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,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");

    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];
    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;
    }

    float kpa = (float)raw[5]*2.5;
    int temp = raw[6]-50;

    snprintf(info->name,sizeof(info->name),"%s","Schrader TPMS");
    snprintf(info->raw,sizeof(info->raw),"%02X%02X%02X%02X%02X%02X%02X%02X",
        raw[0],raw[1],raw[2],raw[3],raw[4],raw[5],
        raw[6],raw[7]);
    snprintf(info->info1,sizeof(info->info1),"Tire ID %01X%02X%02X%02X",
        raw[1]&7,raw[2],raw[3],raw[4]); /* Only 28 bits of ID, not 32. */
    snprintf(info->info2,sizeof(info->info2),"Pressure %.2f kpa", (double)kpa);
    snprintf(info->info3,sizeof(info->info3),"Temperature %d C", temp);
    return true;
}

ProtoViewDecoder SchraderTPMSDecoder = {
    "Schrader TPMS", decode
};