The NFCT peripheral implements a frame assembler in hardware.
When the NFCT peripheral is in the ACTIVE_A state, the software can decide to enter RX or TX mode. For RX mode, see Frame disassembler. For TX mode, the software must indicate the address of the source buffer in Data RAM and its size through programming the PACKETPTR and MAXLEN registers respectively, then issuing a STARTTX task.
MAXLEN must be set so that it matches the size of the frame to be sent.
The STARTED event indicates that the PACKETPTR and MAXLEN registers have been captured by the frame assembler EasyDMA.
When asserting the STARTTX task, the frame assembler module will start reading TXD.AMOUNT.TXDATABYTES bytes (plus one additional byte if TXD.AMOUNT.TXDATABITS > 0) from the RAM position set by the PACKETPTR.
The NFCT peripheral transmits the data as read from RAM, adding framing and the CRC calculated on the fly if set in TXD.FRAMECONFIG. The NFCT peripheral will take (8*TXD.AMOUNT.TXDATABYTES + TXD.AMOUNT.TXDATABITS) bits and assemble a frame according to the settings in TXD.FRAMECONFIG. Both short frames, standard frames, and bit-oriented SDD frames as specified in the NFC Forum, NFC Digital Protocol Technical Specification can be assembled by the correct setting of the TXD.FRAMECONFIG register.
The bytes will be transmitted on air in the same order as they are read from RAM with a rising bit order within each byte, least significant bit (LSB) first. That is, the least significant bit (b0) will be transmitted on air before the second bit (b1), and so on. The bits read from RAM will be coded into symbols as defined in the NFC Forum, NFC Digital Protocol Technical Specification.
The frame assembler can be configured in TXD.FRAMECONFIG to add SoF symbol, calculate and add parity bits, and calculate and add CRC to the data read from RAM when assembling the frame. The total frame will then be longer than what is defined by TXD.AMOUNT.TXDATABYTES. TXDATABITS. DISCARDMODE will select if the first bits in the first byte read from RAM or the last bits in the last byte read from RAM will be discarded if TXD.AMOUNT.TXDATABITS are not equal to zero. Note that if TXD.FRAMECONFIG.PARITY = Parity and TXD.FRAMECONFIG.DISCARDMODE=DiscardStart, a parity bit will be included after the non-complete first byte. No parity will be added after a non-complete last byte.
The frame assemble operation for different settings in TXD.FRAMECONFIG is illustrated in the following table. All shaded bit fields are added by the frame assembler. Some of these bits are optional and appearances are configured in TXD.FRAMECONFIG. Note that the frames illustrated do not necessarily comply with the NFC specification. The figure only illustrates the behavior of the NFCT peripheral.
The accurate timing for transmitting the frame on air is set using the frame timing controller settings.