LM3S301 Luminary Micro, Inc, LM3S301 Datasheet - Page 312

• Manufacturer Part Number: LM3S301
• Description: Lm3s301 Arm Microcontroller
• Manufacturer: Luminary Micro, Inc

Pulse Width Modulator (PWM)
15.2.4
15.2.5
15.2.6
15.2.7
312
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 15-5 shows the effect of the dead-band generator on
an input PWM signal.
Figure 15-5. PWM Dead-Band Generator
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 synchronously reset any or all of the counters in 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
conditions: the output signals can be forced into an inactive state and/or the PWM timers can be
stopped.
PWMA
PWMB
Input
Rising Edge
Preliminary
Delay
Falling Edge
Delay
July 5, 2006