Pwm Initialization - Freescale Semiconductor MC68HC908MR16 Datasheet

Timer Interface B (TIMB)
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.

17.3.4.3 PWM Initialization

To ensure correct operation when generating unbuffered or buffered PWM signals, use this initialization
procedure:
1. In the TIMB status and control register (TBSC):
a. Stop the TIMB counter by setting the TIMB stop bit, TSTOP.
b. Reset the TIMB counter and prescaler by setting the TIMB reset bit, TRST.
2. In the TIMB counter modulo registers (TBMODH–TBMODL), write the value for the required PWM
period.
3. In the TIMB channel x registers (TBCHxH–TBCHxL), write the value for the required pulse width.
4. In TIMB channel x status and control register (TBSCx):
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
b. Write 1 to the toggle-on-overflow bit, TOVx.
c. Write 1:0 (polarity 1 — to clear output on compare) or 1:1 (polarity 0 — 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
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 TIMB status control register (TBSC), clear the TIMB stop bit, TSTOP.
Setting MS0B links channels 0 and 1 and configures them for buffered PWM operation. The TIMB channel
0 registers (TBCH0H–TBCH0L) initially control the buffered PWM output. TIMB status control register 0
(TBSC0) controls and monitors the PWM signal from the linked channels. MS0B takes priority over MS0A.
Clearing the toggle-on-overflow bit, TOVx, inhibits output toggles on TIMB overflows. Subsequent output
compares try to force the output to a state it is already in and have no effect. The result is a 0 percent duty
cycle output.
Setting the channel x maximum duty cycle bit (CHxMAX) and setting the TOVx bit generates a
100 percent duty cycle output. (See
242
17.7.4 TIMB Channel Status and Control
MC68HC908MR32 • MC68HC908MR16 Data Sheet, Rev. 6.1
NOTE
Table 17-2.)
Table 17-2.)
Registers.)
Freescale Semiconductor