M68HC08 datasheet(134/288 Pages) FREESCALE

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Timer Interface Module (TIM)

Data Sheet

MC68HC908BD48 — Rev. 2.1

134

Timer Interface Module (TIM)

Freescale Semiconductor

NOTE:

In buffered PWM signal generation, do not write new pulse width 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 PWM signals.

10.5.4.3 PWM Initialization

To ensure correct operation when generating unbuffered or buffered

PWM signals, use the following initialization procedure:

1. In the TIM status and control register (TSC):

a. Stop the TIM counter by setting the TIM stop bit, TSTOP.

b. Reset the TIM counter and prescaler by setting the TIM reset

bit, TRST.

2. In the TIM counter modulo registers (TMODH:TMODL), write the

value for the required PWM period.

3. In the TIM channel x registers (TCHxH:TCHxL), write the value for

the required pulse width.

4. In TIM channel x status and control register (TSCx):

a. Write 0:1 (for unbuffered output compare or PWM signals) or

1:0 (for buffered output compare or PWM signals) to the

mode select bits, MSxB:MSxA. (See Table 10-4.)

b. Write 1 to the toggle-on-overflow bit, TOVx.

c. Write 1:0 (to clear output on compare) or 1:1 (to set output on

compare) to the edge/level select bits, ELSxB:ELSxA. The

output action on compare must force the output to the

complement of the pulse width level. (See Table 10-4.)

NOTE:

In PWM signal generation, do not program the PWM channel to toggle

on output compare. Toggling on output compare prevents reliable 0%

duty cycle generation and removes the ability of the channel to self-

correct in the event of software error or noise. Toggling on output

compare can also cause incorrect PWM signal generation when

changing the PWM pulse width to a new, much larger value.

5. In the TIM status control register (TSC), clear the TIM stop bit,

TSTOP.

M68HC08 Datasheet(PDF) 134 Page - Freescale Semiconductor, Inc