LM3S2276 Datasheet, PDF (658/785 Pages) Texas Instruments – Stellaris® LM3S2276 Microcontroller

Pulse Width Modulator (PWM)
17.3
17.3.1
17.3.2
Functional Description
PWM Timer
The timer in each PWM generator runs in one of two modes: Count-Down mode or Count-Up/Down
mode. In Count-Down mode, the timer counts from the load value to zero, goes back to the load
value, and continues counting down. In Count-Up/Down mode, the timer counts from zero up to the
load value, back down to zero, back up to the load value, and so on. Generally, Count-Down mode
is used for generating left- or right-aligned PWM signals, while the Count-Up/Down mode is used
for generating center-aligned PWM signals.
The timers output three signals that are used in the PWM generation process: the direction signal
(this is always Low in Count-Down mode, but alternates between Low and High in Count-Up/Down
mode), a single-clock-cycle-width High pulse when the counter is zero, and a single-clock-cycle-width
High pulse when the counter is equal to the load value. Note that in Count-Down mode, the zero
pulse is immediately followed by the load pulse.
PWM Comparators
There are two comparators in each PWM generator that monitor the value of the counter; when
either match the counter, they output a single-clock-cycle-width High pulse. When in Count-Up/Down
mode, these comparators match both when counting up and when counting down; they are therefore
qualified by the counter direction signal. These qualified pulses are used in the PWM generation
process. If either comparator match value is greater than the counter load value, then that comparator
never outputs a High pulse.
Figure 17-3 on page 658 shows the behavior of the counter and the relationship of these pulses
when the counter is in Count-Down mode. Figure 17-4 on page 659 shows the behavior of the counter
and the relationship of these pulses when the counter is in Count-Up/Down mode.
Figure 17-3. PWM Count-Down Mode
Load
CompA
CompB
Zero
Load
Zero
A
B
Dir
BDown
ADown
658
November 17, 2011
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