While updating my .NET nanoFramework Semtech SX127X library I revisited (because I thought it might still be wrong) how the output power is calculated. I started with the overview of the transmitter architecture in in the datasheet…
The RegPaConfig register has three settings PaSelect(RFO & PA_BOOST), MaxPower(0..7), and OutputPower(0..15). When in RFO mode the pOut has a range of -4 to 15 and PA_BOOST mode has a range of 2 to 20. (The AdaFruit version of the RadioHead library has differences to the Semtech Lora-net/LoRaMac-Node libraries)
The SX127X also has a power amplifier attached to the PA_BOOST pin and a higher power amplifier which is controlled by the RegPaDac register.
The RegOcp (over current protection) has to be relaxed for the higher power modes
I started with the Semtech Lora-net/LoRaMac-Node library which reads the RegPaConfig, RegPaSelect and RegPaDac registers then does any updates required.
void SX1276SetRfTxPower( int8_t power )
{
uint8_t paConfig = 0;
uint8_t paDac = 0;
paConfig = SX1276Read( REG_PACONFIG );
paDac = SX1276Read( REG_PADAC );
paConfig = ( paConfig & RF_PACONFIG_PASELECT_MASK ) | SX1276GetPaSelect( power );
if( ( paConfig & RF_PACONFIG_PASELECT_PABOOST ) == RF_PACONFIG_PASELECT_PABOOST )
{
if( power > 17 )
{
paDac = ( paDac & RF_PADAC_20DBM_MASK ) | RF_PADAC_20DBM_ON;
}
else
{
paDac = ( paDac & RF_PADAC_20DBM_MASK ) | RF_PADAC_20DBM_OFF;
}
if( ( paDac & RF_PADAC_20DBM_ON ) == RF_PADAC_20DBM_ON )
{
if( power < 5 )
{
power = 5;
}
if( power > 20 )
{
power = 20;
}
paConfig = ( paConfig & RF_PACONFIG_OUTPUTPOWER_MASK ) | ( uint8_t )( ( uint16_t )( power - 5 ) & 0x0F );
}
else
{
if( power < 2 )
{
power = 2;
}
if( power > 17 )
{
power = 17;
}
paConfig = ( paConfig & RF_PACONFIG_OUTPUTPOWER_MASK ) | ( uint8_t )( ( uint16_t )( power - 2 ) & 0x0F );
}
}
else
{
if( power > 0 )
{
if( power > 15 )
{
power = 15;
}
paConfig = ( paConfig & RF_PACONFIG_MAX_POWER_MASK & RF_PACONFIG_OUTPUTPOWER_MASK ) | ( 7 << 4 ) | ( power );
}
else
{
if( power < -4 )
{
power = -4;
}
paConfig = ( paConfig & RF_PACONFIG_MAX_POWER_MASK & RF_PACONFIG_OUTPUTPOWER_MASK ) | ( 0 << 4 ) | ( power + 4 );
}
}
SX1276Write( REG_PACONFIG, paConfig );
SX1276Write( REG_PADAC, paDac );
}
I also reviewed the Arduino-LoRa Semtech library which only writes to the RegPaConfig, RegPaSelect and RegPaDac registers.
void LoRaClass::setTxPower(int level, int outputPin)
{
if (PA_OUTPUT_RFO_PIN == outputPin) {
// RFO
if (level < 0) {
level = 0;
} else if (level > 14) {
level = 14;
}
writeRegister(REG_PA_CONFIG, 0x70 | level);
} else {
// PA BOOST
if (level > 17) {
if (level > 20) {
level = 20;
}
// subtract 3 from level, so 18 - 20 maps to 15 - 17
level -= 3;
// High Power +20 dBm Operation (Semtech SX1276/77/78/79 5.4.3.)
writeRegister(REG_PA_DAC, 0x87);
setOCP(140);
} else {
if (level < 2) {
level = 2;
}
//Default value PA_HF/LF or +17dBm
writeRegister(REG_PA_DAC, 0x84);
setOCP(100);
}
writeRegister(REG_PA_CONFIG, PA_BOOST | (level - 2));
}
}
I updated the output power configuration code in the Initialise method of the SX127X library. After reviewing the SX127X datasheet I extended the way the pOut is calculated in RFO mode. The code uses two values for MaxPower(RegPAConfigMaxPower.Min & RegPAConfigMaxPower.Max) so that the full RTO output power range was available.
// Set RegPAConfig & RegPaDac if powerAmplifier/OutputPower settings not defaults
if ((powerAmplifier != Configuration.RegPAConfigPASelect.Default) || (outputPower != Configuration.OutputPowerDefault))
{
if (powerAmplifier == Configuration.RegPAConfigPASelect.PABoost)
{
byte regPAConfigValue = (byte)Configuration.RegPAConfigPASelect.PABoost;
// Validate the minimum and maximum PABoost outputpower
if ((outputPower < Configuration.OutputPowerPABoostMin) || (outputPower > Configuration.OutputPowerPABoostMax))
{
throw new ApplicationException($"PABoost {outputPower}dBm Min power {Configuration.OutputPowerPABoostMin} to Max power {Configuration.OutputPowerPABoostMax}");
}
if (outputPower < Configuration.OutputPowerPABoostPaDacThreshhold)
{
// outputPower 0..15 so pOut is 2=17-(15-0)...17=17-(15-15)
regPAConfigValue |= (byte)Configuration.RegPAConfigMaxPower.Default;
regPAConfigValue |= (byte)(outputPower - 2);
_registerManager.WriteByte((byte)Configuration.Registers.RegPAConfig, regPAConfigValue);
_registerManager.WriteByte((byte)Configuration.Registers.RegPaDac, (byte)Configuration.RegPaDac.Normal);
}
else
{
// outputPower 0..15 so pOut is 5=20-(15-0)...20=20-(15-15) // See https://github.com/adafruit/RadioHead/blob/master/RH_RF95.cpp around line 411 could be 23dBm
regPAConfigValue |= (byte)Configuration.RegPAConfigMaxPower.Default;
regPAConfigValue |= (byte)(outputPower - 5);
_registerManager.WriteByte((byte)Configuration.Registers.RegPAConfig, regPAConfigValue);
_registerManager.WriteByte((byte)Configuration.Registers.RegPaDac, (byte)Configuration.RegPaDac.Boost);
}
}
else
{
byte regPAConfigValue = (byte)Configuration.RegPAConfigPASelect.Rfo;
// Validate the minimum and maximum RFO outputPower
if ((outputPower < Configuration.OutputPowerRfoMin) || (outputPower > Configuration.OutputPowerRfoMax))
{
throw new ApplicationException($"RFO {outputPower}dBm Min power {Configuration.OutputPowerRfoMin} to Max power {Configuration.OutputPowerRfoMax}");
}
// Set MaxPower and Power calculate pOut = PMax-(15-outputPower), pMax=10.8 + 0.6*MaxPower
if (outputPower > Configuration.OutputPowerRfoThreshhold)
{
// pMax 15=10.8+0.6*7 with outputPower 0...15 so pOut is 15=pMax-(15-0)...0=pMax-(15-15)
regPAConfigValue |= (byte)Configuration.RegPAConfigMaxPower.Max;
regPAConfigValue |= (byte)(outputPower + 0);
}
else
{
// pMax 10.8=10.8+0.6*0 with output power 0..15 so pOut is -4=10-(15-0)...10.8=10.8-(15-15)
regPAConfigValue |= (byte)Configuration.RegPAConfigMaxPower.Min;
regPAConfigValue |= (byte)(outputPower + 4);
}
_registerManager.WriteByte((byte)Configuration.Registers.RegPAConfig, regPAConfigValue);
_registerManager.WriteByte((byte)Configuration.Registers.RegPaDac, (byte)Configuration.RegPaDac.Normal);
}
}
The formula for pOut and pMax in RegPaConfig documentation is included in the source code so I could manually calculate (including edge cases) the values as part of my testing. I ran the SX127XLoRaDeviceClient and inspected the PaConfig & RegPaDac in the Visual Studio 2022 debugger.
PABoost
Output power = 1
Output power = 21
Exception
Output power = 2
PaConfig = 192
RegPaDac = normal
1100 0000
Output power = 16
PaConfig = 206
RegPaDac = normal
1100 1110
Output power = 17
PaConfig = 204
RegPacDac = Normal
1100 1100
Output power = 18
PaConfig = 205
RegPacDac = Boost
1100 1101
Output power = 19
PaConfig = 206
RegPacDac = Boost
1100 1110
Output power = 20
PaConfig = 207
RegPacDac = Boost
1100 1111
RFO
Output power = -5
Output power = 16
Exception
Output power = -4
PAConfig = 0
0000 0000
Output power = -1
PAConfig = 3
0000 0011
Output power = 0
PAConfig = 4
0000 0100
Output power = 1
PAConfig = 113
0111 0001
OutputPower = 14
PAConfig = 126
0111 1110
OutputPower = 15
PAConfig = 127
0111 1111
I need to borrow some test gear to check my implementation
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