PIC16F74-ESLASHML datasheet(35/174 Pages) MICROCHIP

35 / 174 page

 2002 Microchip Technology Inc.

DS30325B-page 33

PIC16F7X

4.2

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).

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 4-3:

BLOCK DIAGRAM OF

RB3:RB0 PINS

Four of the PORTB pins (RB7:RB4) have an inter-

rupt-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 inter-

rupt-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 ORed 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

Stroke” (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 12.11.1.

FIGURE 4-4:

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.

To enable weak pull-ups, set the appropriate TRIS

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

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 in Serial Programming mode

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>).

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