On the EZRadioPro and EZRadio devices the Rx (Receive) state machine can automatically change the device’s state upon three different events:

1. RX TIMEOUT
2. RX VALID
3. RX INVALID

Let’s have a look at these events and the mechanisms that can make them occur!

RX TIMEOUT: Generally speaking this is an event that signals that there has been no signal detection in the current receive process (i.e., from the time instant the receiver got started / re-started). There are a few mechanism that can provide this signal.

• No preamble is detected within the given preamble detection timeout period. This mechanism is the default configuration when preamble detection is enabled in the receiver. On EZRadioPro devices the preamble detection timeout can be configured in API property PREAMBLE_CONFIG_STD_2. On EZRradio devices this timeout period is set to 15 nibbles (60 bits) by default and cannot be changed.

• The Latched RSSI value is below a threshold defined in API property MODEM_RSSI_THRESH. This option is only available on EZRadioPro devices (not on EZRradio devices). For this mechanism to take effect bit field “CHECK_THRESH_AT_LATCH” must be set in API property MODEM_RSSI_CONTROL. The latching event itself can also be configured in the very same API property.

• The Digital Signal Arrival (DSA) detector reports a “no signal” condition. This option is only available on EZRadioPRO Si446x revC2A/revA2A devices. The DSA detector can typically report a “no signal” condition by observing the channel for 6Tb times from the time instant the receiver has got started. This time is is not fixed, however and can change statistically (with some low probability) to longer times. The important thing is that asserting the TIMEOUT signal is event driven in this case as opposed to checking a certain condition at a well-defined time instant (wrt to receive start) as in the above two cases. For activating this feature field “low_duty” must be set to a non-zero value in API property MODEM_DSA_MISC. This seemingly random enabling mechanism is only one thing of many others that have to be configured for DSA operation therefore the configuration is better left for the WDS calculator. Note that at the time of writing this article this feature is not yet publicly available in WDS (3.2.9.0).

RX VALID: This event simply signals that a packet has been received without a CRC error. Note that if no CRC check is configured in the packet handler, all received packets (albeit erroneous) will be also deemed valid packets.

RX INVALID: This event simply signals that a packet has been received with a CRC error. Note that if no CRC error check is configured in the packet handler, this event will never come about.

Now, in the START_RX command a state can be defined for all above described events (look for arguments RXTIMEOUT_STATE, RXVALID_STATE, RXINVALID_STATE at the START_RX API command) where the automatic state machine will bring the part upon the occurrences of these events.

Let’s go through the typical state transition use cases:

0: NO CHANGE: There will not be any change in the receiver’s state whatsoever. This is the recommended value for the RX TIMEOUT event in normal (i.e., not LDC or PSM) continuous receive mode. One might say the 8: RX_STATE is as good a state transition as no change in such a case. When RX_STATE is selected however, the receiver does get re-started making it blind for a few 100 us while the calibrations are completed. Note also that in automatic frequency hopping Rx application at NO CHANGE the receiver will still hop onto the next channel.

1: SLEEP: This is the recommended value for the RX_TIMEOUT state when in automatic LDC RX mode. The part will be sent back to SLEEP state if no signal is detected in the channel.

3: READY: This state is recommended after a (valid) packet has been received. There are a few advantages of using READY state as opposed to restarting the receiver straight away. (1) The latched RSSI and frequency error measurement values are reset upon entering into RX state therefore if the host FW is not quick enough at reading them they may get lost if automatic RX STATE transition is selected. (2) If the receiver is sent back to RX STATE automatically another packet may be being received while the previous one is being retrieved from the FIFO calling for very careful FIFO handling from the host not to lose parts of packets.

8: RX: This state transition can be used after a (valid/invalid) packet has been received taken above considerations (at READY state) into account.

9: IDLE: This state transition is used by the Preamble Sense Mode (PSM) operation where the receiver is duty cycling in a short term basis. The part will transition into this state of no signal has been detected by the DSA block.

Independently of all above described there is another automatic state machine running in the demodulator while the part is Rx state. (Let’s call this state machine the demodulator state machine and one discussed above the Rx state machine.) It has got three states: (1) Preamble search, (2) Sync word search (3) Packet Receive. Note that the demodulator state machine always gets reset when the Rx state machine transitions into Rx state.

Let’s have a look at the two commonly used packet based receive processes!

• The most typical application is receiving a packet with a long enough preamble to do a robust preamble detection on (i.e., preamble length >= 32 bits). In such a scenario once the receiver is started the demodulator state machine will start off in preamble search state. It will stay in this state as long as preamble is detected. (Note that RX state machine can restart the receiver at this phase dependent on the TIMEOUT event next state setting.) Once preamble has been detected the demodulator will transition to sync word search state upon the second erroneous preamble bit (this is how it determines the end of the preamble pattern) and kicks off the sync word timeout timer. If no sync word is found within the sync word timeout period (typically 2 x sync word length) the state machine will transition back to preamble search state. If the sync word has been found the state machine will transition into packet receive state. It will be staying in packet receive state until all the predefined number of bytes have been loaded into the FIFO. Once the packet has been received the demodulator state machine takes no further actions. In order to reset the sate machine the receiver must be restarted either by the Rx state machine or manually. The following state diagram shows the operation for this example.

• When the preamble is not long enough for robust preamble detection the demodulator state machine will start off in sync word search state and the operation simplifies to the following diagram. In this mode of operation sync word timeout has no meaning and is not operational. It follows than that no INVALID_SYNC interrupts can fire either. It is also worth noting that the VALID_PREAMBLE signal (if observed on one of the GPIOs) will always be high.

All the considerations on the demodulator state machine hold for both the EZRadioPRO and EZRadio devices.