PIC16F877AISLASHML datasheet(43/234 Pages) MICROCHIP

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 2003 Microchip Technology Inc.

DS39582B-page 41

PIC16F87XA

4.0

I/O PORTS

Some pins for these I/O ports are multiplexed with an

alternate function for the peripheral features on the

device. In general, when a peripheral is enabled, that

pin may not be used as a general purpose I/O pin.

Additional information on I/O ports may be found in the

PICmicro™ Mid-Range Reference Manual (DS33023).

4.1

PORTA and the TRISA Register

PORTA is a 6-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. All

write operations are read-modify-write operations.

Therefore, a write to a port implies that the port pins are

read, the value is modified and then written to the port

data latch.

Pin RA4 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 PORTA pins have TTL input levels and full

CMOS output drivers.

Other PORTA pins are multiplexed with analog inputs

and the analog VREF input for both the A/D converters

and the comparators. The operation of each pin is

selected by clearing/setting the appropriate control bits

in the ADCON1 and/or CMCON registers.

The TRISA register controls the direction of the port

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 4-1:

INITIALIZING PORTA

FIGURE 4-1:

BLOCK DIAGRAM OF

RA3:RA0 PINS

Note:

On a Power-on Reset, these pins are con-

figured as analog inputs and read as ‘0’.

The comparators are in the off (digital)

state.

BCF

STATUS, RP0

;

BCF

STATUS, RP1

; Bank0

CLRF

PORTA

; Initialize PORTA by

; clearing output

; data latches

BSF

STATUS, RP0

; Select Bank 1

MOVLW

0x06

; Configure all pins

MOVWF

ADCON1

; as digital inputs

MOVLW

0xCF

; Value used to

; initialize data

; direction

MOVWF

TRISA

; Set RA<3:0> as inputs

; RA<5:4> as outputs

; TRISA<7:6>are always

; read as '0'.

Data

Bus

PORTA

TRISA

Data Latch

TRIS Latch

RD PORTA

VSS

VDD

I/O pin(1)

Note 1: I/O pins have protection diodes to VDD and VSS.

Analog

Input

Mode

TTL

Input

Buffer

To A/D Converter or Comparator

TRISA

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