MC68HC908MR8 Datasheet, PDF (206/372 Pages) Motorola, Inc – Microcontrollers

Timer Interface A (TIMA)
counter reaches the value in the registers of an output compare channel,
the TIMA can set, clear, or toggle the channel pin. Output compares can
generate TIMA CPU interrupt requests.
11.4.3.1 Unbuffered Output Compare
Any output compare channel can generate unbuffered output compare
pulses as described in 11.4.3 Output Compare. The pulses are
unbuffered because changing the output compare value requires writing
the new value over the old value currently in the TIMA channel registers.
An unsynchronized write to the TIMA channel registers to change an
output compare value could cause incorrect operation for up to two
counter overflow 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 counter overflow period. Also,
using a TIMA overflow interrupt routine to write a new, smaller output
compare value may cause the compare to be missed. The TIMA may
pass the new value before it is written.
Use these methods to synchronize unbuffered changes in the output
compare value on channel x:
• When changing to a smaller value, 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 output compare pulse. The interrupt routine has until
the end of the counter overflow period to write the new value.
• When changing to a larger output compare value, 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 counter overflow 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 counter
overflow period.
11.4.3.2 Buffered Output Compare
Channels 0 and 1 can be linked to form a buffered output compare
channel whose output appears on the PTB3/TCH0A pin. The TIMA
channel registers of the linked pair alternately control the output.
Technical Data
206
Timer Interface A (TIMA)
MC68HC908MR8 — Rev 4.1
Freescale Semiconductor