DSPIC30F3010-30I Datasheet, PDF (90/226 Pages) Microchip Technology – dsPIC30F3010/3011 Data Sheet

dsPIC30F3010/3011
14.1 Quadrature Encoder Interface
Logic
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.
14.2 16-Bit Up/Down Position Counter
Mode
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.
When selecting the INDX signal to reset the Position
Counter (POSCNT), the user has to specify the states
on QEA and QEB input pins. These states have to be
matched in order for a Reset to occur. These states are
selected by the IMV<1:0> bits in the DFLTCON
register.
The IMV<1:0> (Index Match Value) bits allow the user
to specify the state of the QEA and QEB input pins
during an index pulse when the POSCNT register is to
be reset.
In x4 Quadrature Count mode:
IMV1 = Required state of Phase B input signal for
match on index pulse
IMV0 = Required state of Phase A input signal for
match on index pulse
In x2 Quadrature Count mode:
IMV1 = Selects phase input signal for index state
match (0 = Phase A, 1 = Phase B)
IMV0 = Required state of the selected phase input
signal for match on index pulse
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 relationship
between Phase A and Phase B. In addition to the out-
put pin, the state of this internal UPDN signal is
supplied to a SFR bit, UPDN (QEICON<11>), as a
read-only bit.
Note:
QEI pins are multiplexed with analog inputs.
The user must insure that all QEI associ-
ated pins are set as digital inputs in the
ADPCFG register.
DS70141E-page 88
© 2008 Microchip Technology Inc.