2.1 RXMode Issue with EOT Before SOT

When using the “Stay in RX after EOT”, there is a minor chance that subsequent receptions will fail. This happens when the internal firmware and hardware state machines fall out of sync, which disables proper signal reception due to an unintentionally set status flag.

In this condition, the system technically still detects signals, but the reception is aborted immediately after the Start of Telegram (SOT) state is reached. The standard notifications/events (WCO, SOT, and EOT) are still generated, but the receive buffer fill level remains at zero (0x00).

The system enters this suspect state if an End of Telegram (EOT) event occurs shortly before the system reaches the Start of Telegram (SOT). In other words, there is a break in the reception shortly before reaching the SOT.

The critical time window for entering this state is less than 6 μs. If the EOT occurs 6 μs or less before reaching the SOT, the software and hardware state machines may fall out of sync during the restart of the receive path. As a result, the system can no longer properly receive RF messages (payload data).

Under very specific conditions, the time window may be extended to 40 μs, specifically for very strong input signals (>−39 dBm) at the LNA input. If the input signal level varies around this threshold during EOT detection, the internal antenna‑damping state machine initiates a restart of the channel filter, resulting in additional latency before the receive path is reactivated. This can occur when the signal level crosses the threshold during the EOT event, that is, when it transitions from below −39 dBm to above −39 dBm or vice versa.

All of the following conditions must be met:

  • RXMode active
  • “Stay in RX after EOT” option enabled
  • Automatic Antenna Damping enabled
  • Carrier Check EOT2 enabled
  • One of the following receiver bandwidths is selected: 71 kHz, 80 kHz, 93 kHz, 165 kHz, 185 kHz, 219 kHz, or 243 kHz (bandwidths resulting in a baseband clock of 215.1 kHz or 502 kHz)

Countermeasure/Detection

There are multiple ways to address or detect the issue:
  • The unintended state can be suspected if multiple consecutive receive attempts return no payload in the receive buffer.
  • Final confirmation is obtained by reading register SSMFCR (address 0x00FF). If bit 1 is set, the system is in the desynchronized state.

Work Around

Once the desynchronized state is confirmed, restart RXMode to resynchronize hardware and software.

There are multiple options available for fixing or addressing the issue:

  • Change the receiver bandwidth to a value not listed above. The demodulator clocking scheme depends on the channel filter bandwidth. Changing it alters several clock domains (frequency and phase relationship), preventing the system from entering the desynchronized state.
  • Disable Carrier Check EOT2. This check can trigger an EOT shortly before SOT. Disabling it removes the condition that leads to desynchronization.
  • Disable “Stay in RX after EOT”. Transitioning to Idle mode after EOT allows the main microcontroller to process the received data and restart RXMode when required. A fresh RXMode start ensures hardware and software are always synchronized.
  • Disable Automatic Antenna Damping. This speeds up internal EOT processing. With faster EOT handling, an SOT cannot occur at the critical time, preventing the scenario of the desynchronization.

Affected Silicon Revisions

DEF
XXX