24 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 TM4C1294NCPDT 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 TM4C1294NCPDT microcontroller includes one QEI module with the following features:
■ Position integrator that tracks the encoder position
■ Programmable noise filter on the inputs
■ Velocity capture using built-in timer
■ The input frequency of the QEI inputs may be as high as 1/4 of the processor frequency (for
example, 12.5 MHz for a 50-MHz system)
■ Interrupt generation on:
– Index pulse
– Velocity-timer expiration
– Direction change
– Quadrature error detection
24.1 Block Diagram
Figure 24-1 on page 1749 provides an internal block diagram of a TM4C1294NCPDT QEI module.
The PhA and PhB inputs shown in this diagram are the internal signals that enter the Quadrature
Encoder after the external signals, PhAn and PhBn, have passed through inversion and swapping
logic shown in Figure 24-2 on page 1750. The QEI module has the option of inverting and/or swapping
the incoming signals.
Note: Any references in this chapter to PhA and PhB refer to the internal PhA and PhB inputs that
enter the Quadrature Encoder after the external signals, PhAn and PhBn, have passed
through inversion and swapping logic that is enabled through the QEI Control (QEICTL)
register.
June 18, 20141748
Texas Instruments-Production Data
Quadrature Encoder Interface (QEI)
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