LM3S8G62 Datasheet, PDF (884/1096 Pages) Texas Instruments – Stellaris® LM3S8G62 Microcontroller

OBSOLETE: TI has discontinued production of this device.
Pulse Width Modulator (PWM)
Figure 19-4. PWM Count-Up/Down Mode
LOAD
COMPA
COMPB
0
load
zero
cmpA
cmpB
dir
19.3.3
BUp
AUp
BDown
ADown
PWM Signal Generator
Each PWM generator takes the load, zero, cmpA, and cmpB pulses (qualified by the dir signal) and
generates two internal PWM signals, pwmA and pwmB. In Count-Down mode, there are four events
that can affect these signals: zero, load, match A down, and match B down. In Count-Up/Down
mode, there are six events that can affect these signals: zero, load, match A down, match A up,
match B down, and match B up. The match 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 884 shows the use of Count-Up/Down mode to generate a pair of
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 19-5. PWM Generation Example In Count-Up/Down Mode
LOAD
COMPA
COMPB
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.
884
July 24, 2012
Texas Instruments-Production Data