16F648 datasheet(38/180 Pages) MICROCHIP

38 / 180 page

PIC16F627A/628A/648A

DS40044G-page 38

5.2

PORTB and TRISB Registers

PORTB is an 8-bit wide bidirectional port. The

corresponding data direction register is TRISB. A ‘1’ in

the TRISB register puts the corresponding output driver

in a High-impedance mode. A ‘0’ in the TRISB register

puts the contents of the output latch on the selected

pin(s).

PORTB is multiplexed with the external interrupt,

USART, CCP module and the TMR1 clock input/output.

The standard port functions and the alternate port

functions are shown in Table 5-3. Alternate port

functions may override the TRIS setting when enabled.

Reading PORTB 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.

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

(

≈200 μA typical). A single control bit can turn on all the

pull-ups. This is done by clearing the RBPU

(OPTION<7>) bit. The weak pull-up is automatically

turned off when the port pin is configured as an output.

The pull-ups are disabled on Power-on Reset.

Four of PORTB’s pins, RB<7:4>, have an interrupt-on-

change feature. Only pins configured as inputs can

cause this interrupt to occur (i.e., any RB<7:4> pin

configured as an output is excluded from the interrupt-

on-change comparison). The input pins (of RB<7:4>)

are compared with the old value latched on the last

read of PORTB. The “mismatch” outputs of RB<7:4>

are OR’ed together to generate the RBIF interrupt (flag

latched in 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.

This interrupt on mismatch feature, together with

software configurable pull-ups on these four pins allow

easy interface to a key pad and make it possible for

wake-up on key-depression (See Application Note

AN552 “Implementing Wake-up on Key Strokes”

(DS00552).

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.

FIGURE 5-8:

BLOCK DIAGRAM OF

RB0/INT PIN

Note:

If a change on the I/O pin should occur

when a read operation is being executed

(start of the Q2 cycle), then the RBIF

interrupt flag may not get set.

Data Bus

WR PORTB

WR TRISB

RD PORTB

Data Latch

TRIS Latch

RB0/INT

INT

RD TRISB

RBPU

VDD

VSS

Weak Pull-up

Schmitt

TTL

Input

Buffer

Trigger

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