
Timer Interface Module (TIM)
MC68HC08LK60 MC68HC908LK60 Advance Information Data Sheet, Rev. 1.1
186
Freescale Semiconductor
output after the timer overflows. At each subsequent overflow, the timer channel registers (2 or 3) that
control the output are the ones written to last. TSC2 controls and monitors the buffered output compare
function, and timer channel 3 status and control register (TSC3) is unused. 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.
16.3.4 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 timer counter modulo registers determines the period of the PWM signal. The
channel pin toggles when the counter reaches the value in the timer counter modulo registers. The time
between overflows is the period of the PWM signal.
As
Figure 16-3 shows, the output compare value in the timer 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 state of the PWM pulse is logic 1. Program the TIM to
set the pin if the state of the PWM pulse is logic 0.
Figure 16-3. PWM Period and Pulse Width
The value in the timer counter modulo registers and the selected prescaler output determine the
frequency of the PWM output. The frequency of an 8-bit PWM signal is variable in 256 increments. Writing
$00FF (255) to the timer counter modulo registers produces a PWM period of 256 times the internal bus
clock period.
The value in the timer channel registers determines the pulse width of the PWM output. The pulse width
of an 8-bit PWM signal is variable in 256 increments. Writing $0080 (128) to the timer channel registers
produces a duty cycle of 128/256 or 50%.
NOTE
This device does not have channel pins for PWM.
PTEx/TCHx
PERIOD
PULSE
WIDTH
OVERFLOW
OUTPUT
COMPARE
OUTPUT
COMPARE
OUTPUT
COMPARE