yield the radio using RAIL_Idle().

To summarize:

• In RAIL single protocol, the default state is always idle, and auto state transitions should be used to always return the radio to Rx. In DMP, background Rx should be used for this.
• In single protocol mode, Rx state started by auto state transition is the same as Rx state started by StartRx. In DMP, auto state transitions are intended for ACK, as they inherit the (usually high) priority of the preceding task.

For more details, see UG305 chapter 6

MultiTimers

RAIL includes a timer virtualization service, called MultiTimer. Since its memory footprint is not negligible, this feature is disabled by default in single protocol mode. However, in multiprotocol mode, it is always enabled, since multiple timers are required for the multiple protocols. Therefore, using MultiTimers in your protocol implementations has no drawback in DMP.

Error handling

In single protocol RAIL, you generally need to places to handle errors:

• RAIL_StartXYZ()/RAIL_ScheduleXYZ() might return an error. If no error were returned, the operation was either finished, or an event will be triggered
• The event handler, where either a success or error event can be triggered, e.g. RAIL_EVENT_TX_PACKET_SENT or RAIL_EVENT_TX_CHANNEL_BUSY

In DMP, there's a third error case to handle above the previous two: You should always handle RAIL_EVENT_SCHEDULER_STATUS, where you might receive an error by calling RAIL_GetSchedulerStatus(). It's usually obvious which API call failed, e.g. RAIL_SCHEDULER_STATUS_SINGLE_TX_FAIL means a RAIL_StartTx() failed, but it might not be possible to know the original call resulted in error.

This was implemented because when you call e.g. RAIL_StartTx, the radio scheduler just creates task. When that task is running, it will actually call the single protocol RAIL_StartTx, and if that returns an error, it will trigger an RAIL_EVENT_SCHEDULER_STATUS event.

The event RAIL_EVENT_SCHEDULER_STATUS is also used when a higher priority task interrupts an ongoing radio operation, in which case, RAIL_SCHEDULER_STATUS_EVENT_INTERRUPTED will be returned by RAIL_GetSchedulerStatus().

Debugging

When debugging DMP, Rx/Tx PRS channels are still very useful, see the tutorial about debugging for details.

For DMP based code, writing out to a GPIO when a protocol is scheduled/unscheduled is really useful. This can be easily done by setting a GPIO on RAIL_EVENT_CONFIG_SCHEDULED and clearing it on RAIL_EVENT_CONFIG_UNSCHEDULED.

Recommendations and practices

Although these practices are very important for DMP applications, you should consider applying them in single protocol applications as well, to simplify a potential future port to DMP:

• Only use RAIL_Idle() when it's necessary - In RAIL 1.x almost all APIs required that they be called from idle mode. This requirement was removed in RAIL 2.x, so it's very rarely needed, and since RAIL_Idle() also yields the radio, it should be very rarely used in DMP.
• Use the scheduling Rx/Tx features as much as possible - i.e. don't use timers and start Rx/Tx at timer interrupt, use the corresponding RAIL schedule API instead. This helps the radio scheduler to resolve conflicts with the configured slipTimes, or at least promptly let the application know about an unavoidable conflict.
• Set the slipTime/transactionTime correctly. Again, this helps the radio scheduler to resolve conflicts.

How not to use DMP

Using multiple DMP instances for multiple radio tasks (e.g. one protocol for advertising, one for connection, or one for Tx, one for Rx) is bad design: while it works, each protocol instance has a significant memory footprint, and switching time between protocols is much slower than switching Rx/Tx inside a protocol. At the moment, it is recommended to use DMP only for serving separate protocol stacks.

Using DMP to handle multiple PHYs for the same protocol stack is also not recommended: Multi-PHY is a much better solution for that, see the introduction to Multi-PHY and Multiprotocol tutorial for more details.

Related documentation

• UG305: Dynamic Multiprotocol User's Guide - You can skip the chapters that won't affect your design, e.g. if you don't plan to use zigbee, you can skip the zigbee chapters. This article somewhat overlaps with UG305 as we provide a short summary of the basics here.
• AN1134: Dynamic Multiprotocol Development with Bluetooth and Proprietary Protocols on RAIL - Read it if you plan to use Bluetooth - RAIL DMP, as it presents how to create Bluetooth - RAIL examples.
• The Multiprotocol description in the API doc - It is the concentrated (compared to UG305) description of the Radio Scheduler, and also overlaps with this article.

If you find any conflicts between this article and the above documents, give those documents priority as they will more quickly receive updates to track new RAIL DMP features and guidance.

Introduction

The goal of this tutorial series is to help you get started with RAIL, the C API for embedded software for Silicon Labs' EFR32 Wireless Geckos.

To read it, you should have some experience with embedded C - you should know how to handle interrupts, what volatile means, etc.

While we try to avoid using it, you might need some experience with radios as well. It definitely helps if you know what preamble or sync word is.

The tutorials should be read together with the RAIL API documentation.

If you find any contradiction between the tutorials and the API documentation, just always remember that the API documentation is more accurate.

This tutorial series focuses on the embedded code running on Wireless Geckos. Configuring the radio correctly is equally important, see AN971 for details on that for Studio v4 and AN1253 for Studio v5.

The tutorial series is currently under review and update for the Studio v5 updates. Although RAIL itself haven't changed much, some critical tools did. For details, see AN1254. The tutorials are mostly accurate for both v4 and v5. On those articles where an update will be needed in the near future, a note is added to the first couple of paragraphs.

Getting Started

You should start with these tutorials. All others are based on these, as these are the minimum to develop useful applications on RAIL.

Studio v5 series

1. Introduction, RAIL components and the Empty Example
2. Transmitting a Packet
3. Event Handling and Automatic State Transitions
4. Combining Transmit and Receive

Studio v4 series

1. Introduction and init
2. Transmitting a Packet
3. Event Handling
4. Combining Transmit and Receive

Basic Tutorials

These tutorials are common and basic features, you should read them before you start working on a project which uses RAIL.

• Time, Timestamping and Scheduling
• Calibrations
• Debugging
• RAIL Utility, Initialization: how it works, how to init RAIL without it - coming soon (studio5 specific)
• Power manager with RAIL - coming soon (studio 5 specific)

Advanced Tutorials

These tutorials describe features that are rarely needed - they are available and can be used when necessary, but you probably won't use them all in your application. Generally, you should read them only if you think you need them.

• Special Data Modes (FIFO, Direct, IQ)
• Understanding RAIL Config Files
• Tx/Rx Options
• Timer synchronization and sleep

Multi-PHY and Multi protocol Tutorials

These tutorials are about the Multi-PHY and multiprotocol capabilites of RAIL. If you plan to use that feature, we recommend to read the introduction then what you need, depending on your application.

• Introduction to Multi-PHY and Multiprotocol
• Multi-PHY usecases and examples
• Dynamic Multiprotocol (DMP)