LM3S2776 Datasheet, PDF (660/785 Pages) Texas Instruments – Stellaris® LM3S2776 Microcontroller

Pulse Width Modulator (PWM)
17.3.4
Dead-Band Generator
The two PWM signals produced by the PWM generator are passed to the dead-band generator. If
disabled, the PWM signals simply pass through unmodified. If enabled, the second PWM signal is
lost and two PWM signals are generated based on the first PWM signal. The first output PWM signal
is the input signal with the rising edge delayed by a programmable amount. The second output
PWM signal is the inversion of the input signal with a programmable delay added between the falling
edge of the input signal and the rising edge of this new signal.
This is therefore 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 17-6 on page 660 shows the effect of the dead-band generator on an input
PWM signal.
Figure 17-6. PWM Dead-Band Generator
Input
PWMA
PWMB
Rising Edge
Delay
Falling Edge
Delay
17.3.5
17.3.6
Interrupt/ADC-Trigger Selector
The 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
within the PWM signal. Note that interrupts and ADC triggers are based on the raw events; delays
in the PWM signal edges caused by the dead-band generator are not taken into account.
Synchronization Methods
The PWM unit provides four PWM generators providing eight PWM outputs that may be used in a
wide variety of applications. Generally speaking, this falls into combinations of two categories of
operation:
■ Unsynchronized. The PWM generator and its two output signals are used by itself, independent
of other PWM generators.
■ Synchronized. The PWM generator and its two outputs signals are used in conjunction with
other PWM generators using a common, unified time base.
If multiple PWM generators are configured with the same counter load value, this can be used to
guarantee that they also have the same count value (this does imply that the PWM generators must
be configured before they are synchronized). With this, more than two PWM signals can be produced
with a known relationship between the edges of those signals since the counters always have the
same values. Other states in the unit provide mechanisms to maintain the common time base and
mutual synchronization.
660
November 17, 2011
Texas Instruments-Production Data