The 8-bit PWM value that you set when you call the analogWrite function:
analogWrite(myPWMpin, 128); // Outputs a square wave
is compared against the value in an 8-bit counter. When the counter is less than the PWM value, the pin outputs a HIGH; when the counter is greater than the PWM value, the pin outputs a LOW. In the example above, a square wave is generate because the pin is HIGH from counts 0 to 127, and LOW from counts 128 to 255, so it is HIGH for the same amount of time it is LOW.
It follows logically that the frequency of the PWM signal is determined by the speed of the counter. Assuming you are using an Atmega168 with the Arduino Diecimila bootloader burned on it (which is exactly what you are using if you bought an Arduino Diecimila), this counter's clock is equal to the sytem clock divided by a prescaler value. The prescaler is a 3-bit value stored in the three least significant bits of the Timer/Counter register: CS02, CS01, and CS00. There are three such Timer/Counter registers: TCCR0B, TCCR1B, and TCCR2B.
Since there are three different prescalers, the six PWM pins are broken up into three pairs, each pair having its own prescaler. For instance, Arduion pins 6 and 5 are both controlled by TCCR0B, so you can set Arduino pins 6 and 5 to output a PWM signal at one frequency. Arduino pins 9 and 10 are controlled by TCCR1B, so they can be set at a different frequency from pins 6 and 5. Arduino pins 11 and 3 are controlled by TCCR2B, so they may be set at a third frequency. But you can't set different frequencies for pins that are controlled by the same prescaler (e.g. pins 6 and 5 must be at the same frequency).
If you use the default values set by the Arduino Diecimila's bootloader, these are your PWM frequencies:
Arduino Pins 5 and 6: 1kHz
Arduino Pins 9, 10, 11, and 3: 500Hz
How do you change the PWM frequency?
In the void setup() part of your Arduino code, set or clear the CS02, CS01, and CS00 bits in the relevant TCCRnB register. For example, if you want to set pins 5 and 6 to output a PWM signal at the highest possible frequency (see "List of Possible Frequencies" at the end of this post), insert the following code in the void setup() section of your Arduino code:
//First clear all three prescaler bits:
int prescalerVal = 0x07; //create a variable called prescalerVal and set it equal to the binary number "00000111"
TCCR0B &= ~prescalerVal; //AND the value in TCCR0B with binary number "11111000"
//Now set the appropriate prescaler bits:
int prescalerVal = 1; //set prescalerVal equal to binary number "00000001"
TCCR0B |= prescalerVal; //OR the value in TCCR0B with binary number "00000001"
The above code cleared bits CS02 and CS01, and set bit CS00.
Another way to do this, is with the following code:
//First clear the three prescaler bits
TCCR0B &= ~(1<<cs02);>
Invert that to get "11111011". AND that with TCCR0B.
TCCR0B &= ~(1<<cs01);>
TCCR0B &= ~(1<
//Now set the appropriate prescaler bits
TCCR0B |= (1<<cs00);>
OR that with TCCR0B.
If the above code is confusing, try the Bit math explanation here: http://www.arduino.cc/playground/Code/BitMath
List of Possible Frequencies:
For pins 6 and 5 (OC0A and OC0B):
If TCCR0B = xxxxx001, frequency is 64kHz
If TCCR0B = xxxxx010, frequency is 8 kHz
If TCCR0B = xxxxx011, frequency is 1kHz (this is the default from the Diecimila bootloader)
If TCCR0B = xxxxx100, frequency is 250Hz
If TCCR0B = xxxxx101, frequency is 62.5 Hz
For pins 9, 10, 11 and 3 (OC1A, OC1B, OC2A, OC2B):
If TCCRnB = xxxxx001, frequency is 32kHz
If TCCRnB = xxxxx010, frequency is 4 kHz
If TCCRnB = xxxxx011, frequency is 500Hz (this is the default from the Diecimila bootloader)
If TCCRnB = xxxxx100, frequency is 125Hz
If TCCRnB = xxxxx101, frequency is 31.25 Hz