LM3S818 Luminary Micro, Inc, LM3S818 Datasheet - Page 344

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

Quadrature Encoder Interface (QEI)
16
16.1
16.2
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Quadrature Encoder Interface (QEI)
A quadrature encoder, also known as a 2-channel incremental encoder, converts linear
displacement into a pulse signal. By monitoring both the number of pulses and the relative phase
of the two signals, you can track the position, direction of rotation, and speed. In addition, a third
channel, or index signal, can be used to reset the position counter.
The Stellaris quadrature encoder interface (QEI) module interprets the code produced by a
quadrature encoder wheel to integrate position over time and determine direction of rotation. In
addition, it can capture a running estimate of the velocity of the encoder wheel.
The Stellaris quadrature encoder has the following features:
Block Diagram
Figure 16-1 provides a block diagram of the QEI module.
Figure 16-1. QEI Block Diagram
Functional Description
The QEI module interprets the two-bit gray code produced by a quadrature encoder wheel to
integrate position over time and determine direction of rotation. In addition, it can capture a running
estimate of the velocity of the encoder wheel.
The position integrator and velocity capture can be independently enabled, though the position
integrator must be enabled before the velocity capture can be enabled. The two phase signals,
PHA and PHB, can be swapped in software before being interpreted by the QEI module to change
Position integrator that tracks the encoder position
Velocity capture using built-in timer
Interrupt generation on:
–
–
–
–
Index pulse
Velocity-timer expiration
Direction change
Quadrature error detection
PhA
IDX
PhB
Quadrature
Encoder
Control & Status
Preliminary
QEISTAT
dir
QEICTL
clk
Predivider
Velocity
Velocity Accumulator
Position Integrator
Interrupt Control
Velocity Timer
QEIMAXPOS
QEICOUNT
QEISPEED
QEIINTEN
QEILOAD
QEITIME
QEIPOS
QEIRIS
QEIISC
Interrupt
February 6, 2007