LM3S317 Datasheet, PDF (319/379 Pages) List of Unclassifed Manufacturers – Microcontroller

LM3S317 Data Sheet
15
15.1
15.2
15.2.1
15.2.2
Pulse Width Modulator (PWM)
Pulse width modulation (PWM) is a powerful technique for digitally encoding analog signal levels.
High-resolution counters are used to generate a square wave, and the duty cycle of the square
wave is modulated to encode an analog signal. Typical applications include switching power
supplies and motor control.
The LM3S317 PWM module consists of PWM generator block and a control block. PWM
generator block contains one timer (16-bit down or up/down counter), two comparators, a PWM
signal generator, a dead-band generator, and an interrupt selector. The control block determines
the polarity of the PWM signals, and which signals are passed through to the pins.
PWM generator block produces two PWM signals that can either be independent signals (other
than being based on the same timer and therefore having the same frequency) or a single pair of
complementary signals with dead-band delays inserted. The output of the PWM generation block
managed by the output control block before being passed to the device pins.
The LM3S317 PWM module provides a great deal of flexibility. It can generate simple PWM
signals, such as those required by a simple charge pump. It can also generate paired PWM
signals with dead-band delays, such as those required by a half-H bridge driver.
Block Diagram
Figure 15-1 provides a block diagram of a Stellaris PWM module.
Figure 15-1. PWM Module Block Diagram
Functional Description
PWM Timer
The timer 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 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 15-2 shows the behavior of the counter and the relationship of these pulses when the
counter is in Count-Down mode. Figure 15-3 shows the behavior of the counter and the
relationship of these pulses when the counter is in Count-Up/Down mode.
May 4, 2007
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Preliminary