A COMM CLEAR is sent on the RX pin of the base device. It does not send to the stack devices. RX cannot be disabled and a COMM CLEAR can be sent at any time regardless of the TX status. Ensure that the COMM CLEAR does not exceed the maximum value of t_UART(CLR) bit periods, as this may result in recognition of other communication pings.

Figure 8-19 UART COMM CLEAR

Use the COMM CLEAR command to clear the receiver and instruct the UART engine to look for a new start of frame. The next byte following the COMM CLEAR is always considered a start-of-frame byte. When detected, a COMM CLEAR sets the FAULT_COMM1[COMMCLR_DET] flag. The host must wait at least t_UART(RXMIN) after the COMM CLEAR to start sending a new frame. It should be noted that in addition to the [COMMCLR_DET] flag, the FAULT_COMM1[STOP_DET] flag is also set because the COMM CLEAR timing violates the typical byte timing and the STOP bit is seen as 0.

A SLEEPtoACTIVE ping/tone also clears the UART receiver. This ping/tone sets the [COMMCLR_DET] flag when transiting from SLEEP to ACTIVE mode. If this ping/tone is sent during ACTIVE mode, the [COMMCLR_DET] and [STOP_DET] flags are set.

COMM CLEAR sent during daisy-chain communication:

When a read command is sent, but the response has not yet completely returned to the host, if a COMM CLEAR is received in the base device at this condition, the device response is discarded. In addition, the stack devices do not see the COMM CLEAR and continue to send their responses which are forwarded to the host, resulting in host receiving unexpected response frames. Hence, host should avoid this condition by waiting until all responses are received from the stack before sending a COMM CLEAR.

If the above condition occurs, the base device low-level communication debug register DEBUG_UART_RR_TR[TR_WAIT] (indicating device is waiting to transmit response) or DEBUG_UART_RR_TR[TR_SOF] (indicating a COMM CLEAR is received while device is transmitting data) bits can be set depending on the timing in receiving the COMM CLEAR signal.

When using the multidrop configuration, a COMM CLEAR signal must be used before every frame to ensure consistent communication.