LM3S818 Luminary Micro, Inc, LM3S818 Datasheet - Page 311

• Manufacturer Part Number: LM3S818
• Description: Lm3s818 Arm Microcontroller
• Manufacturer: Luminary Micro, Inc

15
15.1
15.2
15.2.1
February 6, 2007
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 LM3S818 PWM module consists of three PWM generator blocks and a control block. Each
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.
Each 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
blocks are managed by the output control block before being passed to the device pins.
The LM3S818 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. It can also
generate the full six channels of gate controls required by a 3-Phase inverter bridge.
Block Diagram
Figure 15-1 provides a block diagram of a Stellaris PWM module. The LM3S818 controller
contains three generator blocks (PWM0, PWM1, and PWM2) and generates six independent
PWM signals or three paired PWM signals with dead-band delays inserted.
Figure 15-1. PWM Module Block Diagram
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
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
LM3S818 Data Sheet
PWMENABLE
PWMINVERT
PWMFAULT
Control
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