Advance Information MC68HC908MR16/MC68HC908MR32 — Rev. 4.0
208 Timer Interface A (TIMA) MOTOROLA
Timer Interface A (TIMA)
11.4.4 Pulse-Width Modulation (PWM)
By using the toggle-on-overflow feature with a n output compare channel,
the TIMA can generate a PWM signal. The value in the TIMA counter
modulo registers determines the period of the PWM signal. The channel
pin toggles when the counter reaches the value in the TIMA counter
modulo registers. The time between overflow s is the period o f the PW M
signal.
As Figure 11-3 shows, the output compare value in the TIMA channel
registers determines the pulse width of the PWM signal. The time
between overflow and output compare is the pulse width. Program the
TIMA to clear the channel pin on output compare if the state of the PWM
pulse is logic 1. Program the TIMA to set the pin if the state of the PW M
pulse is logic 0.
Figure 11-3. PWM Period and Pulse Width
The value in the TIMA counter modulo registers and the selected
prescaler output determines the frequency of the PWM output. The
frequency of an 8-bit PWM signal is variable in 25 6 i n crements. Writing
$00FF (255) to the TIMA counter modulo registers produces a PWM
period of 256 times the internal bus clock period if the prescaler select
value is $000 (see 11.8.1 TIMA Status and Control Register).
The value in the TIMA channel registers determines the pulse width of
the PWM output. The pulse width of an 8-bit PWM si gnal is variable in
256 increments. Writing $0080 (128) to the TIMA channel registers
produces a duty cycle of 128/256 or 50 percent.
PTEx/TCHx
PERIOD
PULSE
WIDTH
OVERFLOW OVERFLOW OVERFLOW
OUTPUT
COMPARE OUTPUT
COMPARE
OUTPUT
COMPARE