mayhem-firmware/firmware/common/tpms_packet.cpp

/*
 * Copyright (C) 2015 Jared Boone, ShareBrained Technology, Inc.
 * Copyright (C) 2023 Mark Thompson
 *
 * This file is part of PortaPack.
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2, or (at your option)
 * any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; see the file COPYING.  If not, write to
 * the Free Software Foundation, Inc., 51 Franklin Street,
 * Boston, MA 02110-1301, USA.
 */

#include "tpms_packet.hpp"

#include "crc.hpp"

namespace tpms {

Timestamp Packet::received_at() const {
    return packet_.timestamp();
}

FormattedSymbols Packet::symbols_formatted() const {
    return format_symbols(decoder_);
}

Optional<Reading> Packet::reading_fsk_19k2_schrader() const {
    const auto length = crc_valid_length();

    switch (length) {
        case 64:
            return Reading{
                Reading::Type::FLM_64,
                (uint32_t)reader_.read(0, 32),
                Pressure{static_cast<int>(reader_.read(32, 8)) * 4 / 3},
                Temperature{static_cast<int>(reader_.read(40, 8) & 0x7f) - 56}};

        case 72:
            return Reading{
                Reading::Type::FLM_72,
                (uint32_t)reader_.read(0, 32),
                Pressure{static_cast<int>(reader_.read(40, 8)) * 4 / 3},
                Temperature{static_cast<int>(reader_.read(48, 8)) - 56}};

        case 80:
            return Reading{
                Reading::Type::FLM_80,
                (uint32_t)reader_.read(8, 32),
                Pressure{static_cast<int>(reader_.read(48, 8)) * 4 / 3},
                Temperature{static_cast<int>(reader_.read(56, 8)) - 56}};

        default:
            return {};
    }
}

Optional<Reading> Packet::reading_ook_8k192_schrader() const {
    /*
     * Preamble: 11*2, 01*14, 11, 10
     * Function code: 3 Manchester symbols
     * ID: 24 Manchester symbols (one variant seen with 21 symbols?)
     * Pressure: 8 Manchester symbols
     * Checksum: 2 Manchester symbols (2 LSBs of sum incl this field == 3)
     */
    const auto flags = reader_.read(0, 3);
    const auto checksum = reader_.read(35, 2);

    uint32_t checksum_calculated = reader_.read(0, 1);
    for (size_t i = 1; i < 37; i += 2) {
        checksum_calculated += reader_.read(i, 2);
    }

    if ((checksum_calculated & 3) == 3) {
        return Reading{
            Reading::Type::Schrader,
            (uint32_t)reader_.read(3, 24),
            Pressure{static_cast<int>(reader_.read(27, 8)) * 4 / 3},
            {},
            Flags{static_cast<Flags>((flags << 4) | checksum)}};
    } else {
        return {};
    }
}

Optional<Reading> Packet::reading_ook_8k4_schrader() const {
    /*
     * Preamble: 01*40
     * System ID: 01100101, ??*20 (not really sure what this data is)
     * ID: 32 Manchester symbols
     * Value: 8 Manchester symbols (pressure)
     * Value: 8 Manchester symbols (temperature)
     * Checksum: 8 Manchester symbols (uint8_t sum of bytes starting with system ID)
     */
    /* NOTE: First four bits of packet are consumed in preamble detection.
     * Those bits assumed to be 0b0100", which may not be entirely true...
     */
    constexpr uint8_t first_nibble = 0x4;
    // const auto system_id = (first_nibble << 20) | reader_.read(0, 20);
    const auto id = reader_.read(20, 32);
    const auto value_0 = reader_.read(52, 8);
    const auto value_1 = reader_.read(60, 8);
    const auto checksum = reader_.read(68, 8);

    uint8_t checksum_calculated = (first_nibble << 4) | reader_.read(0, 4);
    for (size_t i = 4; i < 68; i += 8) {
        checksum_calculated += reader_.read(i, 8);
    }

    if (checksum_calculated == checksum) {
        return Reading{
            Reading::Type::GMC_96,
            (uint32_t)id,
            Pressure{static_cast<int>(value_0) * 11 / 4},
            Temperature{static_cast<int>(value_1) - 61}};
    } else {
        return {};
    }
}

Optional<Reading> Packet::reading() const {
    switch (signal_type()) {
        case SignalType::FSK_19k2_Schrader:
            return reading_fsk_19k2_schrader();
        case SignalType::OOK_8k192_Schrader:
            return reading_ook_8k192_schrader();
        case SignalType::OOK_8k4_Schrader:
            return reading_ook_8k4_schrader();
        default:
            return {};
    }
}

size_t Packet::crc_valid_length() const {
    constexpr uint32_t checksum_bytes = 0b1111111;
    constexpr uint32_t crc_72_bytes = 0b111111111;
    constexpr uint32_t crc_80_bytes = 0b1111111110;

    std::array<uint8_t, 10> bytes;
    for (size_t i = 0; i < bytes.size(); i++) {
        bytes[i] = reader_.read(i * 8, 8);
    }

    uint32_t checksum = 0;
    CRC<8> crc_72{0x01, 0x00};
    CRC<8> crc_80{0x01, 0x00};

    for (size_t i = 0; i < bytes.size(); i++) {
        const uint32_t byte_mask = 1 << i;
        const auto byte = bytes[i];

        if (checksum_bytes & byte_mask) {
            checksum += byte;
        }
        if (crc_72_bytes & byte_mask) {
            crc_72.process_byte(byte);
        }
        if (crc_80_bytes & byte_mask) {
            crc_80.process_byte(byte);
        }
    }

    if (crc_80.checksum() == 0) {
        return 80;
    } else if (crc_72.checksum() == 0) {
        return 72;
    } else if ((checksum & 0xff) == bytes[7]) {
        return 64;
    } else {
        return 0;
    }
}

} /* namespace tpms */