
Timer Interface Module (TIM)
MC68HC908QL4  MC68HC908QL3  MC68HC908QL2 Data Sheet, Rev. 4
176
Freescale Semiconductor
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).
Figure 15-3. PWM Period and Pulse Width
PERIOD
PULSE
WIDTH
OVERFLOW
OVERFLOW
OVERFLOW
OUTPUT
COMPARE
OUTPUT
COMPARE
OUTPUT
COMPARE
POLARITY = 1
(ELSxA = 0)
POLARITY = 0
(ELSxA = 1)
TCHx
TCHx