LM3S812_16 Datasheet, PDF (500/590 Pages) Texas Instruments – Stellaris LM3S812 Microcontroller

Pulse Width Modulator (PWM)
15.3.4
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 15-6 on page 500 shows the effect of the dead-band generator on an input
PWM signal.
Figure 15-6. PWM Dead-Band Generator
Input
PWMA
PWMB
Rising Edge
Delay
Falling Edge
Delay
15.3.5
15.3.6
15.3.7
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
There is a global reset capability that can reset the counter of the PWM generator.
The counter load values and comparator match values of the PWM generator can be updated in
two ways. The first is immediate update mode, where a new value is used as soon as the counter
reaches zero. By waiting for the counter to reach zero, a guaranteed behavior is defined, and overly
short or overly long output PWM pulses are prevented.
The other update method is synchronous, where the new value is not used until a global synchronized
update signal is asserted, at which point the new value is used as soon as the counter reaches
zero. This second mode allows multiple items to be updated simultaneously without odd effects
during the update; everything runs from the old values until a point at which they all run from the
new values.
Fault Conditions
There are two external conditions that affect the PWM block; the signal input on the Fault pin and
the stalling of the controller by a debugger. There are two mechanisms available to handle such
500
July 14, 2014
Texas Instruments-Production Data