Disabling RX parity processing with ST25R3916
Hello,
I am trying to implement the Mifare Classic protocol with ST25R3916 and RFAL.
For that CRC and parity bits processing need to be disabled, which ST25R3916 support, so I tried to experiment with disabling CRC and parity framing on a message sent by the tag to see how it is encoded.
I am having a hard time figuring out exactly what's the encoding, and the datasheet of ST25R3916 doesn't say exactly what's in the FIFO.
I set up X-CUBE-NFC6 with STM32CubeMX and simply modified the polling demo to send the command 6000 + CRC to the tag, which should respond with a 4-bytes nonce with standard ISO-14443-3A parity and no CRC.
This is the only part of the code I added, after selecting the ISO-14443-3A tag:
uint32_t flags =
// CRC stuff
RFAL_TXRX_FLAGS_CRC_TX_AUTO |
RFAL_TXRX_FLAGS_CRC_RX_MANUAL |
RFAL_TXRX_FLAGS_CRC_RX_KEEP |
// parity bits
RFAL_TXRX_FLAGS_PAR_RX_KEEP |
RFAL_TXRX_FLAGS_PAR_TX_AUTO |
// analog
RFAL_TXRX_FLAGS_AGC_ON;
uint8_t tx_buf[8] = {0x60, 0x00}; // use key A on block 0
uint8_t rx_buf[32] = {0};
uint16_t rx_buf_rcvd_len = 0;
ReturnCode ret = rfalTransceiveBlockingTxRx(tx_buf, 2, rx_buf, 32, &rx_buf_rcvd_len, flags, rfalConvMsTo1fc(100U));
if (ret != RFAL_ERR_NONE && ret != RFAL_ERR_CRC) {
platformLog("Error: %d\r\n", ret);
} else {
platformLog("Received nonce: (%d) ", rx_buf_rcvd_len);
for (int i = 0; i < 32; i++) {
platformLog("%02X", rx_buf[i]);
}
platformLog("\r\n");
}In parallel to this I am monitoring the communication between the ST25R3916 and the tag with a RFID debugging tool, so I know what the card is actually sending.
When the card sends 23 50 81 FD, what I read from the FIFO is 23 A0 06 EE 07. It is 36 bits as expected (only the last 4 bits of the last byte are actual data), and the first byte always matches, but the rest of it is a mystery.
I assumed what I read from the FIFO was byte1 + parity1 + byte2 + parity2 + byte3 + parity3 + byte4 + parity4 for a total of 36 bits, but it is not the case.
Still in this same example, if I add the parity bits to what the card actually sends I get:
00100011 0 01010000 1 10000001 1 11111101 0
But if I read what I got from the FIFO, I get:
00100011 1 01000000 0 00011011 1 01110011 1
Could someone shine some light on this?
Thank you