MC908AZ32ACFUE Datasheet, PDF (211/324 Pages) Freescale Semiconductor, Inc – Microcontrollers

Functional Description
to clear the channel pin on output compare if the state of the PWM pulse is logic 1. Program the TIMB to
set the pin if the state of the PWM pulse is logic 0.
OVERFLOW
OVERFLOW
OVERFLOW
PERIOD
PTEx/TCHx
PULSE
WIDTH
OUTPUT
COMPARE
OUTPUT
COMPARE
Figure 19-3. PWM Period and Pulse Width
OUTPUT
COMPARE
The value in the TIMB 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 TIMB counter modulo registers produces a PWM period of 256 times the internal bus
clock period if the prescaler select value is $000 19.8.1 TIMB Status and Control Register.
The value in the TIMB 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 TIMB channel registers
produces a duty cycle of 128/256 or 50%.
19.3.4.1 Unbuffered PWM Signal Generation
Any output compare channel can generate unbuffered PWM pulses as described in 19.3.4 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 TIMB channel registers.
An unsynchronized write to the TIMB 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 TIMB overflow interrupt routine to write a new, smaller pulse width value may cause
the compare to be missed. The TIMB may pass the new value before it is written to the TIMB channel
registers.
Use the following methods to synchronize unbuffered changes in the PWM pulse width on channel x:
• When changing to a shorter pulse width, enable channel x output compare interrupts and write the
new value in the output compare interrupt routine. The output compare interrupt occurs at the end
of the current pulse. The interrupt routine has until the end of the PWM period to write the new
value.
• When changing to a longer pulse width, enable TIMB overflow interrupts and write the new value
in the TIMB overflow interrupt routine. The TIMB overflow interrupt occurs at the end of the current
PWM period. Writing a larger value in an output compare interrupt routine (at the end of the current
pulse) could cause two output compares to occur in the same PWM period.
NOTE
In PWM signal generation, do not program the PWM channel to toggle on
output compare. Toggling on output compare prevents reliable 0% duty
cycle generation and removes the ability of the channel to self-correct in the
MC68HC908AZ32A Data Sheet, Rev. 2
Freescale Semiconductor
211