added External Clock by configuring the EXTCLK bit

Author: hoffmannjan

Adafruit_PWMServoDriver.cpp

@@ -45,21 +45,54 @@ Adafruit_PWMServoDriver::Adafruit_PWMServoDriver(TwoWire *i2c, uint8_t addr) {
 /*!
  *  @brief  Setups the I2C interface and hardware
  */
-void Adafruit_PWMServoDriver::begin(void) {
+void Adafruit_PWMServoDriver::begin(uint8_t prescale) {
   _i2c->begin();
   reset();
   // set a default frequency
-  setPWMFreq(1000);
+  if (prescale) {
+    setExtClk(prescale);
+  } else {
+    // set a default frequency
+    setPWMFreq(1000);
+  }
 }
 
 /*!
  *  @brief  Sends a reset command to the PCA9685 chip over I2C
  */
-void Adafruit_PWMServoDriver::reset(void) {
+void Adafruit_PWMServoDriver::reset() {
   write8(PCA9685_MODE1, 0x80);
   delay(10);
 }
 
+/**************************************************************************/
+/*!
+    @brief  Sets EXTCLK pin to use the external clock
+       @param  prescale Configures the prescale value to be used by the external
+   clock
+*/
+/**************************************************************************/
+void Adafruit_PWMServoDriver::setExtClk(uint8_t prescale) {
+  uint8_t oldmode = read8(PCA9685_MODE1);
+  uint8_t newmode = (oldmode & 0x7F) | 0x10; // sleep
+  write8(PCA9685_MODE1, newmode); // go to sleep, turn off internal oscillator
+
+  // This sets both the SLEEP and EXTCLK bits of the MODE1 register to switch to
+  // use the external clock.
+  write8(PCA9685_MODE1, (newmode |= 0x40));
+
+  write8(PCA9685_PRESCALE, prescale); // set the prescaler
+
+  delay(5);
+  write8(PCA9685_MODE1,
+         (newmode & ~(0x10)) | 0xa0); // clear the SLEEP bit to start
+
+#ifdef ENABLE_DEBUG_OUTPUT
+  Serial.print("Mode now 0x");
+  Serial.println(read8(PCA9685_MODE1), HEX);
+#endif
+}
+
 /*!
  *  @brief  Sets the PWM frequency for the entire chip, up to ~1.6 KHz
  *  @param  freq Floating point frequency that we will attempt to match

Adafruit_PWMServoDriver.h

@@ -49,8 +49,9 @@
 class Adafruit_PWMServoDriver {
  public:
   Adafruit_PWMServoDriver(TwoWire *I2C = &Wire, uint8_t addr = 0x40);
-  void begin(void);
-  void reset(void);
+  void begin(uint8_t prescale = 0);
+  void reset();
+  void setExtClk(uint8_t prescale);
   void setPWMFreq(float freq);
   void setPWM(uint8_t num, uint16_t on, uint16_t off);
   void setPin(uint8_t num, uint16_t val, bool invert=false);