Timer Interface Module (TIM)
$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 9.9.1 TIM Status and Control Register.
OVERFLOW
OVERFLOW
OVERFLOW
PERIOD
PULSE
WIDTH
TCHx
OUTPUT
COMPARE
OUTPUT
COMPARE
OUTPUT
COMPARE
Figure 9-3. 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 increments. Writing $0080 (128) to the TIM channel registers
produces a duty cycle of 128/256 or 50%.
9.4.4.1 Unbuffered PWM Signal Generation
Any output compare channel can generate unbuffered PWM pulses as described in 9.4.4 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 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 cause the
compare to be missed. The TIM may pass the new value before it is written.
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 TIM overflow interrupts and write the new value in
the TIM overflow interrupt routine. The TIM 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.
MC68HC908AP Family Data Sheet, Rev. 4
140
Freescale Semiconductor
FREESCALE [ Freescale ]
