PIC16F87X datasheet(33/218 Pages) MICROCHIP

33 / 218 page

 2001 Microchip Technology Inc.

DS30292C-page 31

PIC16F87X

3.2

PORTB and the TRISB Register

PORTB is an 8-bit wide, bi-directional port. The corre-

sponding data direction register is TRISB. Setting a

TRISB bit (= 1) will make the corresponding PORTB pin

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

Hi-Impedance mode). Clearing a TRISB bit (= 0) will

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

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

Three pins of PORTB are multiplexed with the Low

Voltage Programming function: RB3/PGM, RB6/PGC

and RB7/PGD. The alternate functions of these pins

are described in the Special Features Section.

Each of the PORTB pins has a weak internal pull-up. A

single control bit can turn on all the pull-ups. This is per-

formed by clearing bit RBPU (OPTION_REG<7>). The

weak pull-up is automatically turned off when the port

pin is configured as an output. The pull-ups are dis-

abled on a Power-on Reset.

FIGURE 3-3:

BLOCK DIAGRAM OF

RB3:RB0 PINS

Four of the PORTB pins, RB7:RB4, have an interrupt-

on-change feature. Only pins configured as inputs can

cause this interrupt to occur (i.e., any RB7:RB4 pin

configured as an output is excluded from the interrupt-

on-change comparison). The input pins (of RB7:RB4)

are compared with the old value latched on the last

read of PORTB. The “mismatch” outputs of RB7:RB4

are OR’ed together to generate the RB Port Change

Interrupt with flag bit RBIF (INTCON<0>).

This interrupt can wake the device from SLEEP. The

user, in the Interrupt Service Routine, can clear the

interrupt in the following manner:

Any read or write of PORTB. This will end the

mismatch condition.

Clear flag bit RBIF.

A mismatch condition will continue to set flag bit RBIF.

Reading PORTB will end the mismatch condition and

allow flag bit RBIF to be cleared.

The interrupt-on-change feature is recommended for

wake-up on key depression operation and operations

where PORTB is only used for the interrupt-on-change

feature. Polling of PORTB is not recommended while

using the interrupt-on-change feature.

This interrupt-on-mismatch feature, together with soft-

ware configureable pull-ups on these four pins, allow

easy interface to a keypad and make it possible for

wake-up on key depression. Refer to the Embedded

Control Handbook, “Implementing Wake-up on Key

Strokes” (AN552).

RB0/INT is an external interrupt input pin and is config-

ured using the INTEDG bit (OPTION_REG<6>).

RB0/INT is discussed in detail in Section 12.10.1.

FIGURE 3-4:

BLOCK DIAGRAM OF

RB7:RB4 PINS

Data Latch

RBPU(2)

VDD

Data Bus

WR Port

WR TRIS

RD TRIS

RD Port

Weak

Pull-up

RD Port

RB0/INT

I/O

pin(1)

TTL

Input

Buffer

Schmitt Trigger

Buffer

TRIS Latch

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

2: To enable weak pull-ups, set the appropriate TRIS

bit(s) and clear the RBPU bit (OPTION_REG<7>).

RB3/PGM

Data Latch

From other

RBPU(2)

VDD

I/O

Data Bus

WR Port

WR TRIS

Set RBIF

TRIS Latch

RD TRIS

RD Port

RB7:RB4 pins

Weak

Pull-up

RD Port

Latch

TTL

Input

Buffer

pin(1)

Buffer

RB7:RB6

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

2: To enable weak pull-ups, set the appropriate TRIS

bit(s) and clear the RBPU bit (OPTION_REG<7>).

In Serial Programming Mode

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