UM10360 All information provided in this document is subject to legal disclaimers. © NXP B.V. 2013. All rights reserved.
User manual Rev. 3 — 19 December 2013 546 of 841
NXP Semiconductors
UM10360
Chapter 26: LPC176x/5x Quadrature Encoder Interface (QEI)
26.4 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 the velocity of the encoder wheel.
26.4.1 Input signals
The QEI module supports two modes of signal operation: quadrature phase mode and
clock/direction mode. In quadrature phase mode, the encoder produces two clocks that
are 90 degrees out of phase; the edge relationship is used to determine the direction of
rotation. In clock/direction mode, the encoder produces a clock signal to indicate steps
and a direction signal to indicate the direction of rotation.).
This mode is determined by the SigMode bit of the QEI Configuration (QEICONF) register
(See Table 485
). When the SigMode bit = 1, the quadrature decoder is bypassed and the
PhA pin functions as the direction signal and PhB pin functions as the clock signal for the
counters, etc. When the SigMode bit = 0, the PhA pin and PhB pins are decoded by the
quadrature decoder. In this mode the quadrature decoder produces the direction and
clock signals for the counters, etc. In both modes the direction signal is subject to the
effects of the direction invert (DIRINV) bit.
26.4.1.1 Quadrature input signals
When edges on PhA lead edges on PhB, the position counter is incremented. When
edges on PhB lead edges on PhA, the position counter is decremented. When a rising
and falling edge pair is seen on one of the phases without any edges on the other, the
direction of rotation has changed.
[1] All other state transitions are illegal and should set the ERR bit.
Interchanging of the PhA and PhB input signals are compensated by complementing the
DIR bit. When set = 1, the direction inversion bit (DIRINV) complements the DIR bit.
Table 479. Encoder states
Phase A Phase B state
101
112
013
004
Table 480. Encoder state transitions
[1]
from state to state Direction
1 2 positive
23
34
41
4 3 negative
32
21
14
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