S2-LP datasheet(62/90 Pages) STMICROELECTRONICS

62 / 90 page

Link layer protocol

S2-LP

62/90

DocID029944 Rev 1

8.1.1

Automatic acknowledgment with piggybacking

The receiver can fill the ACK packet with data. The mode piggybacking must be set and the

TX FIFO must be filled with the payload to transmit.

When the transmitter uses the piggybacking to fill the ACK packet, a further automatic

acknowledgment and/or retransmission are not explicitly supported. The transmitter, can

determine if its piggybacked packet was received or not by the fact that the initiator will

retransmit the original packet or not. Simply stated:



If the receiver does not retransmit its packet, it means that he has correctly received

the acknowledgment, hence the piggybacked packet, so everything is fine.



If the initiator retransmits its packet, then the destination shall re-acknowledge it and

just resend the piggyback packet again.

8.1.2

Automatic retransmission

If the transmitter does not receive the requested ACK packet, it can be configured to do

other transmissions. The maximum number of transmission configurable is 16 and it is

specified in the register NMAX_RETX (allowed value are from 0 to 15, setting it to ‘0’

disables the feature). The current number of TX attempts is readable in the N_RETX

contain the effective number of attempts done (NMAX_RETX + 1 at most, in this case the

interrupt “Max Re-TX reached” is generated and the TX FIFO is not cleared, it will be

responsibility of the MCU decides to flush the TX FIFO or not).

The TX FIFO does not need to be filled again for the retransmission, but must be loaded

with a single write FIFO operation.

If the automatic retransmission is enabled (transmitter side), the RX command is not

supported and must not be used.

In Figure 12: "Automatic retransmission scenario" a possible scenario is shown:

The receiver does not fulfill the ack request of the first transmission because does not

receive the packet.

The transmitter send again the packet, but in this case is the ack packet to be lost. So,

the communication fails again.

The transmitter send again the packet and receive the ack packet. The communication

is working correctly this time.

S2-LP Datasheet(PDF) 62 Page - STMicroelectronics