PIC18LF4331 datasheet(164/392 Pages) MICROCHIP

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PIC18F2331/2431/4331/4431

DS39616D-page 164

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17.2.3.3

Reset and Update Events

The position counter will continue to increment or dec-

rement until one of the following events takes place.

The type of event and the direction of rotation when it

happens determines if a register Reset or update

occurs.

An index pulse is detected on the INDX input

(QEIM<2:0> = 001).

If the encoder is traveling in the forward direc-

tion, POSCNT is reset (00h) on the next clock

edge after the index marker, INDX, has been

detected. The position counter resets on the

QEA or QEB edge once the INDX rising edge

has been detected.

If the encoder is traveling in the reverse direc-

tion, the value in the MAXCNT register is loaded

into POSCNT on the next quadrature pulse

edge (QEA or QEB) after the falling edge on

INDX has been detected.

A POSTCNT/MAXCNT period match occurs

(QEIM<2:0> = 010).

If the encoder is traveling in the forward direc-

tion, POSCNT is reset (00h) on the next clock

edge when POSCNT = MAXCNT. An interrupt

event is triggered on the next TCY after the Reset

(see Figure 17-10)

If the encoder is traveling in the reverse

direction and the value of POSCNT reaches

00h, POSCNT is loaded with the contents of the

MAXCNT register on the next clock edge. An

interrupt event is triggered on the next TCY after

the load operation (see Figure 17-10).

The value of the position counter is not affected during

QEI mode changes, nor when the QEI is disabled

altogether.

17.2.4

QEI INTERRUPTS

The position counter interrupt occurs and the interrupt

flag (IC2QEIF) is set, based on the following events:

• A POSCNT/MAXCNT period match event

(QEIM<2:0> = 010 or 110)

• A POSCNT rollover (FFFFh to 0000h) in Period

mode only (QEIM<2:0> = 010 or 110)

• An index pulse detected on INDX

The interrupt timing diagrams for IC2QEIF are shown in

Figure 17-10 and Figure 17-11.

When the direction has changed, the direction change

interrupt flag (IC3DRIF) is set on the following TCY

clock (see Figure 17-10).

If the position counter rolls over in Index mode, the

QERR bit will be set.

17.2.5

QEI SAMPLE TIMING

The quadrature input signals, QEA and QEB, may vary

in quadrature frequency. The minimum quadrature

input period, TQEI, is 16 TCY.

The position count rate, FPOS, is directly proportional to

the rotor’s RPM, line count D and QEI Update mode (x2

versus x4); that is,

EQUATION 17-1:

The maximum position count rate (i.e., x4 QEI

Update mode, D = 1024) with FCY = 10 MIPS is equal

to 2.5 MHz, which corresponds to FQEI of 625 kHz.

Figure 17-9 shows QEA and QEB quadrature input

timing when sampled by the noise filter.

Note:

The number of incremental lines in the

position encoder is typically set at D = 1024

and the QEI Update mode = x4.

FPOS = 4D • RPM

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