
NON-DISCLOSURE
AGREEMENT
REQUIRED
Timer Interface Module (TIM)
Advance Information
MC68HC(9)08LJ60 — Rev. 1.0
272
Timer Interface Module (TIM)
MOTOROLA
The value in the timer 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 timer channel registers
produces a duty cycle of 128/256 or 50%.
NOTE:
This device does not have channel pins for PWM.
16.4.4.1 Unbuffered PWM Signal Generation
Any output compare channel can generate unbuffered PWM pulses as
unbuffered because changing the pulse width requires writing the new
pulse width value over the old value currently in the timer channel
registers.
An unsynchronized write to the timer 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 timer overflow interrupt routine to
write a new, smaller pulse width value may cause the compare to be
missed. The timer may pass the new value before it is written.
Use these 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 channel x timer
overflow interrupts and write the new value in the timer overflow
interrupt routine. The timer 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.
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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