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/* ISO 14443 B Rx (PCD->PICC) implementation
* (C) 2006 by Harald Welte <hwelte@hmw-consulting.de>
*
* speed(kbps) 106 212 424 848
* etu 128/fc 64/fc 32/fc 16/fc
* etu(usec) 9.4 4.7 2.35 1.18
*
* NRZ-L coding with logic level
*
* logic 1: carrier high field amplitude (no modulation)
* logic 0: carrier low field amplitude
*
* Character transmission format:
* start bit: logic 0
* data: eight bits, lsb first
* stop bit logic 1
*
* Frame Format:
*
* SOF char [EGT char, ...] EOF
*
* SOF: falling edge, 10..11 etu '0', rising edge in 1etu, 2..3etu '1'
* EGT: between 0 and 57uS
* EOF: falling edge, 10..11 etu '0', rising edge in 1etu
*
*
* Sampling
* - sample once per bit clock, exactly in the middle of it
* - synchronize CARRIER_DIV TC0 to first falling edge
* - Configure CARRIER_DIV RA compare (rising edge) to be at
* etu/2 carrier clocks.
* - problem: SOF 12..14etu length, therefore we cannot specify
* SOF as full start condition and then sample with 10bit
* frames :(
*
*/
#include <errno.h>
#include <sys/types.h>
#include <os/dbgu.h>
#include <picc/decoder.h>
/* currently this code will only work with oversampling_rate == 1 */
#define OVERSAMPLING_RATE 1
static u_int32_t get_next_bytesample(struct decoder_state *st,
u_int8_t *parity_sample)
{
u_int32_t ret = 0;
u_int8_t bits_per_sampled_char = st->algo->bits_per_sampled_char;
u_int8_t bytesample_mask = st->algo->bytesample_mask;
/* FIXME: shift start and stop bit into parity_sample and just
* return plain 8-bit data word */
/* first part of 10-databit bytesample */
ret = (*(st->buf32) >> st->bit_ofs) & bytesample_mask;
if (st->bit_ofs > 32 - bits_per_sampled_char) {
/* second half of 10-databit bytesample */
st->buf32++;
ret |= (*(st->buf32) << (32 - st->bit_ofs));
}
st->bit_ofs = (st->bit_ofs + bits_per_sampled_char) % 32;
return ret & bytesample_mask;
}
static int nrzl_decode_sample(const u_int32_t sample, u_int8_t *data)
{
*data = (sample >> 1) & 0xff;
if (!(sample & 0x01)) {
DEBUGPCRF("invalid start bit 0!");
return -EIO;
}
if (sample & 0x20) {
DEBUGPCRF("invalid stop bit 1!");
return -EIO;
}
return 0;
}
static struct decoder_algo nrzl_decoder = {
.oversampling_rate = OVERSAMPLING_RATE,
.bits_per_sampled_char = 10 * OVERSAMPLING_RATE,
.bytesample_mask = 0x3ff,
.decode_sample = &nrzl_decode_sample,
.get_next_bytesample = &get_next_bytesample,
};
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