mc68hc908kx8 Freescale Semiconductor, Inc, mc68hc908kx8 Datasheet - Page 160

• Manufacturer Part Number: mc68hc908kx8
• Description: M68hc08 Microcontrollers
• Manufacturer: Freescale Semiconductor, Inc

Timer Interface Module (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 on overflow if the state of the PWM pulse is logic 0.
The value in the TIM 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 256 increments. Writing
$00FF (255) to the TIM counter modulo registers produces a PWM period of 256 times the internal bus
clock period if the prescaler select value is $000. See
The value in the TIM 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 TIM channel registers
produces a duty cycle of 128/256 or 50 percent.
15.4.7 Unbuffered PWM Signal Generation
Any output compare channel can generate unbuffered PWM pulses as described in
Modulation
pulse width value over the old value currently in the TIM channel registers.
An unsynchronized write to the TIM channel registers to change a pulse width value could cause incorrect
operation for up to two PWM periods. For example, writing a new value before the counter reaches the
old value but after the counter reaches the new value prevents any compare during that PWM period.
Also, using a TIM overflow interrupt routine to write a new, smaller pulse width value may cause the
compare to be missed. The TIM may pass the new value before it is written.
Use these methods to synchronize unbuffered changes in the PWM pulse width on channel x:
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When changing to a shorter pulse width, enable channel x output compare interrupts and write the
new value in the output compare interrupt routine. The output compare interrupt occurs at the end
of the current pulse. The interrupt routine has until the end of the PWM period to write the new
value.
When changing to a longer pulse width, enable TIM overflow interrupts and write the new value in
the TIM overflow interrupt routine. The TIM overflow interrupt occurs at the end of the current PWM
period. Writing a larger value in an output compare interrupt routine (at the end of the current pulse)
could cause two output compares to occur in the same PWM period.
(PWM). The pulses are unbuffered because changing the pulse width requires writing the new
PT
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
A
x/TCH
MC68HC908KX8 • MC68HC908KX2 • MC68HC08KX8 Data Sheet, Rev. 2.1
OVERFLOW
Figure 15-4. PWM Period and Pulse Width
PULSE
WIDTH
PERIOD
COMPARE
OUTPUT
OVERFLOW
NOTE
15.8.1 TIM Status and Control
COMPARE
OUTPUT
OVERFLOW
Freescale Semiconductor
COMPARE
OUTPUT
15.4.6 Pulse-Width
Register.