diff --git a/Adafruit_PWMServoDriver.cpp b/Adafruit_PWMServoDriver.cpp
index e972fdf..0c74e51 100755
--- a/Adafruit_PWMServoDriver.cpp
+++ b/Adafruit_PWMServoDriver.cpp
@@ -104,29 +104,41 @@ void Adafruit_PWMServoDriver::wakeup() {
 }
 
 /*!
- *  @brief  Sets EXTCLK pin to use the external clock
+ *  @brief  Sets prescaler for PWM output frequency
  *  @param  prescale
- *          Configures the prescale value to be used by the external clock
+ *          Defines the frequency at which the outputs modulate
+ *  @param  extclk
+ *          Sets EXTCLK pin to use the external clock
  */
-void Adafruit_PWMServoDriver::setExtClk(uint8_t prescale) {
-  uint8_t oldmode = read8(PCA9685_MODE1);
-  uint8_t newmode = (oldmode & ~MODE1_RESTART) | MODE1_SLEEP; // sleep
-  write8(PCA9685_MODE1, newmode); // go to sleep, turn off internal oscillator
+void Adafruit_PWMServoDriver::setPrescale(uint8_t prescale, bool extclk) {
+  if (prescale < PCA9685_PRESCALE_MIN)
+    return;
+  // if (prescale > PCA9685_PRESCALE_MAX) return;
+
+  uint8_t newmode1 = read8(PCA9685_MODE1);
+  newmode1 = (newmode1 & ~MODE1_RESTART) | MODE1_SLEEP; // sleep
+  write8(PCA9685_MODE1, newmode1); // 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 |= MODE1_EXTCLK));
+  if (extclk) {
+    write8(PCA9685_MODE1, (newmode1 |= MODE1_EXTCLK));
+  }
 
   write8(PCA9685_PRESCALE, prescale); // set the prescaler
 
   delay(5);
   // clear the SLEEP bit to start
-  write8(PCA9685_MODE1, (newmode & ~MODE1_SLEEP) | MODE1_RESTART | MODE1_AI);
+  write8(PCA9685_MODE1, (newmode1 & ~MODE1_SLEEP) | MODE1_RESTART | MODE1_AI);
+}
 
-#ifdef ENABLE_DEBUG_OUTPUT
-  Serial.print("Mode now 0x");
-  Serial.println(read8(PCA9685_MODE1), HEX);
-#endif
+/*!
+ *  @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) {
+  setPrescale(prescale, true);
 }
 
 /*!
@@ -156,14 +168,7 @@ void Adafruit_PWMServoDriver::setPWMFreq(float freq) {
   Serial.println(prescale);
 #endif
 
-  uint8_t oldmode = read8(PCA9685_MODE1);
-  uint8_t newmode = (oldmode & ~MODE1_RESTART) | MODE1_SLEEP; // sleep
-  write8(PCA9685_MODE1, newmode);                             // go to sleep
-  write8(PCA9685_PRESCALE, prescale); // set the prescaler
-  write8(PCA9685_MODE1, oldmode);
-  delay(5);
-  // This sets the MODE1 register to turn on auto increment.
-  write8(PCA9685_MODE1, oldmode | MODE1_RESTART | MODE1_AI);
+  setPrescale(prescale);
 
 #ifdef ENABLE_DEBUG_OUTPUT
   Serial.print("Mode now 0x");
diff --git a/Adafruit_PWMServoDriver.h b/Adafruit_PWMServoDriver.h
index e0857d8..92705ca 100644
--- a/Adafruit_PWMServoDriver.h
+++ b/Adafruit_PWMServoDriver.h
@@ -81,6 +81,7 @@ class Adafruit_PWMServoDriver {
   void reset();
   void sleep();
   void wakeup();
+  void setPrescale(uint8_t prescale, bool extclk = false);
   void setExtClk(uint8_t prescale);
   void setPWMFreq(float freq);
   void setOutputMode(bool totempole);
diff --git a/examples/pwmtest/pwmtest.ino b/examples/pwmtest/pwmtest.ino
index 3c2c4b6..09af7b4 100644
--- a/examples/pwmtest/pwmtest.ino
+++ b/examples/pwmtest/pwmtest.ino
@@ -36,6 +36,8 @@ void setup() {
   // you get the frequency you're expecting!
   pwm.setOscillatorFrequency(27000000);  // The int.osc. is closer to 27MHz
   pwm.setPWMFreq(1600);  // This is the maximum PWM frequency
+  // or as alternative set the prescale manual: freq = oscillator-Hz / 4096*(prescale+1)
+  // pwm.setPrescale(0x1e);  // PCA9685 sets itself default to 0x1e, approx 200Hz
 
   // if you want to really speed stuff up, you can go into 'fast 400khz I2C' mode
   // some i2c devices dont like this so much so if you're sharing the bus, watch
diff --git a/keywords.txt b/keywords.txt
index 2831dae..1b40b63 100644
--- a/keywords.txt
+++ b/keywords.txt
@@ -12,6 +12,7 @@ begin	KEYWORD2
 reset	KEYWORD2
 sleep	KEYWORD2
 wakeup	KEYWORD2
+setPrescale KEYWORD2
 setExtClk	KEYWORD2
 setPWMFreq	KEYWORD2
 setOutputMode	KEYWORD2