Asynchonous XModem Receiver

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This is a minimal C implementation of the XModem transfer protocol. It is designed to be used in bare-metal embedded systems with no underlying operating system. It is asynchonous (non-blocking), and all data is either directly supplied or sent out via callbacks, with no OS-level dependencies. It does not allocate any dynamic memory, using only 164B (1080B if large packet sizes are used) of memory while the transfer is in progress, with a very shallow stack (3-calls deep maximum). It is approximately 1kB of ARM-thumb2 compiled code.

Usage

This is a simple single .c & .h file, designed to be directly imported into most existing code bases without issue.

For most usage, there are four functions which are of interest

• xmodem_server_init - initialise the state, and provide the callback for transmitting individual response bytes
• xmodem_server_rx_byte - insert a byte of incoming data into the server (typically from a UART)
• xmodem_server_process - check for timeouts, and extract the next packet if available
• xmodem_server_is_done - indicates when the transfer is completed
• xmodem_server_get_state - get the specific state of the transfer (including success/failure)

Example

struct xmodem_server xdm;

xmodem_server_init(&xdm, uart_tx_char, NULL);
while (!xmodem_server_is_done(&xdm)) {
    uint8_t resp[XMODEM_MAX_PACKET_SIZE];
    uint32_t block_nr;
    int rx_data_len;

    if (uart_has_data())
        xmodem_server_rx_byte(uart_read());
    rx_data_len = xmodem_server_process(&xdm, resp, &block_nr, ms_time());
    if (rx_data_len > 0)
        handle_incoming_packet(resp, rx_data_len, block_nr);
}
if (xmodem_server_get_state(&xdm) == XMODEM_STATE_FAILURE)
    handle_transfer_failure();

License

This code is licensed using the Unlicense - do what you want with it.