LM3S301 Luminary Micro, Inc, LM3S301 Datasheet - Page 309

• Manufacturer Part Number: LM3S301
• Description: Lm3s301 Arm Microcontroller
• Manufacturer: Luminary Micro, Inc

15
15.1
15.2
15.2.1
July 5, 2006
PWM Clock
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 LM3S301 PWM module consists of one PWM generator block and a control block. The 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/ADC-trigger selector. The control block
determines the polarity of the PWM signals, and which signals are passed through to the pins.
The 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 is managed by the output control block before being passed to the device pins.
The LM3S301 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. The LM3S301 controller
contains one generator block (PWM0) and generates two independent PWM signals or one paired
PWM signal with dead-band delays inserted.
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.
PWMnCOUNT
PWMnLOAD
16
Timer
Comparator A
Comparator B
PWMnCMPA
PWMnCMPB
zero
load
dir
cmpA
cmpB
Preliminary
Trigger Generate
Interrupt and
PWMnINTEN
PWMnGENA
PWMnGENB
Generator
PWMnRIS
PWMnISC
PWM
pwma
pwmb
PWM Generator Block
PWMnDBRISE
PWMnDBFALL
PWMnDBCTL
Dead-Band
Generator
Fault
Interrupt
ADC Trigger
PWM Output
LM3S301 Data Sheet
PWMENABLE
PWMINVERT
PWMFAULT
Control
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