PIC18F8585T-I/PT【PDF 32/424 页】IC PIC MCU FLASH 24KX16 80TQFP

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2004 Microchip Technology Inc.

DS30491C-page 125

PIC18F6585/8585/6680/8680

10.0

I/O PORTS

Depending on the device selected, there are either seven

or nine I/O ports available on PIC18F6X8X/8X8X

devices. Some of their pins are multiplexed with one or

more alternate functions from the other peripheral fea-

tures on the device. In general, when a peripheral is

enabled, that pin may not be used as a general purpose

I/O pin.

Each port has three registers for its operation. These

registers are:

TRIS register (data direction register)

PORT register (reads the levels on the pins of the

device)

LAT register (output latch)

The Data Latch register (LAT) is useful for read-modify-

write operations on the value that the I/O pins are

driving.

A simplified version of a generic I/O port and its

operation is shown in Figure 10-1.

FIGURE 10-1:

SIMPLIFIED BLOCK

DIAGRAM OF

PORT/LAT/TRIS

OPERATION

10.1

PORTA, TRISA and LATA

Registers

PORTA is a 7-bit wide, bidirectional port. The corre-

sponding data direction register is TRISA. Setting a

TRISA bit (= 1) will make the corresponding PORTA pin

an input (i.e., put the corresponding output driver in a

high-impedance mode). Clearing a TRISA bit (= 0) will

make the corresponding PORTA pin an output (i.e., put

the contents of the output latch on the selected pin).

Reading the PORTA register reads the status of the

pins, whereas writing to it will write to the port latch.

The Data Latch register (LATA) is also memory

mapped. Read-modify-write operations on the LATA

PORTA.

The RA4 pin is multiplexed with the Timer0 module

clock input to become the RA4/T0CKI pin. The

RA4/T0CKI pin is a Schmitt Trigger input and an open-

drain output. All other RA port pins have TTL input

levels and full CMOS output drivers.

The RA6 pin is only enabled as a general I/O pin in

ECIO and RCIO Oscillator modes.

The other PORTA pins are multiplexed with analog

inputs and the analog VREF+ and VREF- inputs. The

operation of each pin is selected by clearing/setting the

control bits in the ADCON1 register (A/D Control

The TRISA register controls the direction of the RA pins

even when they are being used as analog inputs. The

user must ensure the bits in the TRISA register are

maintained set when using them as analog inputs.

EXAMPLE 10-1:

INITIALIZING PORTA

WR LAT +

Data Latch

I/O pin

RD Port

WR Port

TRIS

RD LAT

Data Bus

Note:

On a Power-on Reset, RA5 and RA3:RA0

are configured as analog inputs and read

as ‘0’. RA6 and RA4 are configured as

digital inputs.

CLRF

PORTA

; Initialize PORTA by

; clearing output

; data latches

CLRF

LATA

; Alternate method

; to clear output

; data latches

MOVLW

0Fh

; Configure A/D

MOVWF

ADCON1

; for digital inputs

MOVLW

0CFh

; Value used to

; initialize data

; direction

MOVWF

TRISA

; Set RA<3:0> as inputs

; RA<5:4> as outputs

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