BitFlow NEO-PCE-CLB, Hardware Reference Manual

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Quadrature Encoder Introduction
Version G.5 BitFlow, Inc. NEO-4-1
Quadrature Encoder
Chapter 4
4.1 Introduction
This section discusses support for quadrature encoders. A quadrature encoder is an
encoder that outputs two signals A and B. Both signals are used as a line trigger. How-
ever, the signals are 90 degrees out of phase. By comparing the A and B signals, the
direction of the encoder motion can be determined. There are a number of ways that
quadrature encoders can be used to control acquisition. The following sections cover
all of the support methods.
Most of the quadrature encoder system is based around a 24-bit counter. This nor-
mally starts at zero and then counts up or down every time the encoder moves. The
counter can be observed at any time via the QENC_COUNT register. This registers is
the heart of the encoder system. For example, triggers values can be programmed to
start and end acquisition of lines. Also, as the counter tracks the motion of the stage
attached to the encoder exactly, the system can be programmed to only acquire for-
ward only or backward only stage movements. The system can be be programmed to
only acquire one line for each encoder count that corresponds to a physical location
on the stage. The encoder counter can be used in many different ways, described in
more details below.
4.1.1 Simple Encoder Mode
The most basic method of using a quadrature encoder is to use it like a standard sig-
nal phase encoder. In this mode, the quadrature encoder provides a higher resolution
signal, as both the A and B signals can be used to trigger lines. Also, by setting
QENC_DECODE = 1, both the rising and the falling edges of both the A and B signals
are used to trigger lines, providing a 4x increase in resolution over a signal phase
encoder.
In this mode, every encoder edge triggers a line, the direction information from the
encoder is ignored.
4.1.2 Positive or Negative Only Acquisition
The board can be programmed to only acquired lines when the encoder moves for-
ward (increase the encoder count in a positive direction) or moves backwards
(decrease the encoder count in a negative direction). This mode is useful in situations
where a stage is moving back and forth, and lines need only be acquired if the stage
is moving in one direction only. The direction of acquisition is controlled by the
QENC_AQ_DIR register.