PIC18F2331 Datasheet, PDF (174/396 Pages) Microchip Technology – 28/40/44-Pin Enhanced Flash Microcontrollers with nanoWatt Technology, High Performance PWM and A/D

PIC18F2331/2431/4331/4431
16.2.2 QEI MODES
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.
TABLE 16-4: QEI MODES
QEIM2: Mode/
QEIM0 Reset
Description
000
—
QEI disabled⁽¹⁾
001 x2 update/ Two clocks per QEA pulse.
index pulse INDX resets POSCNT.
010 x2 update/ Two clocks per QEA pulse.
period POSCNT reset by the period
match match (MAXCNT).
011
—
unused
100
—
unused
101 x4 update/ Four clocks per QEA and
index QEB pulse pair.
INDX resets POSCNT.
110 x4 update/ Four clocks per QEA and
period QEB pulse pair.
match POSCNT reset by the period
match (MAXCNT).
111
—
unused
Note 1: QEI module is disabled. The position
counter and the velocity measurement
functions are fully disabled in this mode.
16.2.2.1 QEI x2 Update Mode
QEI x2 Update mode is selected by setting the QEI
Mode Select bits (QEIM2:QEIM0) 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.
The position counter can be reset by either an input on
the INDX pin (QEIM2:QEIM0 = 001), or by a
period-match, even when the POSCNT register pair
equals MAXCNT (QEIM2:QEIM0 = 010).
16.2.2.2 QEI 4x Update Mode
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.
Like QEI x2 mode, the position counter can be reset by
an input on the pin (QEIM2:QEIM0 = 101), or by the
period-match event (QEIM2:QEIM0 = 010).
16.2.3 QEI OPERATION
The Position Counter register pair (POSCNTH:
POSCNTL) acts as an integrator, whose value is pro-
portional to the position of the sensor rotor that corre-
sponds to the number of active edges detected.
POSCNT can either increment or decrement, depend-
ing 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.
16.2.3.1 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.
In QEI x2 Update mode, the position counter incre-
ments or decrements on every QEA edge based on the
phase relationship of the QEA and QEB signals.
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.
16.2.3.2 Direction of Count
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.
TABLE 16-5:
Current
Signal
Detected
QEA rising
QEA falling
QEB rising
QEB falling
DIRECTION OF ROTATION
Previous Signal
Detected
Rising
Falling
Pos.
Cntrl.⁽¹⁾
QEA QEB QEA QEB
x
INC
x
DEC
x DEC
x
INC
x
INC
x
DEC
x
INC
x
DEC
Note 1: When UP/DOWN = 1, the position
counter is incremented; when UP/DOWN
= 0, the position counter is decremented.
DS39616B-page 172
Preliminary
 2003 Microchip Technology Inc.