The NFCT peripheral includes a frame timing controller that continuously keeps track of the number of the 13.56 MHz RF carrier clock periods since the end of the EoF of the last received frame.
The NFCT peripheral can be programmed to send a responding frame within a time window or at an exact count of RF carrier periods. In case of FRAMEDELAYMODE = Window, a STARTTX task triggered before the frame timing controller counter is equal to FRAMEDELAYMIN will force the transmission to halt until the counter is equal to FRAMEDELAYMIN. If the counter is within FRAMEDELAYMIN and FRAMEDELAYMAX when the STARTTX task is triggered, the NFCT peripheral will start the transmission straight away. In case of FRAMEDELAYMODE = ExactVal, a STARTTX task triggered before the frame delay counter is equal to FRAMEDELAYMAX will halt the actual transmission start until the counter is equal to FRAMEDELAYMAX.
In case of FRAMEDELAYMODE = WindowGrid, the behaviour is similar to the FRAMEDELAYMODE = Window, but the actual transmission between FRAMEDELAYMIN and FRAMEDELAYMAX starts on a bit grid as defined for NFC-A Listen frames (slot duration of 128 RF carrier periods).
An ERROR event (with FRAMEDELAYTIMEOUT cause in ERRORSTATUS) will be asserted if the frame timing controller counter reaches FRAMEDELAYMAX without any STARTTX task triggered. This may happen even when the response is not required as per NFC Forum, NFC Digital Protocol Technical Specification. Any commands handled by the automatic collision resolution that don't involve a response being generated may also result in an ERROR event (with FRAMEDELAYTIMEOUT cause in ERRORSTATUS). The FRAMEDELAYMIN and FRAMEDELAYMAX values shall only be updated before the STARTTX task is triggered. Failing to do so may cause unpredictable behaviour.
The frame timing controller operation is illustrated in the following figure. The frame timing controller automatically adjusts the frame timing counter based on the last received data bit according to NFC-A technology in the NFC Forum, NFC Digital Protocol Technical Specification.