PXS20RM Datasheet, PDF (660/1368 Pages) Freescale Semiconductor, Inc – PXS20 Microcontroller

Flexible Motor Control Pulse Width Modulator Module (FlexPWM)
During deadtime, load inductance
drives voltage with polarity that keeps
inductive current flowing through diodes.
PWMA
V+
i+
PWMB
Actual load
voltage
(for i+)
Actual load
voltage
(for i-)
i-
to PWMX input
Actual load voltage
V- pulse width is measured
Figure 25-11. Output Pulse Width Measurement Possible with the E-Capture Circuit
25.3.2.7 Synchronous Switching of Multiple Outputs
Before the PWM signals are routed to the output pins, they are processed by a hardware block that permits
all submodule outputs to be switched synchronously. This feature can be extremely useful in commutated
motor applications where the next commutation state can be laid in ahead of time and then immediately
switched to the outputs when the appropriate condition or time is reached. Not only do all the changes
occur synchronously on all submodule outputs, but they occur IMMEDIATELY after the trigger event
occurs eliminating any interrupt latency.
The synchronous output switching is accomplished via a signal called FORCE_OUT. This signal
originates from the local FORCE bit within the submodule, from submodule0, or from external to the
PWM module and, in most cases, is supplied from an external timer channel configured for output
compare. In a typical application, software sets up the desired states of the output pins in preparation for
the next FORCE_OUT event. This selection lays dormant until the FORCE_OUT signal transitions and
then all outputs are switched simultaneously. The signal switching is performed upstream from the
deadtime generator so that any abrupt changes that might occur do not violate deadtime on the power stage
when in complementary mode.
Figure 25-12 shows a popular application that can benefit from this feature. On a brushless DC motor it
is desirable on many cases to spin the motor without need of hall-effect sensor feedback. Instead, the back
EMF of the motor phases is monitored and this information is used to schedule the next commutation
event. The top waveforms of Figure 25-12 are a simplistic representation of these back EMF signals.
25-14
PXS20 Microcontroller Reference Manual, Rev. 1
Freescale Semiconductor