MC68HC908RK2 Datasheet, PDF (125/158 Pages) Motorola, Inc – Microcontroller Unit

Functional Description
11.4.8 Buffered PWM Signal Generation
Channels 0 and 1 can be linked to form a buffered PWM channel whose output appears on the TCH0 pin.
The TIM channel registers of the linked pair alternately control the pulse width of the output.
Setting the MS0B bit in TIM channel 0 status and control register (TSC0) links channel 0 and channel 1.
The TIM channel 0 registers initially control the pulse width on the TCH0 pin. Writing to the TIM channel 1
registers enables the TIM channel 1 registers to synchronously control the pulse width at the beginning
of the next PWM period. At each subsequent overflow, the TIM channel registers (0 or 1) that control the
pulse width are the ones written to last. TSC0 controls and monitors the buffered PWM function, and TIM
channel 1 status and control register (TSC1) is unused.
NOTE
In buffered PWM signal generation, do not write new pulse width values to
the currently active channel registers. Writing to the active channel
registers is the same as generating unbuffered PWM signals.
11.4.9 PWM Initialization
To ensure correct operation when generating unbuffered or buffered PWM signals, use this 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 by setting the TIM reset bit, TRST.
2. In the TIM counter modulo registers (TMODH and TMODL), write the value for the required PWM
period.
3. In the TIM channel x registers (TCHxH and 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 and MSxA. See Table 11-3.
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 and ELSxA. The output action on compare must force the output to the
complement of the pulse width level. (See Table 11-3.)
NOTE
In PWM signal generation, do not program the PWM channel to toggle on
output compare. Toggling on output compare prevents reliable 0 percent
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.
Setting MS0B links channels 0 and 1 and configures them for buffered PWM operation. The TIM
channel 0 registers (TCH0H and TCH0L) initially control the buffered PWM output. TIM status control
register 0 (TSCR0) controls and monitors the PWM signal from the linked channels.
MC68HC908RK2 Data Sheet, Rev. 5.1
Freescale Semiconductor
125