PXS20RM Datasheet, PDF (659/1368 Pages) Freescale Semiconductor, Inc – PXS20 Microcontroller

Flexible Motor Control Pulse Width Modulator Module (FlexPWM)
Figure 25-10. Capture Capabilities of the E-Capture Circuit
When a submodule is being used for PWM generation, its timer counts up to the modulus value used to
specify the PWM frequency and then is re-initialized. Therefore, using this timer for input captures on one
of the other pins (e.g, PWMX) has limited utility since it does not count through all of the numbers and
the timer reset represents a discontinuity in the 16 bit number range. However, when measuring a signal
that is synchronous to the PWM frequency, the timer modulus range is perfectly suited for the application.
As an example, consider Figure 25-11. In this application the output of a PWM power stage is connected
to the PWMX pin that is configured for free running input captures. Specifically, the CVAL0 capture
circuitry is programmed for rising edges and the CVAL1 capture circuitry is set for falling edges. This will
result in new load pulse width data being acquired every PWM cycle. To calculate the pulse width, simply
subtract the CVAL0 register value from the CVAL1 register value. This measurement is extremely
beneficial when performing dead-time distortion correction on a half bridge circuit driving an inductive
load. Also, these values can be directly compared to the VALx registers responsible for generating the
PWM outputs to obtain a measurement of system propagation delays.
Freescale Semiconductor
PXS20 Microcontroller Reference Manual, Rev. 1
25-13