Device-Level Timers
MSC8144E Reference Manual, Rev. 3
Freescale Semiconductor
21-11
21.1.5.1
Compare Preload Registers
The TMRxCMPLD1, TMRxCMPLD2 and TMRxCOMSC registers offer a high degree of
flexibility for loading compare registers with user-defined values on different compare events. To
ensure correct functionality, use the loading method described in this section.
The compare preload feature speeds updating of the compare registers. The compare preload
feature allows you to calculate new compare values and store them into the comparator preload
registers. When a compare event occurs, the new compare values in the comparator preload
registers are directly written to the compare registers, eliminating the use of software to do this.
The compare preload feature is used in variable frequency PWM mode. See Section 21.1.4.4,
Variable Frequency PWM Mode, on page 21-8. The TMRxCMP1 register determines the pulse
width for the logic low part of the timer output, and TMRxCMP2 determines the pulse width for
the logic high part of the timer output. The period of the waveform is determined by the
TMRxCMP1 and TMRxCMP2 values and the frequency of the primary clock source.
See Figure 21-4.
Figure 21-4. Variable PWM Waveform
To update the duty cycle or period of the waveform, update the TMRxCMP1 and TMRxCMP2
values using the compare preload feature.
21.1.5.2
Capture Register Use
The capture register, TMRxCAP (page 21-23), stores a copy of the timer value when an input
edge (positive, negative, or both) on the secondary input signal is detected. The capture mode,
programmable via TMRxSCTL[CM] (page 21-19), is one of the following:
CM = 00. Disabled.
CM = 01. Load the capture register on the rising edge of the signal.
CM = 10. Load the capture register on the falling edge of the signal.
CM = 11. Load the capture register on either edge of the signal.
When a capture event occurs, there are no further updates of TMRxCAP until the input edge flag
(IEF) is cleared by writing a value of 0 to the TMRxSCTL[IEF] bit (page 21-19).
TMRxCMP1
TMRxCMP2
PWM Period
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