LM3S1439_16 Datasheet, PDF (559/699 Pages) Texas Instruments – Stellaris LM3S1439 Microcontroller

Stellaris^® LM3S1439 Microcontroller
Table 16-2. PWM Signals (108BGA) (continued)
Pin Name
Pin Number Pin Type Buffer Type^a Description
PWM3
C8
O
TTL
PWM 3. This signal is controlled by PWM Generator 1.
PWM4
J11
O
TTL
PWM 4. This signal is controlled by PWM Generator 2.
PWM5
J12
O
TTL
PWM 5. This signal is controlled by PWM Generator 2.
a. The TTL designation indicates the pin has TTL-compatible voltage levels.
16.3 Functional Description
16.3.1
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.
16.3.2
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 16-3 on page 560 shows the behavior of the counter and the relationship of these pulses
when the counter is in Count-Down mode. Figure 16-4 on page 560 shows the behavior of the counter
and the relationship of these pulses when the counter is in Count-Up/Down mode.
July 15, 2014
559
Texas Instruments-Production Data