Table of Contents
Advertisement
Functional Description
alignment between the signals is not restricted to symmetry around the zero count value,
as any other number would also work. However, centering on zero provides the greatest
range in signed mode and also simplifies the calculations.
37.5.1.2 Edge Aligned PWMs
When the turn on edge for each pulse is specified to be the INIT value, then edge aligned
operation results, as the following figure shows. Therefore, only the turn off edge value
needs to be periodically updated to change the pulse width.
VAL1 ($0100)
VAL5
VAL0 ($0000)
VAL3
INIT ($FF00)
PWM_A
PWM_B
Figure 37-225. Edge Aligned Example (INIT=VAL2=VAL4)
With edge aligned PWMs, another example of the benefits of signed mode can be seen. A
common way to drive an H-bridge is to use a technique called "bipolar" PWMs where a
50% duty cycle results in zero volts on the load. Duty cycles less than 50% result in
negative load voltages and duty cycles greater than 50% generate positive load voltages.
If the module is set to signed mode operation (the INIT and VAL1 values are the same
number with opposite signs), then there is a direct proportionality between the PWM turn
off edge value and the voltage, INCLUDING the sign. So once again, signed mode of
operation simplifies the software interface to the PWM module since no offset
calculations are required to translate the output variable control algorithm to the voltage
on an H-Bridge load.
828
KV4x Reference Manual, Rev. 2, 02/2015
Preliminary
Freescale Semiconductor, Inc.
Advertisement
Table of Contents
