nrf23L01+ lost ACK payload packets

[angelatlarge]

I am a bit confused about what happens to ACK_PLD when ack payload packets are lost. I am assuming that in case of ack payload packet loss MAX_RT bit in STATUS is set.

However, beyond that, is failed ack payload packet then cleared from ACK_PLD, or does it stay there and a retransmit will be attempted when MAX_RT is cleared?

Also, I am not 100% clear on the relationship between ACK_PLD and TX_EMPTY bit in FIFO_STATUS: does ACK_PLD data affect TX_EMPTY

[Håkon Alseth]

Hi,

ACK_PLD is the internal buffer which is filled by SPI command "W_ACK_PAYLOAD". If you have filled the TX FIFO with one payload (regardless of ACK-payload or normal payload), the TX_EMPTY bit in the FIFO_STATUS register will be '0'.

An ACK-payload is uploaded using the SPI command "W_ACK_PAYLOAD", which is written into the nRF just as a normal payload. MAX_RT is an interrupt that only a transmitter will see. A device configured as a receiver will not see this interrupt.

The problem with an ACK payload is that you will not get an ACK on the ACK-payload (weird sentence, I know). How this is handled inside the radio state machine is that there is a PID field which is a 2 bit counter. This counter will increment for each new packet so that the receiver can will be able to interpret a new packet from a re-transmission. This way, the ACK payload will give the receiver a "TX_DS" interrupt, however; the TX_DS signal will only occur on the next payload:

• PTX sends payload #1
• PRX receives, and sends ACK-payload

In this scenario, the PRX does not know if the ACK-payload was successfully. On the next payload sent, this will happen:

• PTX sends payload #2 and PID counter is incremented.
• PRX receives, both "RX_DR" and "TX_DS" will be set in the STATUS register.

If the PTX lost the ACK-payload from the PRX, the PTX will send a re-transmit which the PRX will interpret and re-send the ACK-payload.

The PRX will only forget the ACK-payload once the PID field has incremented (the next PTX payload) and reports "TX_DS" up to the application, meaning that the PTX has successfully received the previous ACK-payload.

Hope this makes sense.

Cheers, Håkon

[jrowe]

To be clear, do I need to manually re-send my ack packets if the TX_DS bit is not set on the PRX device, or will the PRX re-send the data automatically if the bit is not set? Thanks again.