Update int.osc. notes in examples

In theory the data sheet says the internal oscillator (clock) is 25MHz but in testing the internal oscillator for the PCA9685 chip is a range between about 23-27MHz.
Adafruit testing for implimenting the int.osc. showed and upper level of about 27 Mhz
My own testing with 4 different Adafruit PCA9685 16-Channel Servo Drivers gave 24060000, 24090000, 24600000, and 25000000 Mhz when tuning the int.osc. frequency to get aproximatly 50Hz PWM signals. Actual pwm signals were 50.6xx to 50.1xx Hz across the four chips tested as the  PCA9685 chip output is not super precise.

This PR expands the notes on the PWM frequency and gives rough instructions on checking the PWM frequenty for tuning the int.osc. value.

Author: photodude

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);