Microchip Technology MA300015 Datenbogen

DS70150E-page 92

© 2011 Microchip Technology Inc.

A typical incremental (a.k.a. optical) encoder has three
outputs: Phase A, Phase B, and an index pulse. These
signals are useful and often required in position and
speed control of ACIM and SR motors. 

The two channels, Phase A (QEA) and Phase B (QEB),
have a unique relationship. If Phase A leads Phase B,
then the direction (of the motor) is deemed positive or
forward. If Phase A lags Phase B, then the direction (of
the motor) is deemed negative or reverse. 

A third channel, termed index pulse, occurs once per
revolution and is used as a reference to establish an
absolute position. The index pulse coincides with
Phase A and Phase B, both low. 

The 16-bit up/down counter counts up or down on
every count pulse, which is generated by the difference
of the Phase A and Phase B input signals. The counter
acts as an integrator, whose count value is proportional
to position. The direction of the count is determined by
the UPDN signal, which is generated by the
Quadrature Encoder Interface logic.

14.2.1

POSITION COUNTER ERROR 
CHECKING

Position count error checking in the QEI is provided for
and indicated by the CNTERR bit (QEICON<15>). The
error checking only applies when the position counter
is configured for Reset on the Index Pulse modes
(QEIM<2:0> = ‘110’ or ‘100’). In these modes, the
contents of the POSCNT register are compared with
the values (0xFFFF or MAXCNT + 1, depending on
direction). If these values are detected, an error condi-
tion is generated by setting the CNTERR bit and a QEI
count error interrupt is generated. The QEI count error
interrupt can be disabled by setting the CEID bit
(DFLTCON<8>). The position counter continues to
count encoder edges after an error has been detected.
The POSCNT register continues to count up/down until
a natural rollover/underflow. No interrupt is generated
for the natural rollover/underflow event. The CNTERR
bit is a read/write bit and reset in software by the user.

14.2.2

POSITION COUNTER RESET

The Position Counter Reset Enable bit, POSRES
(QEI<2>), controls whether the position counter is reset
when the index pulse is detected. This bit is only
applicable when QEIM<2:0> = 100 or 110.

If the POSRES bit is set to ‘1’, then the position counter
is reset when the index pulse is detected. If the
POSRES bit is set to ‘0’, then the position counter is not
reset when the index pulse is detected. The position
counter will continue counting up or down, and will be
reset on the rollover or underflow condition. 

The interrupt is still generated on the detection of the
index pulse and not on the position counter overflow/
underflow.

14.2.3

COUNT DIRECTION STATUS

As mentioned in the previous section, the QEI logic
generates an UPDN signal, based upon the relation-
ship between Phase A and Phase B. In addition to the
output pin, the state of this internal UPDN signal is
supplied to a SFR bit, UPDN (QEICON<11>) as a read-
only bit. To place the state of this signal on an I/O pin,
the SFR bit, PCDOUT (QEICON<6>), must be 1.

There are two measurement modes which are
supported and are termed x2 and x4. These modes are
selected by the QEIM<2:0> mode select bits located in
SFR QEICON<10:8>. 

When control bits QEIM<2:0> = 100 or 101, the x2
Measurement mode is selected and the QEI logic only
looks at the Phase A input for the position counter
increment rate. Every rising and falling edge of the
Phase A signal causes the position counter to be incre-
mented or decremented. The Phase B signal is still
utilized for the determination of the counter direction,
just as in the x4 mode.

Within the x2 Measurement mode, there are two
variations of how the position counter is reset:

1.

Position counter reset by detection of index
pulse, QEIM<2:0> = 100.

2.

Position counter reset by match with MAXCNT,
QEIM<2:0> = 101.

When control bits QEIM<2:0> = 110 or 111, the x4
Measurement mode is selected and the QEI logic looks
at both edges of the Phase A and Phase B input sig-
nals. Every edge of both signals causes the position
counter to increment or decrement.

Within the x4 Measurement mode, there are two
variations of how the position counter is reset:

1.

Position counter reset by detection of index
pulse, QEIM<2:0> = 110.

2.

Position counter reset by match with MAXCNT,
QEIM<2:0> = 111.

The x4 Measurement mode provides for finer resolu-
tion data (more position counts) for determining motor
position.