portapack-mayhem/firmware/application/rfm69.cpp

/*
 * Copyright (C) 2017 Jared Boone, ShareBrained Technology, Inc.
 * Copyright (C) 2019 Furrtek
 *
 * 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 "rfm69.hpp"
#include "crc.hpp"
#include "portapack_shared_memory.hpp"

uint32_t RFM69::gen_frame(std::vector<uint8_t>& payload) {
    CRC<16> crc{0x1021, 0x1D0F, 0xFFFF};
    std::vector<uint8_t> frame{};
    uint8_t byte_out = 0;

    // Preamble
    // Really is 0xAA but the RFM69 skips the very last bit (bug ?)
    // so the whole preamble is shifted right to simulate that
    frame.insert(frame.begin(), num_preamble_, 0x55);

    // Sync word
    frame.push_back(sync_word_ >> 8);
    frame.push_back(sync_word_ & 0xFF);

    // Payload length
    payload.insert(payload.begin(), payload.size());

    crc.process_bytes(payload.data(), payload.size());

    if (CRC_) {
        payload.push_back(crc.checksum() >> 8);
        payload.push_back(crc.checksum() & 0xFF);
    }

    // Manchester-encode payload
    if (manchester_) {
        for (auto byte : payload) {
            for (uint32_t b = 0; b < 8; b++) {
                byte_out <<= 2;

                if (byte & 0x80)
                    byte_out |= 0b10;
                else
                    byte_out |= 0b01;

                if ((b & 3) == 3)
                    frame.push_back(byte_out);

                byte <<= 1;
            }
        }
    } else {
        frame.insert(frame.end(), payload.begin(), payload.end());
    }

    memcpy(shared_memory.bb_data.data, frame.data(), frame.size());

    // Copy for baseband
    return frame.size();
}