PIC16LC71 datasheet(27/176 Pages) MICROCHIP

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DS30272A-page 27

PIC16C71X

5.2

PORTB and TRISB Registers

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

sponding data direction register is TRISB. Setting a bit

in the TRISB register puts the corresponding output

driver in a hi-impedance input mode. Clearing a bit in

the TRISB register puts the contents of the output latch

on the selected pin(s).

EXAMPLE 5-2:

INITIALIZING PORTB

BCF

STATUS, RP0

;

CLRF

PORTB

; Initialize PORTB by

; clearing output

; data latches

BSF

STATUS, RP0

; Select Bank 1

MOVLW

0xCF

; Value used to

; initialize data

; direction

MOVWF

TRISB

; Set RB<3:0> as inputs

; RB<5:4> as outputs

; RB<7:6> as inputs

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

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

performed by clearing bit RBPU (OPTION<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 5-3:

BLOCK DIAGRAM OF

RB3:RB0 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

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

2: TRISB = ’1’ enables weak pull-up if

RBPU = ’0’ (OPTION<7>).

Schmitt Trigger

Buffer

TRIS Latch

Four of PORTB’s 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 con-

ﬁgured 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 Inter-

rupt with ﬂag bit RBIF (INTCON<0>).

This interrupt can wake the device from SLEEP. The

user, in the interrupt service routine, can clear the inter-

rupt 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 soft-

ware conﬁgurable 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).

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.

Note:

For the PIC16C71

if a change on the I/O pin should occur

when the read operation is being executed

(start of the Q2 cycle), then interrupt ﬂag bit

RBIF may not get set.

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