Example of framing with UART protocol

framing_example.c

// 
#define FRAME_SOF               (0xFE)
#define FRAME_TYPE_WIFI_CONNECT (0x02)

char frame_buf[254];

typedef struct {
	unsigned char sof;
	unsigned char flen;
	unsigned char ftype;
} frame_t;

// Returns length of frame for marshaling out to UART or other
int frame_make_wifi(char* buffer, char* ssid, unsigned char ssid_len, char* pass, unsigned char pass_len) {
	char *payload;
	frame_t *frame;
	int length;

	unsigned char chksum = 0;

	frame = (frame*)(buffer); // Allow struct access to directly modify buffer data
	payload = buffer + sizeof(frame_t);
	
	frame->sof = FRAME_SOF;
	frame->ftype = FRAME_TYPE_WIFI_CONNECT;

	*payload++ = ssid_len;
	memcpy(payload, ssid, (size_t) ssid_len);
	payload += ssid_len;
	
	*payload++ = pass_len;
	memcpy(payload, pass, (size_t) pass_len);
	 payload += pass_len;

  	// Length is FTYPE + 254b PAYLOAD; we remove SOF and FLEN (CHKSUM not included, but not counted so not reomved)
	frame->flen = (payload - 2 - buffer); 
	
	payload = buffer + 1; // Skip SOF for checksum, which is (FLEN..PAYLOAD)
        for (unsigned char c = 0; c < (frame->flen + 1); c++) // Add back FLEN field into length
		chksum = *payload++;

  	// `payload` points to byte just past PAYLOAD field, where CHKSUM resides
	*payload++ = chksum;
	
	return (int)(payload - buffer); // includes all frame_t, payload, and CHKSUM 
	
}

int main(void) {
  /// do stuff.
  int frame_length = frame_make_wifi(frame_buf, (char*)ssid /* from JSON */, (unsigned char)ssid_len, (char*)pass /* from JSON */, (unsigned char)pass_len);
  uart_send(frame_buf, frame_length); // or something.
}