MC68HC908GR16 Datasheet, PDF (252/310 Pages) Motorola, Inc – Microcontrollers

Freescale Semiconductor, Inc.
Timer Interface Module (TIM)
OVERFLOW
OVERFLOW
PERIOD
OVERFLOW
TCHx
PULSE
WIDTH
OUTPUT
COMPARE
OUTPUT
COMPARE
Figure 18-4. PWM Period and Pulse Width
OUTPUT
COMPARE
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%.
18.4.4.1 Unbuffered PWM Signal Generation
Any output compare channel can generate unbuffered PWM pulses as described
in 18.4.4 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.
Data Sheet
252
Timer Interface Module (TIM)
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MC68HC908GR16 — Rev. 1.0
MOTOROLA