firmware/src/picc/decoder_nrzl.c

   1 /* NRZ-L decoder implementation for OpenPICC
   2  * (C) 2006 by Harald Welte <hwelte@hmw-consulting.de>
   3  *
   4  *  This program is free software; you can redistribute it and/or modify
   5  *  it under the terms of the GNU General Public License as published by
   6  *  the Free Software Foundation; either version 2 of the License, or
   7  *  (at your option) any later version.
   8  *
   9  *  This program is distributed in the hope that it will be useful,
  10  *  but WITHOUT ANY WARRANTY; without even the implied warranty of
  11  *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  12  *  GNU General Public License for more details.
  13  *
  14  *  You should have received a copy of the GNU General Public License
  15  *  along with this program; if not, write to the Free Software
  16  *  Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
  17  *
  18  */
  19
  20 /*
  21  * speed(kbps)  106     212     424     848
  22  * etu          128/fc  64/fc   32/fc   16/fc
  23  * etu(usec)    9.4     4.7     2.35    1.18
  24  *
  25  * NRZ-L coding with logic level
  26  *
  27  * logic 1:     carrier high field amplitude (no modulation)
  28  * logic 0:     carrier low field amplitude
  29  *
  30  * Character transmission format:
  31  *      start bit:      logic 0
  32  *      data:           eight bits, lsb first
  33  *      stop bit        logic 1
  34  *
  35  * Frame Format:
  36  *
  37  *              SOF char [EGT char, ...] EOF
  38  *
  39  *      SOF: falling edge, 10..11 etu '0', rising edge in 1etu, 2..3etu '1'
  40  *      EGT: between 0 and 57uS
  41  *      EOF: falling edge, 10..11 etu '0', rising edge in 1etu
  42  *
  43  *
  44  * Sampling
  45  * - sample once per bit clock, exactly in the middle of it
  46  * - synchronize CARRIER_DIV TC0 to first falling edge
  47  * - Configure CARRIER_DIV RA compare (rising edge) to be at
  48  *   etu/2 carrier clocks.
  49  * - problem: SOF 12..14etu length, therefore we cannot specify
  50  *            SOF as full start condition and then sample with 10bit
  51  *            frames :(
  52  *
  53  */
  54
  55 #include <errno.h>
  56 #include <sys/types.h>
  57 #include <os/dbgu.h>
  58 #include <picc/decoder.h>
  59
  60 /* currently this code will only work with oversampling_rate == 1 */
  61 #define OVERSAMPLING_RATE 1
  62
  63 static u_int32_t get_next_bytesample(struct decoder_state *st,
  64                                      u_int8_t *parity_sample)
  65 {
  66         u_int32_t ret = 0;
  67         u_int8_t bits_per_sampled_char = st->algo->bits_per_sampled_char;
  68         u_int8_t bytesample_mask = st->algo->bytesample_mask;
  69
  70         /* FIXME: shift start and stop bit into parity_sample and just
  71          * return plain 8-bit data word */
  72
  73         /* first part of 10-databit bytesample */
  74         ret = (*(st->buf32) >> st->bit_ofs) & bytesample_mask;
  75
  76         if (st->bit_ofs > 32 - bits_per_sampled_char) {
  77                 /* second half of 10-databit bytesample */
  78                 st->buf32++;
  79                 ret |= (*(st->buf32) << (32 - st->bit_ofs));
  80         }
  81         st->bit_ofs = (st->bit_ofs + bits_per_sampled_char) % 32;
  82
  83         return ret & bytesample_mask;
  84 }
  85
  86 static int nrzl_decode_sample(const u_int32_t sample, u_int8_t *data)
  87 {
  88         *data = (sample >> 1) & 0xff;
  89
  90         if (!(sample & 0x01)) {
  91                 DEBUGPCRF("invalid start bit 0!");
  92                 return -EIO;
  93         }
  94         if (sample & 0x20) {
  95                 DEBUGPCRF("invalid stop bit 1!");
  96                 return -EIO;
  97         }
  98
  99         return 0;
 100 }
 101
 102 static struct decoder_algo nrzl_decoder = {
 103         .oversampling_rate = OVERSAMPLING_RATE,
 104         .bits_per_sampled_char = 10 * OVERSAMPLING_RATE,
 105         .bytesample_mask = 0x3ff,
 106         .decode_sample = &nrzl_decode_sample,
 107         .get_next_bytesample = &get_next_bytesample,
 108 };