PIC18F2331/2431/4331/4431
17.2.2
QEI MODES
17.2.3
QEI OPERATION
Position measurement resolution depends on how
often the Position Counter register, POSCNT, is
incremented. There are two QEI Update modes to
measure the rotor’s position: QEI x2 and QEI x4.
The Position Counter register pair (POSCNTH:
POSCNTL) acts as an integrator, whose value is propor-
tional to the position of the sensor rotor that corresponds
to the number of active edges detected. POSCNT can
either increment or decrement, depending on a number
of selectable factors which are decoded by the QEI logic
block. These include the Count mode selected, the
phase relationship of QEA to QEB (“lead/lag”), the
direction of rotation and if a Reset event occurs. The
logic is detailed in the sections that follow.
TABLE 17-4: QEI MODES
Mode/
QEIM<2:0>
Description
Reset
000
001
—
QEI disabled.(1)
x2 update/ Two clocks per QEA
index pulse pulse. INDX resets
POSCNT.
17.2.3.1
Edge and Phase Detect
In the first step, the active edges of QEA and QEB are
detected, and the phase relationship between them is
determined. The position counter is changed based on
the selected QEI mode.
010
x2 update/ Two clocks per QEA pulse.
period
match
POSCNT is reset by the
period match (MAXCNT).
In QEI x2 Update mode, the position counter
increments or decrements on every QEA edge based
on the phase relationship of the QEA and QEB signals.
011
100
101
—
—
Unused.
Unused.
x4 update/ Four clocks per QEA and
index pulse QEB pulse pair.
INDX resets POSCNT.
In QEI x4 Update mode, the position counter
increments or decrements on every QEA and QEB
edge based on the phase relationship of the QEA and
QEB signals. For example, if QEA leads QEB, the
position counter is incremented by ‘1’. If QEB lags
QEA, the position counter is decremented by ‘1’.
110
x4 update/ Four clocks per QEA and
period
match
QEB pulse pair.
POSCNT is reset by the
period match (MAXCNT).
17.2.3.2
Direction of Count
111
—
Unused.
The QEI control logic generates a signal that sets the
UP/DOWN bit (QEICON<5>); this, in turn, determines
the direction of the count. When QEA leads QEB,
UP/DOWN is set (= 1) and the position counter
increments on every active edge. When QEA lags
QEB, UP/DOWN is cleared and the position counter
decrements on every active edge.
Note 1: QEI module is disabled. The position
counter and the velocity measurement
functions are fully disabled in this mode.
17.2.2.1
QEI x2 Update Mode
QEI x2 Update mode is selected by setting the QEI
Mode Select bits (QEIM<2:0>) to ‘001’ or ‘010’. In this
mode, the QEI logic detects every edge on the QEA
input only. Every rising and falling edge on the QEA
signal clocks the position counter.
TABLE 17-5: DIRECTION OF ROTATION
Previous Signal
Detected
Current
Signal
Detected
The position counter can be reset by either an input on
the INDX pin (QEIM<2:0> = 001), or by a period match,
even when the POSCNT register pair equals MAXCNT
(QEIM<2:0> = 010).
Pos.
Rising Falling
Cntrl.(1)
QEA QEB QEA QEB
QEA Rising
QEA Falling
QEB Rising
QEB Falling
x
INC
DEC
DEC
INC
17.2.2.2
QEI x4 Update Mode
x
QEI x4 Update mode provides for a finer resolution of
the rotor position, since the counter increments or
decrements more frequently for each QEA/QEB input
pulse pair than in QEI x2 mode. This mode is selected
by setting the QEI mode select bits to ‘101’ or ‘110’. In
QEI x4, the phase measurement is made on the rising
and the falling edges of both QEA and QEB inputs. The
position counter is clocked on every QEA and QEB
edge.
x
x
x
x
x
x
INC
DEC
INC
DEC
Note 1: When UP/DOWN = 1, the position counter
is incremented. When UP/DOWN = 0, the
position counter is decremented.
Like QEI x2 mode, the position counter can be reset by
an input on the pin (QEIM<2:0> = 101), or by the period
match event (QEIM<2:0> = 010).
2010 Microchip Technology Inc.
DS39616D-page 163
MICROCHIP [ MICROCHIP ]
