The NFCT peripheral implements a frame disassembler in hardware.
When the NFCT peripheral is in the ACTIVE_A state, the software can decide to enter RX or TX mode. For TX mode, see Frame assembler. For RX mode, the software must indicate the address and size of the destination buffer in Data RAM through programming the PACKETPTR and MAXLEN registers before issuing an ENABLERXDATA task.
The STARTED event indicates that the PACKETPTR and MAXLEN registers have been captured by the frame disassembler EasyDMA.
When an incoming frame starts, the RXFRAMESTART event will get issued and data will be written to the buffer in Data RAM. The frame disassembler will verify and remove any parity bits, start of frame (SoF) and end of frame (EoF) symbols on the fly based on RXD.FRAMECONFIG register configuration. It will, however, verify and transfer the CRC bytes into RAM, if the CRC is enabled through RXD.FRAMECONFIG.
When an EoF symbol is detected, the NFCT peripheral will assert the RXFRAMEEND event and write the RXD.AMOUNT register to indicate numbers of received bytes and bits in the data packet. The module does not interpret the content of the data received from the remote NFC device, except for SoF, EoF, parity, and CRC checking, as described above. The frame disassemble operation is illustrated in the following figure.
Per NFC specification, the time between EoF to the next SoF can be as short as 86 μs, and thefore care must be taken that PACKETPTR and MAXLEN are ready and ENABLERXDATA is issued on time after the end of previous frame. The use of a PPI shortcut from TXFRAMEEND to ENABLERXDATA is recommended.