Add method to support SX128x Frequency Error of last received LoRa or Ranging packet. Updated SX128x receive examples.

examples/SX128x/SX128x_Receive/SX128x_Receive.ino

@@ -85,6 +85,12 @@ void loop() {
     Serial.print(radio.getSNR());
     Serial.println(F(" dB"));
 
+    // print the Frequency Error
+    // of the last received packet
+    Serial.print(F("[SX1280] Frequency Error:\t"));
+    Serial.print(radio.getFrequencyError());
+    Serial.println(F(" Hz"));
+
   } else if (state == RADIOLIB_ERR_RX_TIMEOUT) {
     // timeout occurred while waiting for a packet
     Serial.println(F("timeout!"));

examples/SX128x/SX128x_Receive_Interrupt/SX128x_Receive_Interrupt.ino

@@ -136,6 +136,12 @@ void loop() {
       Serial.print(radio.getSNR());
       Serial.println(F(" dB"));
 
+      // print the Frequency Error
+      // of the last received packet
+      Serial.print(F("[SX1280] Frequency Error:\t"));
+      Serial.print(radio.getFrequencyError());
+      Serial.println(F(" Hz"));
+
     } else if (state == RADIOLIB_ERR_CRC_MISMATCH) {
       // packet was received, but is malformed
       Serial.println(F("CRC error!"));

src/modules/SX128x/SX128x.cpp

@@ -1064,6 +1064,39 @@ float SX128x::getSNR() {
   }
 }
 
+double SX128x::getFrequencyError() {
+    // check active modem
+    uint8_t modem = getPacketType();
+    if (!((modem == RADIOLIB_SX128X_PACKET_TYPE_LORA) || (modem == RADIOLIB_SX128X_PACKET_TYPE_RANGING))) {
+        return (0.0);
+    }
+
+    uint8_t efeRaw[3] = {0};
+    // read the raw frequency error register values
+    int16_t state = readRegister(RADIOLIB_SX128X_REG_FEI_MSB, &efeRaw[0], 1);
+    RADIOLIB_ASSERT(state);
+    state = readRegister(RADIOLIB_SX128X_REG_FEI_MID, &efeRaw[1], 1);
+    RADIOLIB_ASSERT(state);
+    state = readRegister(RADIOLIB_SX128X_REG_FEI_LSB, &efeRaw[2], 1);
+    RADIOLIB_ASSERT(state);
+    uint32_t efe = ((uint32_t) efeRaw[0] << 16) | ((uint32_t) efeRaw[1] << 8) | efeRaw[2];
+    efe &= 0x0FFFFF;
+
+    double error;
+
+    // check the first bit
+    if (efe & 0x80000) {
+        // frequency error is negative
+        efe |= (uint32_t) 0xFFF00000;
+        efe = ~efe + 1;
+        error = 1.55 * (double) efe / (1600.0 / (double) _bwKhz) * -1.0;
+    } else {
+        error = 1.55 * (double) efe / (1600.0 / (double) _bwKhz);
+    }
+
+    return (error);
+}
+
 size_t SX128x::getPacketLength(bool update) {
   (void)update;
   uint8_t rxBufStatus[2] = {0, 0};

src/modules/SX128x/SX128x.h

@@ -727,6 +727,13 @@ class SX128x: public PhysicalLayer {
     */
     float getSNR();
 
+    /*!
+      \brief Gets frequency error of the latest received packet.
+
+      \returns Frequency error in Hz.
+    */
+    double getFrequencyError();
+
     /*!
       \brief Query modem for the packet length of received payload.