Asynchonous XModem Receiver

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 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_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_PACKET_SIZE];
	uint32_t block_nr;
	if (xmodem_server_process(&xdm, resp, &block_nr, ms_time()))
		handle_incoming_packet(resp, 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.