PWM Basics

PWM is an abbreviation for Pulse Width Modulation. In this mode, the timer acts as an up/down counter. This means that the counter counts up to its maximum value and then clears to zero. The advantage of the PWM is that the duty cycle relation can be changed in a phase consistent way.

If the PWM is configured to toggle the Output Compare pin (OCx), the signal at this pin can look like shown in the following figure.

Figure 1. Output Signal of PWM

VH: Output Voltage high level

VL: Output Voltage low level

VAV: Average Output Voltage level

x: Duty cycle high level

y: Duty cycle low level

A low-pass filter at the output pin combined with the relative high speed of the PWM will cause a constant voltage level instead of a square wave signal as output signal. Equation 4 shows how this voltage level can be calculated:

where

The fact that this method allows the timer to generate voltage levels between VCC and GND means that a DAC can be implemented using the PWM. Details about this are described in the application notes AVR314: DTMF Transmitter and AVR335: Digital Sound Recorder with AVR and Serial DataFlash.