LM3S3768 Luminary Micro, Inc, LM3S3768 Datasheet - Page 608

• Manufacturer Part Number: LM3S3768
• Description: Lm3s3768 Arm Microcontroller
• Manufacturer: Luminary Micro, Inc

Pulse Width Modulator (PWM)
19.2.4
608
A or match B events are ignored when they coincide with the zero or load events. If the match A
and match B events coincide, the first signal, PWMA, is generated based only on the match A event,
and the second signal, PWMB, is generated based only on the match B event.
For each event, the effect on each output PWM signal is programmable: it can be left alone (ignoring
the event), it can be toggled, it can be driven Low, or it can be driven High. These actions can be
used to generate a pair of PWM signals of various positions and duty cycles, which do or do not
overlap. Figure 19-5 on page 608 shows the use of Count-Up/Down mode to generate a pair of
center-aligned, overlapped PWM signals that have different duty cycles.
Figure 19-5. PWM Generation Example In Count-Up/Down Mode
CompA
CompB
PWMA
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 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 19-6 on page 608 shows the effect of the dead-band generator on an input
PWM signal.
Figure 19-6. PWM Dead-Band Generator
PWMA
PWMB
PWMB
Load
Input
Zero
Rising Edge
Delay
Falling Edge
Delay
Preliminary
June 02, 2008