LM3S2793 Datasheet, PDF (952/1194 Pages) Texas Instruments – Stellaris® LM3S2793 Microcontroller

Pulse Width Modulator (PWM)
center-aligned, overlapped PWM signals that have different duty cycles. This figure shows the pwmA
and pwmB signals before they have passed through the dead-band generator.
Figure 20-5. PWM Generation Example In Count-Up/Down Mode
LOAD
COMPA
COMPB
20.3.4
0
pwmA
pwmB
In this example, the first generator is set to drive High on match A up, drive Low on match A down,
and ignore the other four events. The second generator is set to drive High on match B up, drive
Low on match B down, and ignore the other four events. Changing the value of comparator A
changes the duty cycle of the pwmA signal, and changing the value of comparator B changes the
duty cycle of the pwmB signal.
Dead-Band Generator
The pwmA and pwmB signals produced by each PWM generator are passed to the dead-band
generator. If the dead-band generator is disabled, the PWM signals simply pass through to the
pwmA' and pwmB' signals unmodified. If the dead-band generator is enabled, the pwmB signal is
lost and two PWM signals are generated based on the pwmA signal. The first output PWM signal,
pwmA' is the pwmA signal with the rising edge delayed by a programmable amount. The second
output PWM signal, pwmB', is the inversion of the pwmA signal with a programmable delay added
between the falling edge of the pwmA signal and the rising edge of the pwmB' signal.
The resulting signals are a pair of active High signals where one is always High, except for a
programmable amount of time at transitions where both are Low. These signals are therefore suitable
for driving a half-H bridge, with the dead-band delays preventing shoot-through current from damaging
the power electronics. Figure 20-6 on page 952 shows the effect of the dead-band generator on the
pwmA signal and the resulting pwmA' and pwmB' signals that are transmitted to the output control
block.
Figure 20-6. PWM Dead-Band Generator
pwmA
pwmA’
pwmB’
Rising Edge
Delay
Falling Edge
Delay
20.3.5
Interrupt/ADC-Trigger Selector
Each PWM generator also takes the same four (or six) counter events and uses them to generate
an interrupt or an ADC trigger. Any of these events or a set of these events can be selected as a
source for an interrupt; when any of the selected events occur, an interrupt is generated. Additionally,
the same event, a different event, the same set of events, or a different set of events can be selected
as a source for an ADC trigger; when any of these selected events occur, an ADC trigger pulse is
generated. The selection of events allows the interrupt or ADC trigger to occur at a specific position
952
January 20, 2012
Texas Instruments-Production Data