PIC18LF4331-ESLASHSP datasheet(163/392 Pages) MICROCHIP

163 / 392 page

 2010 Microchip Technology Inc.

DS39616D-page 163

PIC18F2331/2431/4331/4431

17.2.2

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 17-4:

QEI MODES

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.

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).

17.2.2.2

QEI x4 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 (QEIM<2:0> = 101), or by the period

match event (QEIM<2:0> = 010).

17.2.3

QEI OPERATION

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.

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.

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.

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’.

17.2.3.2

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 17-5:

DIRECTION OF ROTATION

QEIM<2:0>

Mode/

Reset

Description

000

—

QEI disabled.(1)

001

x2 update/

index pulse

Two clocks per QEA

pulse. INDX resets

POSCNT.

010

x2 update/

period

match

Two clocks per QEA pulse.

POSCNT is reset by the

period match (MAXCNT).

011

—

Unused.

100

—

Unused.

101

x4 update/

index pulse

Four clocks per QEA and

QEB pulse pair.

INDX resets POSCNT.

110

x4 update/

period

match

Four clocks per QEA and

QEB pulse pair.

POSCNT is 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.

Current

Signal

Detected

Previous Signal

Detected

Pos.

Cntrl.(1)

Rising

Falling

QEA QEB

QEA

QEB

QEA Rising

INC

xDEC

QEA Falling

DEC

xINC

QEB Rising

INC

xDEC

QEB Falling

INC

xDEC

Note 1:

When UP/DOWN = 1, the position counter

is incremented. When UP/DOWN = 0, the

position counter is decremented.

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