SX1276 and LoRa (P2P): Setting and getting payload length in implicit header mode with SF6

[snowrodeo]

Working to move some LoRa (P2P) code from RadioHead. The equipment I am working with uses implicit header mode with SF 6. The payload length is 4 bytes. Using the API, I attempt to set it like this:

rf95State = LORA_RF95.implicitHeader(RF95_PACKETLENGTH);
and that returns status code 0, so all good it seems. Since I not seeing packets, so time to check the config at the modem level.

There doesn't appear to be a method to report the payload length (reg 0x22) with the API - which seems like a (small) oversight. My first step was to read reg 0x22 directly via SPI like this:

payloadLength = LoRa_HW->SPIreadRegister(0x22);

That returns 0x00, which isn't the answer we are looking for. Next up, I attempted to set the reg by SPI like this:

LORA_RF95.standby(); int16_t payloadLengthSet = LoRa_HW->SPIsetRegValue(0x22,0x04); // RegPayloadLength = 4 Serial.printf("Result of setSPI method method %d\n", payloadLengthSet);

Explicitly (I hope) putting the modem into standby mode to write the register. Sadly, a read of the reg give 0x00, so that doesn't seem to work either.

Finally, I tried to write / read to another register (0x24) using the exact same code as to write to 0x22. That worked!

1. Is there no way to get the value of the payload length in implicit header mode using the API?
2. Why on earth can I write/read from reg 0x24 and not to 0x22 with the same code? God mode curse?

[snowrodeo]

It turns out the issue is with startReceive(). When called with no args, it defaults the payload length to 0 instead of using the previously set value from implicitHeader. Unfortunately, this means that you need to set the payload length twice.

I will put in a couple of issues based on this experience.