LMIC_ERROR_TX_NOT_FEASIBLE Error on Arduino/ESP32/LoraMAC-in-C

If you get the LMIC_ERROR_TX_NOT_FEASIBLE error when doing TTNMapper or generally using the arduino-lmic library (my version is 3.3.0) then do this patch in src/lmic/lmic.c. This patch was shown to me by Chris from Accelerando Consulting.

Original code:

u1_t maxFlen = LMICbandplan_maxFrameLen(LMIC.datarate);
if (flen > maxFlen) {
	LMICOS_logEventUint32("frame too long for this bandplan", ((u4_t)dlen << 16) | (flen << 8) | maxFlen);
	return 0;
}

New code adds a check for maxFlen:

u1_t maxFlen = LMICbandplan_maxFrameLen(LMIC.datarate);
if (maxFlen > 0) {
	if (flen > maxFlen) {
		LMICOS_logEventUint32("frame too long for this bandplan", ((u4_t)dlen << 16) | (flen << 8) | maxFlen);
		return 0;
	}
}

There’s something about checking for max frame length that only gets triggered when you’re using the 923Mhz bandplan, and this fixes it. I’m guessing it’s part of the lookup in LMICas923_maxFrameLen in lmic_as923.c which return 0 but I haven’t had time to figure out what’s triggering it.