MC68HC908QL4 Datasheet, PDF (176/226 Pages) Motorola, Inc – Microcontrollers

Timer Interface Module (TIM)
• When changing to a larger output compare value, enable TIM overflow interrupts and write the new
value in the TIM overflow interrupt routine. The TIM 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.
15.3.3.2 Buffered Output Compare
Channels 0 and 1 can be linked to form a buffered output compare channel whose output appears on the
TCH0 pin. The TIM channel registers of the linked pair alternately control the output.
Setting the MS0B bit in TIM channel 0 status and control register (TSC0) links channel 0 and channel 1.
The output compare value in the TIM channel 0 registers initially controls the output on the TCH0 pin.
Writing to the TIM channel 1 registers enables the TIM channel 1 registers to synchronously control the
output after the TIM overflows. At each subsequent overflow, the TIM channel registers (0 or 1) that
control the output are the ones written to last. TSC0 controls and monitors the buffered output compare
function, and TIM channel 1 status and control register (TSC1) is unused. While the MS0B bit is set, the
channel 1 pin, TCH1, 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. User software should track
the currently active channel to prevent writing a new value to the active
channel. Writing to the active channel registers is the same as generating
unbuffered output compares.
15.3.4 Pulse Width Modulation (PWM)
By using the toggle-on-overflow feature with an output compare channel, the TIM can generate a PWM
signal. The value in the TIM counter modulo registers determines the period of the PWM signal. The
channel pin toggles when the counter reaches the value in the TIM counter modulo registers. The time
between overflows is the period of the PWM signal.
As Figure 15-3 shows, the output compare value in the TIM channel registers determines the pulse width
of the PWM signal. The time between overflow and output compare is the pulse width. Program the TIM
to clear the channel pin on output compare if the polarity of the PWM pulse is 1 (ELSxA = 0). Program the
TIM to set the pin if the polarity of the PWM pulse is 0 (ELSxA = 1).
OVERFLOW
OVERFLOW
POLARITY = 1
(ELSxA = 0) _TCHx
POLARITY = 0 TCHx
(ELSxA = 1)
PERIOD
PULSE
WIDTH
OVERFLOW
OUTPUT
COMPARE
OUTPUT
COMPARE
Figure 15-3. PWM Period and Pulse Width
OUTPUT
COMPARE
MC68HC908QL4 Data Sheet, Rev. 7
176
Freescale Semiconductor