M68ICS08GR datasheet(344/408 Pages) FREESCALE

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Timer Interface Module (TIM)

Technical Data

MC68HC908GR8 — Rev 4.0

344

Timer Interface Module (TIM)

MOTOROLA

increments. Writing $0080 (128) to the TIM channel registers produces

a duty cycle of 128/256 or 50%.

22.5.7 Unbuffered PWM Signal Generation

Any output compare channel can generate unbuffered PWM pulses as

described in 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.

Freescale Semiconductor, Inc.

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