PIC18F2331 Datasheet, PDF (175/396 Pages) Microchip Technology – 28/40/44-Pin Enhanced Flash Microcontrollers with nanoWatt Technology, High Performance PWM and A/D

PIC18F2331/2431/4331/4431
16.2.3.3 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.
1. An index pulse is detected on the INDX input
(QEIM2:QEIM0 = 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.
2. A POSTCNT/MAXCNT period match occurs
(QEIM2:QEIM0 = 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 16-10)
If the encoder is traveling in the reverse
direction and the value of POSCNT reaches
00h, POSCNT is loaded with the contents of
MAXCNT register on the next clock edge. An
interrupt event is triggered on the next TCY after
the load operation (see Figure 16-10).
The value of the position counter is not affected during
QEI mode changes, nor when the QEI is disabled
altogether.
16.2.4 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
(QEIM2:QEIM0 = 010 or 110)
• A POSCNT rollover (FFFFh to 0000h) in Period
mode only (QEIM2:QEIM0 = 010 or 110)
• An index pulse detected on INDX.
The interrupt timing diagrams for IC2QEIF are shown in
Figure 16-10 and Figure 16-11.
When the direction has changed, the direction change
Interrupt flag (IC3DRIF) is set on the following TCY
clock (see Figure 16-10).
If the position counter rolls over in Index mode, the
ERROR bit will be set.
16.2.5 QEI SAMPLE TIMING
The quadrature input signals, QEA and QEB, may vary
in quadrature frequency. The minimum quadrature
input period TQEI is 16TCY.
The position count rate, FPOS, is directly proportional to
the rotor’s RPM, line count D and QEI Update mode (x2
vs. x4); that is,
F_POS
=
4----D_------⋅_{---R----P---M----}
60
Note:
The number of incremental lines in the
position encoder is typically set at
D = 1024 and the QEI Update mode = x4.
The maximum position count rate (i.e., 4x QEI
Update mode, D = 1024) with FCY = 10 MIPS is equal
to 2.5 MHz, which corresponds to FQEI of 625 kHz.
Figure 16-9 shows QEA and QEB quadrature inputs
timing when sampled by the noise filter.
 2003 Microchip Technology Inc.
Preliminary
DS39616B-page 173