Datasheet

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 OVERFLOW OVERFLOW
OUTPUT
COMPARE
OUTPUT
COMPARE
OUTPUT
COMPARE
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cale Semiconductor,
I
Freescale Semiconductor, Inc.
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