109 lines
3.1 KiB
C
109 lines
3.1 KiB
C
/* NRZ-L decoder implementation for OpenPICC
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* (C) 2006 by Harald Welte <hwelte@hmw-consulting.de>
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*
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License as published by
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* the Free Software Foundation; either version 2 of the License, or
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* (at your option) any later version.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public License
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* along with this program; if not, write to the Free Software
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* Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
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*
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*/
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/*
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* speed(kbps) 106 212 424 848
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* etu 128/fc 64/fc 32/fc 16/fc
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* etu(usec) 9.4 4.7 2.35 1.18
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*
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* NRZ-L coding with logic level
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*
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* logic 1: carrier high field amplitude (no modulation)
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* logic 0: carrier low field amplitude
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*
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* Character transmission format:
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* start bit: logic 0
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* data: eight bits, lsb first
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* stop bit logic 1
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*
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* Frame Format:
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*
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* SOF char [EGT char, ...] EOF
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*
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* SOF: falling edge, 10..11 etu '0', rising edge in 1etu, 2..3etu '1'
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* EGT: between 0 and 57uS
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* EOF: falling edge, 10..11 etu '0', rising edge in 1etu
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*
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*
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* Sampling
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* - sample once per bit clock, exactly in the middle of it
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* - synchronize CARRIER_DIV TC0 to first falling edge
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* - Configure CARRIER_DIV RA compare (rising edge) to be at
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* etu/2 carrier clocks.
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* - problem: SOF 12..14etu length, therefore we cannot specify
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* SOF as full start condition and then sample with 10bit
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* frames :(
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*
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*/
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#include <errno.h>
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#include <sys/types.h>
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#include <os/dbgu.h>
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#include <picc/decoder.h>
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/* currently this code will only work with oversampling_rate == 1 */
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#define OVERSAMPLING_RATE 1
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static uint32_t get_next_bytesample(struct decoder_state *st,
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uint8_t *parity_sample)
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{
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uint32_t ret = 0;
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uint8_t bits_per_sampled_char = st->algo->bits_per_sampled_char;
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uint8_t bytesample_mask = st->algo->bytesample_mask;
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/* FIXME: shift start and stop bit into parity_sample and just
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* return plain 8-bit data word */
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/* first part of 10-databit bytesample */
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ret = (*(st->buf32) >> st->bit_ofs) & bytesample_mask;
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if (st->bit_ofs > 32 - bits_per_sampled_char) {
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/* second half of 10-databit bytesample */
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st->buf32++;
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ret |= (*(st->buf32) << (32 - st->bit_ofs));
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}
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st->bit_ofs = (st->bit_ofs + bits_per_sampled_char) % 32;
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return ret & bytesample_mask;
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}
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static int nrzl_decode_sample(const uint32_t sample, uint8_t *data)
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{
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*data = (sample >> 1) & 0xff;
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if (!(sample & 0x01)) {
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DEBUGPCRF("invalid start bit 0!");
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return -EIO;
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}
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if (sample & 0x20) {
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DEBUGPCRF("invalid stop bit 1!");
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return -EIO;
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}
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return 0;
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}
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static struct decoder_algo nrzl_decoder = {
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.oversampling_rate = OVERSAMPLING_RATE,
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.bits_per_sampled_char = 10 * OVERSAMPLING_RATE,
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.bytesample_mask = 0x3ff,
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.decode_sample = &nrzl_decode_sample,
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.get_next_bytesample = &get_next_bytesample,
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};
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