MC908AZ32ACFUE Datasheet, PDF (210/324 Pages) Freescale Semiconductor, Inc – Microcontrollers

Timer Interface Module B (TIMB)
19.3.3.1 Unbuffered Output Compare
Any output compare channel can generate unbuffered output compare pulses as described in 19.3.3
Output Compare. The pulses are unbuffered because changing the output compare value requires writing
the new value over the old value currently in the TIMB channel registers.
An unsynchronized write to the TIMB 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 TIMB overflow interrupt routine to write a new, smaller output
compare value may cause the compare to be missed. The TIMB may pass the new value before it is
written.
Use the following 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 TIMB overflow interrupts and write the
new value in the TIMB overflow interrupt routine. The TIMB 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.
19.3.3.2 Buffered Output Compare
Channels 0 and 1 can be linked to form a buffered output compare channel whose output appears on the
PTF4 pin. The TIMB channel registers of the linked pair alternately control the output.
Setting the MS0B bit in TIMB channel 0 status and control register (TBSC0) links channel 0 and
channel 1. The output compare value in the TIMB channel 0 registers initially controls the output on the
PTF4 pin. Writing to the TIMB channel 1 registers enables the TIMB channel 1 registers to synchronously
control the output after the TIMB overflows. At each subsequent overflow, the TIMB channel registers (0
or 1) that control the output are the ones written to last. TBSC0 controls and monitors the buffered output
compare function and TIMB channel 1 status and control register (TBSC1) is unused. While the MS0B bit
is set, the channel 1 pin, PTF5/TBCH1, is available as a general-purpose I/O pin.
NOTE
In buffered output compare operation, do not write new output compare
values to the currently active channel registers. Writing to the active
channel registers is the same as generating unbuffered output compares.
19.3.4 Pulse Width Modulation (PWM)
By using the toggle-on-overflow feature with an output compare channel, the TIMB can generate a PWM
signal. The value in the TIMB counter modulo registers determines the period of the PWM signal. The
channel pin toggles when the counter reaches the value in the TIMB counter modulo registers. The time
between overflows is the period of the PWM signal.
As Figure 19-3 shows, the output compare value in the TIMB channel registers determines the pulse width
of the PWM signal. The time between overflow and output compare is the pulse width. Program the TIMB
MC68HC908AZ32A Data Sheet, Rev. 2
210
Freescale Semiconductor