Timer Interface A (TIMA)
Functional Description
OVERFLOW
OVERFLOW
OVERFLOW
PERIOD
POLARITY = 1
(ELSxA = 0)
TCHx
TCHx
PULSE
WIDTH
POLARITY = 0
(ELSxA = 1)
OUTPUT
COMPARE
OUTPUT
COMPARE
OUTPUT
COMPARE
Figure 16-4. PWM Period and Pulse Width
16.3.4.1 Unbuffered PWM Signal Generation
Any output compare channel can generate unbuffered PWM pulses as described
in 16.3.4 Pulse-Width Modulation (PWM). The pulses are unbuffered because
changing the pulse width requires writing the new pulse width value over the 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 to the TIMA channel registers.
Use this method 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 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.
NOTE:
In PWM signal generation, do not program the PWM channel to toggle on output
compare. Toggling on output compare prevents reliable 0 percent 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.
MC68HC908MR32 • MC68HC908MR16 — Rev. 6.0
MOTOROLA
Data Sheet
241
Timer Interface A (TIMA)
FREESCALE [ Freescale ]
