PIC16F84A datasheet(21/124 Pages) MICROCHIP

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PIC16F8X

DS30430C-page 21

5.0

I/O PORTS

The PIC16F8X has two ports, PORTA and PORTB.

Some port pins are multiplexed with an alternate func-

tion for other features on the device.

5.1

PORTA and TRISA Registers

PORTA is a 5-bit wide latch. RA4 is a Schmitt Trigger

input and an open drain output. All other RA port pins

have TTL input levels and full CMOS output drivers. All

pins have data direction bits (TRIS registers) which can

conﬁgure these pins as output or input.

Setting a TRISA bit (=1) will make the corresponding

PORTA pin an input, i.e., put the corresponding output

driver in a hi-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. So a write

to a port implies that the port pins are ﬁrst read, then this

value is modiﬁed and written to the port data latch.

The RA4 pin is multiplexed with the TMR0 clock input.

FIGURE 5-1:

BLOCK DIAGRAM OF PINS

RA3:RA0

EXAMPLE 5-1:

INITIALIZING PORTA

CLRF

PORTA

; Initialize PORTA by

; setting output

; data latches

BSF

STATUS, RP0

; Select Bank 1

MOVLW

0x0F

; Value used to

; initialize data

; direction

MOVWF

TRISA

; Set RA<3:0> as inputs

; RA4 as outputs

; TRISA<7:5> are always

; read as '0'.

FIGURE 5-2:

BLOCK DIAGRAM OF PIN RA4

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

Data

bus

Port

TRIS

Data Latch

TRIS Latch

RD TRIS

RD PORT

TTL

input

buffer

VSS

VDD

I/O pin

Data

bus

PORT

TRIS

RD PORT

Data Latch

TRIS Latch

RD TRIS

Schmitt

Trigger

input

buffer

VSS

RA4 pin

TMR0 clock input

Note: I/O pin has protection diodes to VSS only.

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