Momentum-Apps/protocols/tpms/schrader.c

/* Schrader TPMS. Usually 443.92 Mhz OOK, 120us pulse len.
 *
 * 500us high pulse + Preamble + Manchester coded bits where:
 * 1 = 10
 * 0 = 01
 *
 * 60 bits of data total (first 4 nibbles is the preamble, 0xF).
 *
 * Used in FIAT-Chrysler, Mercedes, ... */

#include "../../app.h"

#define USE_TEST_VECTOR 0
static const char *test_vector = "000000111101010101011010010110010110101001010110100110011001100101010101011010100110100110011010101010101010101010101010101010101010101010101010";

static bool decode(uint8_t *bits, uint32_t numbytes, uint32_t numbits, ProtoViewMsgInfo *info) {

    if (USE_TEST_VECTOR) { /* Test vector to check that decoding works. */
        bitmap_set_pattern(bits,numbytes,0,test_vector);
        numbits = strlen(test_vector);
    }

    if (numbits < 64) return false; /* Preamble + data. */

    const char *sync_pattern = "1111010101" "01011010";
    uint64_t off = bitmap_seek_bits(bits,numbytes,0,numbits,sync_pattern);
    if (off == BITMAP_SEEK_NOT_FOUND) return false;
    FURI_LOG_E(TAG, "Schrader TPMS gap+preamble found");

    info->start_off = off;
    off += 10; /* Skip just the long pulse and the first 3 bits of sync, so
                  that we have the first byte of data with the sync nibble
                  0011 = 0x3. */

    uint8_t raw[8];
    uint8_t id[4];
    uint32_t decoded =
        convert_from_line_code(raw,sizeof(raw),bits,numbytes,off,
            "01","10"); /* Manchester code. */
    FURI_LOG_E(TAG, "Schrader TPMS decoded bits: %lu", decoded);

    if (decoded < 64) return false; /* Require the full 8 bytes. */

    raw[0] |= 0xf0; // Fix the preamble nibble for checksum computation.
    uint8_t cksum = crc8(raw,sizeof(raw)-1,0xf0,0x7);
    if (cksum != raw[7]) {
        FURI_LOG_E(TAG, "Schrader TPMS checksum mismatch");
        return false;
    }

    info->pulses_count = (off+8*8*2) - info->start_off;

    float kpa = (float)raw[5]*2.5;
    int temp = raw[6]-50;
    id[0] = raw[1]&7;
    id[1] = raw[2];
    id[2] = raw[3];
    id[3] = raw[4];

    fieldset_add_bytes(info->fieldset,"Tire ID",id,4*2);
    fieldset_add_float(info->fieldset,"Pressure kpa",kpa,2);
    fieldset_add_int(info->fieldset,"Temperature C",temp,8);
    return true;
}

ProtoViewDecoder SchraderTPMSDecoder = {
    .name = "Schrader TPMS",
    .decode = decode,
    .get_fields = NULL,
    .build_message = NULL
};