SX128x chips mystery register 0x925 - SF Additional Configuration meaning

[GusBon]

Hi,

After reading the SX1280 datasheet, I saw in the Section 13 the register named "SF Additional Configuration" at the address 0x925. Furthermore by reading the section 14.4.1, it is said that it is mandatory to write to this register some values according to the SF we are using.

By reading the code in the repo, I saw that this is implemented:

RadioLib/src/modules/SX128x/SX128x.cpp

Lines 680 to 709 in 5f73a87

	int16_t SX128x::setSpreadingFactor(uint8_t sf) {
	// check active modem
	uint8_t modem = getPacketType();
	if(modem == RADIOLIB_SX128X_PACKET_TYPE_LORA) {
	// check range for LoRa
	RADIOLIB_CHECK_RANGE(sf, 5, 12, RADIOLIB_ERR_INVALID_SPREADING_FACTOR);
	} else if(modem == RADIOLIB_SX128X_PACKET_TYPE_RANGING) {
	// check range for ranging
	RADIOLIB_CHECK_RANGE(sf, 5, 10, RADIOLIB_ERR_INVALID_SPREADING_FACTOR);
	} else {
	return(RADIOLIB_ERR_WRONG_MODEM);
	}
	// update modulation parameters
	this->spreadingFactor = sf << 4;
	int16_t state = setModulationParams(this->spreadingFactor, this->bandwidth, this->codingRateLoRa);
	RADIOLIB_ASSERT(state);
	// update mystery register in LoRa mode - SX1280 datasheet v3.0 section 13.4.1
	if(modem == RADIOLIB_SX128X_PACKET_TYPE_LORA) {
	uint8_t data = 0;
	if((this->spreadingFactor == RADIOLIB_SX128X_LORA_SF_5) || (this->spreadingFactor == RADIOLIB_SX128X_LORA_SF_6)) {
	data = 0x1E;
	} else if((this->spreadingFactor == RADIOLIB_SX128X_LORA_SF_7) || (this->spreadingFactor == RADIOLIB_SX128X_LORA_SF_8)) {
	data = 0x37;
	} else {
	data = 0x32;
	}
	state = SX128x::writeRegister(RADIOLIB_SX128X_REG_LORA_SF_CONFIG, &data, 1);
	}

Do you have any clue of what this register is, and why is it mandatory to write in it after changing the spreading factor ?
As the SX1280 is able to only operate on one SF at a time, and we specify that SF by using the SetModulationParams() command of the chip. Why is it necessary to write in this register ? We are telling the chip two times the same info.

Thanks in advance !
Gus

[jgromes]

Do you have any clue of what this register is, and why is it mandatory to write in it after changing the spreading factor ?

No idea - that's why I called it a "mystery register" in the SX128x driver. In either case it's a good question to ask Semtech themselves ;) My guess is that this is some workaroud for an issue in the chip itself, that's why it is not part of the SPI command but needs to be done by a direct register write. In any case it seems like I did not implement the last part of the recommendation, which is to write 0x01 to register 0x093C. I will fix that right away.