MC68HC08AZ16 Datasheet, PDF (254/527 Pages) Motorola, Inc – HCMOS Microcontroller Unit

Timer Interface Module A (TIMA)
Functional description
value is $000. See TIMA status and control register (TASC) on page
260.
The value in the TIMA 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 TIMA channel registers
produces a duty cycle of 128/256 or 50%.
Unbuffered PWM
signal generation
Any output compare channel can generate unbuffered PWM pulses as
described in Pulse width modulation (PWM) on page 252. The pulses
are unbuffered because changing the pulse width requires writing the
new pulse width value over the old value currently in the TIMA channel
registers.
An unsynchronized write to the TIMA 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 TIMA overflow interrupt
routine to write a new, smaller pulse width value may cause the compare
to be missed. The TIMA 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 channel x TIMA
overflow interrupts and write the new value in the TIMA overflow
interrupt routine. The TIMA 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.
9-tima
MOTOROLA
Timer Interface Module A (TIMA)
MC68HC08AZ32
253