M68ICS08GR datasheet(343/408 Pages) FREESCALE

343 / 408 page

Timer Interface Module (TIM)

Functional Description

MC68HC908GR8 — Rev 4.0

Technical Data

MOTOROLA

Timer Interface Module (TIM)

343

22.5.6 Pulse Width Modulation (PWM)

By using the toggle-on-overflow feature with an output compare channel,

the TIM can generate a PWM signal. The value in the TIM counter

modulo registers determines the period of the PWM signal. The channel

pin toggles when the counter reaches the value in the TIM counter

modulo registers. The time between overflows is the period of the PWM

signal.

As Figure 22-3 shows, the output compare value in the TIM channel

registers determines the pulse width of the PWM signal. The time

between overflow and output compare is the pulse width. Program the

TIM to clear the channel pin on output compare if the state of the PWM

pulse is logic 1. Program the TIM to set the pin if the state of the PWM

pulse is logic 0.

The value in the TIM counter modulo registers and the selected

prescaler output determines the frequency of the PWM output. The

frequency of an 8-bit PWM signal is variable in 256 increments. Writing

$00FF (255) to the TIM counter modulo registers produces a PWM

period of 256 times the internal bus clock period if the prescaler select

value is $000. See TIM Status and Control Register.

Figure 22-3. PWM Period and Pulse Width

The value in the TIM channel registers determines the pulse width of the

PWM output. The pulse width of an 8-bit PWM signal is variable in 256

PTEx/TCHx

PERIOD

PULSE

WIDTH

OVERFLOW

OUTPUT

COMPARE

OUTPUT

COMPARE

OUTPUT

COMPARE

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