MC68HC08AS32A Datasheet, PDF (247/296 Pages) Motorola, Inc – Microcontrollers

Freescale Semiconductor, Inc.
Timer Interface (TIM)
Functional Description
counter reaches the value in the TIM counter modulo registers. The time between
overflows is the period of the PWM signal.
As Figure 16-4 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 polarity of the PWM pulse is logic 1 (ELSxA = 0). Program
the TIM to set the pin if the polarity of the PWM pulse is logic 0 (ELSxA = 1).
OVERFLOW
OVERFLOW
POLARITY = 1
(ELSxA = 0)
TCHx
POLARITY = 0
(ELSxA = 1) ^TCHx
PERIOD
PULSE
WIDTH
OVERFLOW
OUTPUT
COMPARE
OUTPUT
COMPARE
Figure 16-4. PWM Period and Pulse Width
OUTPUT
COMPARE
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 16.8.1 TIM Status and Control
Register).
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 increments.
Writing $0080 (128) to the TIM channel registers produces a duty cycle of 128/256
or 50%.
16.3.4.1 Unbuffered PWM Signal Generation
Any output compare channel can generate unbuffered PWM pulses as described
in 16.3.4 Pulse Width Modulation (PWM). The pulses are unbuffered because
changing the pulse width requires writing the new pulse width value over the value
currently in the TIM channel registers.
An unsynchronized write to the TIM channel registers to change a pulse width
value could cause incorrect operation for up to two PWM periods. For example,
writing a new value before the counter reaches the old value but after the counter
reaches the new value prevents any compare during that PWM period. Also, using
a TIM overflow interrupt routine to write a new, smaller pulse width value may
MC68HC08AS32A — Rev. 1
MOTOROLA
Timer Interface (TIM)
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Data Sheet
247