MC908GZ60CFUE Datasheet, PDF (268/352 Pages) Freescale Semiconductor, Inc – Standard features, Features of the CPU08

Timer Interface Module (TIM1)
OVERFLOW
OVERFLOW
POLARITY = 1
(ELSxA = 0)
TCHx
POLARITY = 0
(ELSxA = 1) ^TCHx
PERIOD
PULSE
WIDTH
OVERFLOW
OUTPUT
COMPARE
OUTPUT
COMPARE
Figure 18-4. PWM Period and Pulse Width
OUTPUT
COMPARE
18.3.4.1 Unbuffered PWM Signal Generation
Any output compare channel can generate unbuffered PWM pulses as described in 18.3.4 Pulse Width
Modulation (PWM). The pulses are unbuffered because changing the pulse width requires writing the new
pulse width value over the old value currently in the TIM1 channel registers.
An unsynchronized write to the TIM1 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 TIM1 overflow interrupt routine to write a new, smaller pulse width value may cause
the compare to be missed. The TIM1 may pass the new value before it is written to the timer channel
(T1CHxH:T1CHxL) 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 TIM1 overflow interrupts and write the new value
in the TIM1 overflow interrupt routine. The TIM1 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
event of software error or noise. Toggling on output compare also can
cause incorrect PWM signal generation when changing the PWM pulse
width to a new, much larger value.
MC68HC908GZ60 • MC68HC908GZ48 • MC68HC908GZ32 Data Sheet, Rev. 6
268
Freescale Semiconductor