[SX127x] disable syncword generation & detection, add method to set preamble polarity (#834)

* allow syncword to be disabled if length is 0

* add method to change preamble polarity in FSK mode

* add new method 'setPreamblePolarity'

* move RADIOLIB_SX127X_PREAMBLE_POLARITY_55 from ::config to ::begin & ::beginFSK

* [SX127x] Remove FSK preamble config from LoRa init method

* [SX127x] Rename preamble inversion method

---------

Co-authored-by: BayCom GmbH <software@baycom.de>
Co-authored-by: jgromes <jan.gromes@gmail.com>

keywords.txt

@@ -124,6 +124,7 @@ setSyncWord	KEYWORD2
 setOutputPower	KEYWORD2
 setCurrentLimit	KEYWORD2
 setPreambleLength	KEYWORD2
+invertPreamble	KEYWORD2
 setGain	KEYWORD2
 getFrequencyError	KEYWORD2
 getRSSI	KEYWORD2

src/modules/SX127x/SX127x.cpp

@@ -118,6 +118,10 @@ int16_t SX127x::beginFSK(uint8_t* chipVersions, uint8_t numVersions, float freqD
   state = SX127x::setPreambleLength(preambleLength);
   RADIOLIB_ASSERT(state);
 
+  // set preamble polarity
+  state = invertPreamble(false);
+  RADIOLIB_ASSERT(state);
+
   // set default sync word
   uint8_t syncWord[] = {0x12, 0xAD};
   state = setSyncWord(syncWord, 2);
@@ -782,6 +786,25 @@ int16_t SX127x::setPreambleLength(size_t preambleLength) {
   return(RADIOLIB_ERR_UNKNOWN);
 }
 
+int16_t SX127x::invertPreamble(bool enable) {
+  // set mode to standby
+  int16_t state = setMode(RADIOLIB_SX127X_STANDBY);
+  RADIOLIB_ASSERT(state);
+
+  // check active modem
+  uint8_t modem = getActiveModem();
+  if(modem == RADIOLIB_SX127X_LORA) {
+    return(RADIOLIB_ERR_WRONG_MODEM);
+  } else if(modem == RADIOLIB_SX127X_FSK_OOK) {
+    // set preamble polarity
+    uint8_t polarity = enable ? RADIOLIB_SX127X_PREAMBLE_POLARITY_AA : RADIOLIB_SX127X_PREAMBLE_POLARITY_55;
+    state = this->mod->SPIsetRegValue(RADIOLIB_SX127X_REG_SYNC_CONFIG, polarity, 5, 5);
+    return(state);
+  }
+
+  return(RADIOLIB_ERR_UNKNOWN);
+}
+
 float SX127x::getFrequencyError(bool autoCorrect) {
   int16_t modem = getActiveModem();
   if(modem == RADIOLIB_SX127X_LORA) {
@@ -999,6 +1022,13 @@ int16_t SX127x::setSyncWord(uint8_t* syncWord, size_t len) {
   // check active modem
   uint8_t modem = getActiveModem();
   if(modem == RADIOLIB_SX127X_FSK_OOK) {
+
+    // disable sync word in case len is 0
+    if(len == 0) {
+      int16_t state = this->mod->SPIsetRegValue(RADIOLIB_SX127X_REG_SYNC_CONFIG, RADIOLIB_SX127X_SYNC_OFF, 4, 4);
+      return(state);
+    }
+
     RADIOLIB_CHECK_RANGE(len, 1, 8, RADIOLIB_ERR_INVALID_SYNC_WORD);
 
     // sync word must not contain value 0x00
@@ -1428,10 +1458,6 @@ int16_t SX127x::configFSK() {
   state |= this->mod->SPIsetRegValue(RADIOLIB_SX127X_REG_PACKET_CONFIG_2, RADIOLIB_SX127X_DATA_MODE_PACKET | RADIOLIB_SX127X_IO_HOME_OFF, 6, 5);
   RADIOLIB_ASSERT(state);
 
-  // set preamble polarity
-  state =this->mod->SPIsetRegValue(RADIOLIB_SX127X_REG_SYNC_CONFIG, RADIOLIB_SX127X_PREAMBLE_POLARITY_55, 5, 5);
-  RADIOLIB_ASSERT(state);
-
   // set FIFO threshold
   state = this->mod->SPIsetRegValue(RADIOLIB_SX127X_REG_FIFO_THRESH, RADIOLIB_SX127X_TX_START_FIFO_NOT_EMPTY, 7, 7);
   state |= this->mod->SPIsetRegValue(RADIOLIB_SX127X_REG_FIFO_THRESH, RADIOLIB_SX127X_FIFO_THRESH, 5, 0);

src/modules/SX127x/SX127x.h

@@ -878,6 +878,13 @@ class SX127x: public PhysicalLayer {
     */
     int16_t setPreambleLength(size_t preambleLength) override;
 
+    /*!
+      \brief Invert FSK preamble polarity. The default (non-inverted) is 0x55, the inverted is 0xAA.
+      \param enable Preamble polarity in FSK mode - 0xAA when true, 0x55 when false.
+      \returns \ref status_codes
+    */
+    int16_t invertPreamble(bool enable);
+
     /*!
       \brief Gets frequency error of the latest received packet.
       \param autoCorrect When set to true, frequency will be automatically corrected.