MC68HC908RF2 Datasheet, PDF (205/254 Pages) Motorola, Inc – HCMOS Microcontroller Unit

Timer Interface Module (TIM)
Functional Description
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 15.9.1 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 percent.
15.5.7 Unbuffered PWM Signal Generation
Any output compare channel can generate unbuffered PWM pulses as
described in 15.5.6 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 these methods to synchronize unbuffered changes in the PWM
pulse width on channel:
• When changing to a shorter pulse width, enable channel 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 channel TIM
overflow interrupts and write the new value in the TIM overflow
interrupt routine. The TIM overflow interrupt occurs at the end of
MC68HC908RF2 — Rev. 1
MOTOROLA
Timer Interface Module (TIM)
Advance Information
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