Merge pull request #72 from photodude/new-internal-oscillator-notes

Update internal oscillator notes in examples

Author: ladyada

examples/pwmtest/pwmtest.ino

@@ -31,10 +31,23 @@ void setup() {
   Serial.println("16 channel PWM test!");
 
   pwm.begin();
-  // In theory the internal oscillator is 25MHz but it really isn't
-  // that precise. You can 'calibrate' by tweaking this number till
-  // you get the frequency you're expecting!
-  pwm.setOscillatorFrequency(27000000);  // The int.osc. is closer to 27MHz
+  /*
+   * In theory the internal oscillator (clock) is 25MHz but it really isn't
+   * that precise. You can 'calibrate' this by tweaking this number until
+   * you get the PWM update frequency you're expecting!
+   * The int.osc. for the PCA9685 chip is a range between about 23-27MHz and
+   * is used for calculating things like writeMicroseconds()
+   * Analog servos run at ~50 Hz updates, It is importaint to use an
+   * oscilloscope in setting the int.osc frequency for the I2C PCA9685 chip.
+   * 1) Attach the oscilloscope to one of the PWM signal pins and ground on
+   *    the I2C PCA9685 chip you are setting the value for.
+   * 2) Adjust setOscillatorFrequency() until the PWM update frequency is the
+   *    expected value (50Hz for most ESCs)
+   * Setting the value here is specific to each individual I2C PCA9685 chip and
+   * affects the calculations for the PWM update frequency. 
+   * Failure to correctly set the int.osc value will cause unexpected PWM results
+   */
+  pwm.setOscillatorFrequency(27000000);
   pwm.setPWMFreq(1600);  // This is the maximum PWM frequency
 
   // if you want to really speed stuff up, you can go into 'fast 400khz I2C' mode

examples/servo/servo.ino

@@ -45,10 +45,23 @@ void setup() {
   Serial.println("8 channel Servo test!");
 
   pwm.begin();
-  // In theory the internal oscillator is 25MHz but it really isn't
-  // that precise. You can 'calibrate' by tweaking this number till
-  // you get the frequency you're expecting!
-  pwm.setOscillatorFrequency(27000000);  // The int.osc. is closer to 27MHz  
+  /*
+   * In theory the internal oscillator (clock) is 25MHz but it really isn't
+   * that precise. You can 'calibrate' this by tweaking this number until
+   * you get the PWM update frequency you're expecting!
+   * The int.osc. for the PCA9685 chip is a range between about 23-27MHz and
+   * is used for calculating things like writeMicroseconds()
+   * Analog servos run at ~50 Hz updates, It is importaint to use an
+   * oscilloscope in setting the int.osc frequency for the I2C PCA9685 chip.
+   * 1) Attach the oscilloscope to one of the PWM signal pins and ground on
+   *    the I2C PCA9685 chip you are setting the value for.
+   * 2) Adjust setOscillatorFrequency() until the PWM update frequency is the
+   *    expected value (50Hz for most ESCs)
+   * Setting the value here is specific to each individual I2C PCA9685 chip and
+   * affects the calculations for the PWM update frequency. 
+   * Failure to correctly set the int.osc value will cause unexpected PWM results
+   */
+  pwm.setOscillatorFrequency(27000000);
   pwm.setPWMFreq(SERVO_FREQ);  // Analog servos run at ~50 Hz updates
 
   delay(10);