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.

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.

The BT factor on the Gaussian pulse shaping filter is often used for narrowing spectral emissions at the Tx side to fit into a regulatory / applications standard mask requirement. On how to adjust the BT factor on the EZRadioPro familiy you can read here.

Note that adjusting the BT factor will only alter the spectrum emission of the modulated signal, it will not affect discrete spurious emissions that are not part of the modulation, like these.

Adjusting the BT factor, however does have an effect on sensitivity at the Rx side. This is quite often overlooked and may result in a huge loss in the link budget. To that end find below two graphs showing sensitivity versus BT factor parameterized with modulation index. (These measurements were taken on revision revB1B but hold up for revC2A/ A2A.)

Key takeaway:

Don’t ever go to a lower BT factor than 0.25 if the modulation index <= 2. If narrower emissions are required lower the modulation index instead.

Also be prepared that the frequency offset tolerance of the Rx degrades by dropping BT and / or the modulation index.

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.

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.

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

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)

This tutorial builds on the following tutorials:

• RAIL Tutorial 1: Introduction and init
• RAIL Tutorial 2: Transmitting a Packet
• RAIL Tutorial 3: Event Handling
• RAIL Tutorial 4: Combining Transmit and Receive

To get the most out of this tutorial, some familiarity with the radio configurator is required, and it's also recommended that you understand the basics of radio protocol stacks (i.e. the OSI model).

You can find these prerequisite tutorials on the table of contents site.

Introduction

With a customizable radio, the Wireless Gecko is a great option to support any radio configuration you need. You can set it up for standard IEEE 802.15.4, Bluetooth Low Energy, WMBus, or even the protocol (designed 30 years ago) used by your garage door opener. However, what if you need more than one of those standards? With single protocol radios, this requires a distinct 15.4 radio chip, a Bluetooth chip, and so on for each supported standard. Alternatively, with a single Wireless Gecko, you can store multiple "setups" in your device and switch between them. The only limitation is that it's still a single radio - you can't use multiple "setups" on the radio at the same time.

The best way to manage the sharing of this single radio resource depends on the particular usecase targeted by your application. We provide different solutions (discussed below) to help you identify an optimal approach for your design.

Channel-based Multi-PHY

The most common usecase in proprietary wireless is the need to support different PHY configurations. For example:

• You always handle received packets the same way, but you have to receive at multiple bitrates
• You have an asymmetric protocol, i.e. the Rx configuration is not the same as the Tx configuration
• You need wider bandwidth for CSMA/CA than your normal Rx mode to pass regulatory standards
• You have to handle different packet formats, but not at the same time: the upper layers of the stack decide which one to use
• Although the PHYs are technically the same, there is a power limitation on a given channel to pass regulatory standards
• The PHYs are technically the same, but the channel map is uneven - it's not possible to configure it using channel spacing

In these cases, Channel-based Multi-PHY is the best solution. It's called Channel-based, because in RAIL, you only need to change the channel to change the PHY.

You can find a setup guide for some of these usecases in the Multi-PHY usecases and examples tutorial.

Protocol-based Multi-PHY

This is not as widely used as Channel-based Multi-PHY, but it's the next logical step. The typical usecases:

• Your device will support multiple regions, and you choose one during configuration, but the device will rarely switch between those.
• Your device will support multiple protocols, but the device will rarely switch between those.

For this, we recommend Protocol-based Multi-PHY. In this case, you can switch between protocols by calling RAIL_ConfigChannels(), which will load the whole configuration.

Protocol based Multi-PHY is also useful if you want to test multiple configuration setups using RAILTest: You can change between protocols using the setConfigIndex RAILTest command.

You can find a setup guide for some of these usecases in the Multi-PHY usecases and examples tutorial.

Channel-based vs Protocol-based Multi-PHY

In general, you should consider your application requirements: If you want to have (for example) 20 channels, but those 20 channels are defined differently in some application configurations (typically by region), you should use Protocol-based Multi-PHY. A protocol change will be slower than a channel change, but the application doesn't even need to know which protocol is in use, since it still sees the same 20 channels through RAIL.

Otherwise, in almost all cases, Channel-based Multi-PHY is a better choice.

Dynamic Multiprotocol (DMP) between RAIL-based protocols

If you have full protocol stacks that are completely independent, it makes sense to develop the PHY and Link layer independently as well. These layers communicate with RAIL, and the cleanest solution to this problem is Dynamic Multiprotocol. Typical usecase:

• One protocol is used to communicate with key fobs, while another is used to communicate in a home automation network

In Dynamic Multiprotocol, the RAIL scheduler decides which protocol is active.

Dynamic Multiprotocol is more capable than Multi-PHY, but also more complex, so it has some drawbacks when compared to Multi-PHY:

• Protocol change takes 450us on EFR32xG1x. This is a fixed worst-case delay, because the RAIL scheduler must plan time-sharing of the radio resource using calculations on a known sufficient time span in order to accomodate protocol switches.
• It requires the multiprotocol version of the RAIL library, which has a bigger memory footprint (both RAM and code) than the single protocol version.

Writing RAIL application in DMP will be the subject of this tutorial.

Dynamic Multiprotocol between RAIL-based and other Silicon Labs stack protocols

The usecase for this is straight forward:

• If you need Bluetooth and a RAIL-based protocol in the same application

At the moment, Silicon Labs only provides such a DMP solution based on the Bluetooth stack and RAIL. You can access it by installing the Bluetooth SDK and starting one of the DMP applications. See AN1134 for more details, and an upcoming tutorial detailing DMP usage from a RAIL perspective.

Other multiprotocol solutions

You can find other types of multiprotocol mentioned in some Silicon Labs documents, but they are rarely used:

Programmable multiprotocol means multiple firmware images for each protocol, and the protocol is decided during the manufacturing process, i.e. a device is either programmed to be a Bluetooth or a Zigbee device.

Switched multiprotocol means two firmware images, one for each protocol. A bootloader loads the firmware needed for the protocol, and the two firmware can communicate with each other in a shared NVM storage. Protocol switching in switched multiprotocol is rather slow, and using DMP is almost always a better choice.

Concurrent multiprotocol means that the PHY and maybe the lower layers of the stacks are the same, but the upper layers are different. E.g. You can run two proprietary 15.4 protocol on the same channel at the same time, but you don't really need multiprotocol specific APIs to support it.

Related documentation

• AN971: EFR32 Radio Configurator Guide
• UG103.16: Multiprotocol fundamentals