Configuring Device Pins
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SPRUI07–March 2020
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Enhanced Quadrature Encoder Pulse (eQEP)
The encoder count (position) is read once during each unit time event. The quantity [x(k) - x(k-1)] is
formed by subtracting the previous reading from the current reading. Then the velocity estimate is
computed by multiplying by the known constant 1/T (where T is the constant time between unit time
events and is known in advance).
Estimation based on Equation 1 has an inherent accuracy limit directly related to the resolution of the
position sensor and the unit time period T. For example, consider a 500-line per revolution quadrature
encoder with a velocity calculation rate of 400 Hz. When used for position, the quadrature encoder gives a
four-fold increase in resolution; in this case, 2000 counts per revolution. The minimum rotation that can be
detected is therefore 0.0005 revolutions, which gives a velocity resolution of 12 rpm when sampled at 400
Hz. While this resolution may be satisfactory at moderate or high speeds, for example 1% error at 1200
rpm, it would clearly prove inadequate at low speeds. In fact, at speeds below 12 rpm, the speed estimate
would erroneously be zero much of the time.
At low speed, Equation 2 provides a more accurate approach. It requires a position sensor that outputs a
fixed interval pulse train, such as the aforementioned quadrature encoder. The width of each pulse is
defined by motor speed for a given sensor resolution. Equation 2 can be used to calculate motor speed by
measuring the elapsed time between successive quadrature pulse edges. However, this method suffers
from the opposite limitation, as does Equation 1. A combination of relatively large motor speeds and high
sensor resolution makes the time interval ΔT small, and thus more greatly influenced by the timer
resolution. This can introduce considerable error into high-speed estimates.
For systems with a large speed range (that is, speed estimation is needed at both low and high speeds),
one approach is to use Equation 2 at low speed and have the DSP software switch over to Equation 1
when the motor speed rises above some specified threshold.
6.2 Configuring Device Pins
The GPIO mux registers must be configured to connect this peripheral to the device pins.
For proper operation of the eQEP module, input GPIO pins must be configured via the GPxQSELn
registers for synchronous input mode (with or without qualification). The asynchronous mode should not
be used for eQEP input pins. The internal pullups can be configured in the GPyPUD register.
See the GPIO chapter for more details on GPIO mux and settings.
6.3 Description
This section provides the eQEP inputs, memory map, and functional description.
6.3.1 EQEP Inputs
The eQEP inputs include two pins for quadrature-clock mode or direction-count mode, an index (or 0
marker), and a strobe input. The eQEP module requires that the QEPA, QEPB, and QEPI inputs are
synchronized to SYSCLK prior to entering the module. The application code should enable the
synchronous GPIO input feature on any eQEP-enabled GPIO pins (see the System Control and Interrupts
chapter for more details).
• QEPA/XCLK and QEPB/XDIR
These two pins can be used in quadrature-clock mode or direction-count mode.
– Quadrature-clock Mode
The eQEP encoders provide two square wave signals (A and B) 90 electrical degrees out of phase.
This phase relationship is used to determine the direction of rotation of the input shaft and number
of eQEP pulses from the index position to derive the relative position information. For forward or
clockwise rotation, QEPA signal leads QEPB signal and vice versa. The quadrature decoder uses
these two inputs to generate quadrature-clock and direction signals.
– Direction-count Mode
In direction-count mode, direction and clock signals are provided directly from the external source.
Some position encoders have this type of output instead of quadrature output. The QEPA pin
provides the clock input and the QEPB pin provides the direction input.
• QEPI: Index or Zero Marker
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