PWM Generation with the HC08 Timer, Rev. 0

2

Freescale Semiconductor

Introduction

Principle of PWM Generation Using Timers

The generation of a PWM signal using the dedicated HC08 PWM module is based on hardware

comparisons between register values and a free-running hardware counter. The HC08 TIM offers similar

hardware comparison in the form of output compare circuitry. The contents of channel registers are

continuously compared to the master free-running timer. When a match occurs, a hardware output event

can be configured to take place, and/or an interrupt can then call a service routine. The timer module has

from two to six compare registers and can thus be configured to toggle the port pin on each of two to six

comparison values, with no core overhead.

Each PWM signal requires a dedicated timer channel with output compare capability. Any channel with

this function along with the associated interrupt vector may be used to generate a PWM signal, using the

methods described.

Functional Description

Features of the TIM can include

•

Two to six input capture/output compare channels

–

Rising-edge, falling-edge or any-edge input capture trigger

–

Set, clear or toggle output compare action

•

Buffered and unbuffered pulse width modulation (PWM) signal generation

•

Programmable TIM clock input

–

Seven frequency internal bus clock prescaler selection

–

External TIM clock input (4 MHz maximum frequency)

•

Free-running or modulo up-count operation

•

Toggle any channel pin on overflow

•

TIM counter Stop and Reset bits

Block Diagram

Figure 1 shows the structure of the TIM, taken from an MC68HC908GZ60 (two channels represented out

of the six available). The central component of the TIM is the 16-bit counter, which can operate as a free-

running counter or a modulo up-counter. The TIM counter provides the timing reference for the input

capture and output compare functions. The TIM counter modulo registers, TMODH:TMODL, control the

modulo value of the timer counter. Software can read the TIM counter value at any time without affecting

the counting sequence.

In the case of the HC908GZ60, the six timer channels are programmable independently as input capture

or output compare channels. In other devices, the number of channels may vary from two to six; however,

the rest of the module remains identical.