[RTTY] Added support for CC1101 (#4)

examples/RTTY_Transmit/RTTY_Transmit.ino

@@ -7,6 +7,7 @@
    Other modules that can be used for RTTY:
     - SX1272/73/76/77/79
     - RF69
+    - CC1101
 */
 
 // include the library
@@ -41,9 +42,13 @@ void setup() {
 
   // initialize RTTY client
   // NOTE: RTTY frequency shift will be rounded
-  //       to multiples of 61 Hz (hardware limitation)
+  //       to the nearest multiple of frequency step size.
+  //       The exact value depends on the module:
+  //         SX127x - 61 Hz
+  //         RF69 - 61 Hz
+  //         CC1101 - 397 Hz
   Serial.print(F("[RTTY] Initializing ... "));
-  // low frequency:               434.0 MHz
+  // low ("space") frequency:     434.0 MHz
   // frequency shift:             183 Hz
   // baud rate:                   45 baud
   // encoding:                    ASCII (7-bit)

src/modules/RF69.cpp

@@ -1,6 +1,6 @@
 #include "RF69.h"
 
-RF69::RF69(Module* module) {
+RF69::RF69(Module* module) : PhysicalLayer(RF69_CRYSTAL_FREQ, RF69_DIV_EXPONENT)  {
   _mod = module;
   _tempOffset = 0;
 }

src/modules/RF69.h

@@ -6,6 +6,10 @@
 
 #include "../protocols/PhysicalLayer.h"
 
+// RF69 physical layer properties
+#define RF69_CRYSTAL_FREQ                             32.0
+#define RF69_DIV_EXPONENT                             19
+
 // RF69 register map
 #define RF69_REG_FIFO                                 0x00
 #define RF69_REG_OP_MODE                              0x01

src/modules/SX127x.cpp

@@ -1,6 +1,6 @@
 #include "SX127x.h"
 
-SX127x::SX127x(Module* mod) {
+SX127x::SX127x(Module* mod) : PhysicalLayer(SX127X_CRYSTAL_FREQ, SX127X_DIV_EXPONENT) {
   _mod = mod;
 }
 
@@ -820,7 +820,7 @@ int16_t SX127x::setBitRate(float br) {
   // set bit rate
   uint16_t bitRate = 32000 / br;
   state = _mod->SPIsetRegValue(SX127X_REG_BITRATE_MSB, (bitRate & 0xFF00) >> 8, 7, 0);
-  state |= _mod->SPIsetRegValue(SX127X_REG_BITRATE_MSB, bitRate & 0x00FF, 7, 0);
+  state |= _mod->SPIsetRegValue(SX127X_REG_BITRATE_LSB, bitRate & 0x00FF, 7, 0);
   // TODO: fractional part of bit rate setting
   if(state == ERR_NONE) {
     SX127x::_br = br;

src/modules/SX127x.h

@@ -6,6 +6,10 @@
 
 #include "../protocols/PhysicalLayer.h"
 
+// SX127x physical layer properties
+#define SX127X_CRYSTAL_FREQ                           32.0
+#define SX127X_DIV_EXPONENT                           19
+
 // SX127x series common LoRa registers
 #define SX127X_REG_FIFO                               0x00
 #define SX127X_REG_OP_MODE                            0x01

src/protocols/PhysicalLayer.h

@@ -6,7 +6,7 @@
 class PhysicalLayer {
   public:
     // constructor
-    // this class is purely virtual and does not require explicit constructor
+    PhysicalLayer(float crysFreq, uint8_t divExp) { _crystalFreq = crysFreq; _divExponent = divExp; }
     
     // basic methods
     virtual int16_t transmitDirect(uint32_t FRF = 0) = 0;
@@ -17,7 +17,12 @@ class PhysicalLayer {
     
     // configuration methods
     virtual int16_t setFrequencyDeviation(float freqDev) = 0;
+    float getCrystalFreq() { return(_crystalFreq); }
+    uint8_t getDivExponent() { return(_divExponent); }
 
+  private:
+    float _crystalFreq;
+    uint8_t _divExponent;
 };
 
 #endif

src/protocols/RTTY.cpp

@@ -103,18 +103,6 @@ RTTYClient::RTTYClient(PhysicalLayer* phy) {
 }
 
 int16_t RTTYClient::begin(float base, uint16_t shift, uint16_t rate, uint8_t encoding, uint8_t stopBits) {
-  // check supplied values
-  if(shift < 30) {
-    return(ERR_INVALID_RTTY_SHIFT);
-  }
-  
-  // clamp shift to multiples of 61 Hz (SX127x synthesis resolution)
-  if(shift % 61 < 31) {
-    _shift = shift / 61;
-  } else {
-    _shift = (shift / 61) + 1;
-  }
-  
   // save configuration
   _encoding = encoding;
   _stopBits = stopBits;
@@ -136,9 +124,23 @@ int16_t RTTYClient::begin(float base, uint16_t shift, uint16_t rate, uint8_t enc
   // calculate duration of 1 bit
   _bitDuration = (uint32_t)1000000/rate;
   
+  // calculate module carrier frequency resolution
+  uint16_t step = round((_phy->getCrystalFreq() * 1000000) / (uint32_t(1) << _phy->getDivExponent()));
+  
+  // check minimum shift value
+  if(shift < step / 2) {
+    return(ERR_INVALID_RTTY_SHIFT);
+  }
+  
+  // round shift to multiples of frequency step size
+  if(shift % step < (step / 2)) {
+    _shift = shift / step;
+  } else {
+    _shift = (shift / step) + 1;
+  }
+  
   // calculate 24-bit frequency
-  uint32_t mult = 1;
+  _base = (base * (uint32_t(1) << _phy->getDivExponent())) / _phy->getCrystalFreq();
-  _base = (base * (mult << 19)) / 32.0;
 
   // set module frequency deviation to 0
   int16_t state = _phy->setFrequencyDeviation(0);