Pins,฀Ports฀and฀Input/Output - Programming the PIC microcontroller in c.pdf

Since฀every฀useful฀program฀must฀read฀from฀or฀write฀to฀a฀PIC’s฀input/output฀pins,฀let’s฀summarize฀how฀

MBasic฀handles฀pins฀and฀ports.฀It฀can฀be฀confusing฀because฀some฀pins฀have฀triple฀or฀quadruple฀or฀even฀more฀

duties฀and฀because฀MBasic฀provides฀several฀ways฀to฀address฀any฀given฀pin.฀And,฀the฀word฀pin฀itself฀has฀dual฀

usage,฀as฀it฀refers฀to฀the฀physical฀packaging฀(an฀8-pin฀DIP฀package฀for฀example)฀and฀to฀those฀physical฀pins฀

that฀may฀be฀used฀for฀various฀purposes.฀To฀simplify฀our฀discussion฀we฀will฀limit฀ourselves฀to฀PICs฀that฀are฀

supported฀by฀MBasic฀and฀plug฀into฀either฀the฀0818฀or฀2840฀development฀boards.

PICs฀communicate฀with฀external฀circuitry฀through฀intermediary฀“ports.”฀Ports฀are฀treated฀internally฀by฀the฀

PIC’s฀CPU,฀and฀by฀MBasic,฀as฀byte฀(8-bit)฀variables฀with฀each฀bit฀corresponding฀to฀a฀particular฀pin.฀For฀

example,฀the฀most฀signiﬁcant฀bit฀in฀PortB’s฀byte฀value฀corresponds฀to฀pin฀RB7,฀while฀the฀least฀signiﬁcant฀bit฀

corresponds฀to฀RB0.฀(In฀some฀PICs,฀not฀all฀bits฀of฀each฀port฀variable฀have฀physical฀pin฀assignments.) Letters฀from฀A…E฀identify฀ports,฀except฀in฀DIP8฀packaged฀PICs฀which฀have฀only฀one฀port,฀called฀GPIO฀

(general฀purpose฀input/output).฀Thus,฀we฀have฀GPIO,฀PortA,฀PortB…PortE฀as฀predeclared฀variables฀in฀

MBasic.฀(Port฀identiﬁers฀are฀written฀without฀a฀space,฀for฀example,฀PortA,฀not฀Port฀A.)฀Of฀course,฀not฀all฀

PICs฀physically฀support฀all฀of฀these฀ports,฀and฀in฀some฀cases฀not฀

all฀eight฀bits฀of฀a฀port฀have฀associated฀pins.฀For฀example,฀the฀

PIC16F876฀has฀PortA฀(but฀only฀bits฀0…5฀are฀mapped฀to฀pins),฀

PortB฀and฀PortC.฀MBasic’s฀conﬁguration฀ﬁles,฀fortunately,฀ensure฀

that฀only฀legitimate฀port฀variables฀are฀predeclared฀for฀the฀particular฀

PIC฀being฀programmed.

Figure฀2-7฀illustrates,฀as฀an฀example,฀Port฀B฀and฀its฀pin฀assign-ments.฀Each฀general-purpose฀I/O฀pin฀is฀identiﬁed฀with฀a฀consistent฀

naming฀convention.฀For฀example,฀RB0฀is฀PortB,฀bit฀0.฀The฀“R”฀

in฀RB฀stands฀for฀“register,”฀which฀is฀synonymous฀with฀“ﬁle”฀or฀

“variable.”฀MBasic฀also฀pre-deﬁnes฀constants฀associated฀with฀each฀ Figure฀2-7:฀PortB฀to฀pin฀assignments.

of฀these฀pins,฀so฀we฀have฀constants฀B0,฀B1…B7฀available฀to฀us.฀MBasic฀gives฀us฀a฀second฀way฀to฀reference฀

pins฀through฀a฀sequential฀numbering฀system,฀for฀example,฀A0฀=฀0,฀A1฀=฀1฀up฀through฀E7฀=฀39.฀Finally,฀to฀

provide฀backwards฀compatibility฀with฀the฀Basic฀Stamp™,฀MBasic฀includes฀the฀dedicated฀functions฀INxx,฀

OUTxx฀and฀DIRxx฀where฀xx฀is฀the฀bit,฀nibble,฀byte฀or฀word฀identiﬁer฀as฀reﬂected฀in฀Table฀2-2.

Table฀2-2:฀Port฀and฀bit฀I/O฀variables,฀constants฀and฀dedicated฀functions.

Variables Constants Dedicated฀Functions

Port฀at฀a฀

Time

Nibble฀at฀

a฀฀Time

Bit฀at฀a฀

Time ฀ ฀

฀

฀ ฀ ฀ ฀ INS/OUTS/DIRS

Port฀฀

Variable Nibble Port.Bit Pin฀

Constant Pin฀

Value Bit Nibble Byte Word

PortA

PortA.

Nib0 or PortA.

LowNib

PortA.Bit0 RA0 0 IN0/OUT0/DIR0

INA/OUTA/

DIRA

INL/OUTL/

DIRL

INS/

OUTS/

DIRS

PortA.Bit1 RA1 1 IN1/OUT1/DIR1

PortA.Bit2 RA2 2 IN2/OUT2/DIR2

PortA.Bit3 RA3 3 IN3/OUT3/DIR3

PortA.

Nib1 or PortA.

HighNib

PortA.Bit4 RA4 4 IN4/OUT4/DIR4

INB/OUTB/

DIRB

PortA.Bit5 RA5 5 IN5/OUT5/DIR5

PortA.Bit6 RA6 6 IN6/OUT6/DIR6

PortA.Bit7 RA7 7 IN7/OUT7/DIR7

PortB

PortB.Nib0 or PortB.

LowNib

PortB.Bit0 RB0 8 IN8/OUT8/DIR8

INC/OUTC/

DIRC

INHOUTH/

DIRH

PortB.Bit1 RB1 9 IN9/OUT9/DIR9

PortB.Bit2 RB2 10 IN10/OUT10/

DIR10

PortB.Bit3 RB3 11 IN11/OUT11/

DIR11

PortB.Nib1 or PortB.

HighNib

PortB.Bit4 RB4 12 IN12/OUT12/

DIR12

IND/OUTD/

DIRD

PortB.Bit5 RB5 13 IN13/OUT13/

DIR13

PortB.Bit6 RB6 14 IN14/OUT14/

DIR14

PortB.Bit7 RB7 15 IN15/OUT15/

DIR15

PortC

PortC.

Nib0 or PortC.

LowNib

PortC.Bit0 RC0 16 ฀ ฀

PortC.Bit1 RC1 17 ฀ ฀

PortC.Bit2 RC2 18 ฀

PortC.Bit3 RC3 19 ฀

PortC.

Nib1 or PortC.

HighNib

PortC.Bit4 RC4 20 INxx:฀Read฀status,฀whether฀in฀input฀or฀

output฀mode

PortC.Bit5 RC5 21 OUTxx:฀Write฀value฀ ฀

PortC.Bit6 RC6 22 DIRxx:฀Set฀direction฀

1=input,฀0=output ฀

PortC.Bit7 RC7 23 ฀

(continued)

Table฀2-2:฀Port฀and฀bit฀I/O฀variables,฀constants฀and฀dedicated฀functions.

Variables Constants Dedicated฀Functions

Port฀at฀a฀

Time

Nibble฀at฀

a฀฀Time

Bit฀at฀a฀

Time ฀ ฀

฀

฀ ฀ ฀ ฀ INS/OUTS/DIRS

Port฀฀

Variable Nibble Port.Bit Pin฀

Constant Pin฀

Value Bit Nibble Byte Word

PortD

PortD.

Nib0 or PortD.

LowNib

PortD.Bit0 RD0 24 xx฀as฀appropriate฀for฀bit,฀nibble,฀byte฀

or฀word

PortD.Bit1 RD1 25 ฀

PortD.Bit2 RD2 26 Note:฀DIRxx฀command฀is฀reversed฀

from฀Basic฀Stamp

PortD.Bit3 RD3 27 ฀

PortD.

Nib1 or PortD.

HighNib

PortD.Bit4 RD4 28 ฀

PortD.Bit5 RD5 29 ฀

PortD.Bit6 RD6 30 ฀

PortD.Bit7 RD7 31 ฀

PortE

PortE.Nib0 or PortE.

LowNib

PortE.Bit0 RE0 32 ฀ ฀

PortE.Bit1 RE1 33 ฀ ฀

PortE.Bit2 RE2 34 ฀ ฀

PortE.Bit3 RE3 35 ฀ ฀

PortE.Nib1 or PortE.

HighNib

PortE.Bit4 RE4 36 ฀ ฀

PortE.Bit5 RE5 37 ฀ ฀

PortE.Bit6 RE6 38 ฀ ฀

PortE.Bit7 RE7 39 ฀ ฀ ฀ ฀

MBasic฀permits฀us฀to฀reference฀a฀port฀or฀a฀pin฀as฀an฀address฀or฀as฀a฀variable.฀As฀an฀address,฀the฀port฀or฀pin฀

is฀an฀argument฀to฀certain฀functions.฀As฀a฀variable,฀the฀value฀of฀the฀port฀(either฀in฀reading฀or฀writing)฀can฀be฀

used฀like฀any฀other฀variable.฀There฀are฀also฀the฀dedicated฀functions฀identiﬁed฀in฀Table฀2-2฀that฀operate฀on฀

speciﬁc฀ports฀or฀pins฀without฀an฀explicit฀port฀or฀pin฀reference,฀such฀as฀IN0.฀We฀must฀remember฀that฀MBasic฀

automatically฀initializes฀all฀I/O฀pins฀as฀inputs฀and฀that฀before฀reading฀from฀or฀writing฀to฀a฀port฀or฀a฀pin฀we฀

must฀follow฀some฀simple฀rules:

First,฀set฀the฀direction฀of฀the฀port฀or฀pin฀to฀be฀either฀an฀input฀or฀output;฀

Second,฀read฀the฀port฀or฀pin฀if฀an฀input,฀or฀write฀to฀the฀port฀or฀pin฀if฀an฀output;

or,

Read฀from฀or฀write฀to฀a฀port฀or฀pin฀with฀a฀procedure฀that฀automatically฀sets฀the฀direction.

Output฀Mode

Let’s฀see฀how฀many฀different฀ways฀we฀can฀assign฀a฀pin฀to฀an฀output฀and฀to฀make฀its฀value฀0.฀We’ll฀use฀pin฀

RB0฀as฀our฀example.

;Direct฀Pin฀Addressing

;---Output฀B0฀ ฀ ;฀First฀make฀it฀an฀output.฀B0฀is฀a฀constant PortB.Bit0฀=฀0฀฀ ;฀PortB.Bit0฀is฀a฀variable

Dir8฀=฀0฀ ฀ ;฀Special฀purpose฀function,฀DIR8฀is฀for฀pin฀B0 Out8฀=฀0฀ ฀ ;฀Likewise฀for฀Out8

Low฀B0฀ ฀ ฀ ;฀Low฀function฀automatically฀makes฀it฀an฀output

฀ ฀ ฀ ;฀no฀need฀to฀separately฀make฀it฀into฀an฀output Output฀8฀ ฀ ;฀B0฀is฀an฀alias฀for฀8฀so฀can฀use฀8฀directly PortB.Bit0฀=฀0฀฀ ;฀Make฀the฀variable฀assignment

Low฀8฀ ฀ ฀ ;฀B0฀is฀an฀alias฀for฀8฀so฀can฀use฀8฀directly

฀ ฀ ฀ ;฀LOW฀switches฀to฀output฀mode฀and฀outputs฀0 TRISB.Bit0฀=฀0฀฀ ;฀TRISB฀variable฀controls฀PortB฀I/O฀direction,฀

฀ ฀ ฀ ;฀0=output฀&฀1=input.฀

PortB.Bit0฀=฀0฀฀ ;฀PortB.Bit0฀is฀a฀variable

;Byte฀at฀a฀time฀addressing฀to฀deal฀with฀multiple฀pins

;in฀one฀instruction

;---TRISB฀=฀%00000000฀ ;฀Sets฀all฀8฀pins฀to฀0,฀i.e.,฀output PortB฀=฀%00000000฀฀ ;฀Assign฀all฀8฀bits฀(pins)฀to฀0.฀

DIRH฀=฀%00000000฀฀฀ ;฀Make฀all฀8฀Pins฀in฀PortB฀output OUTH฀=฀%00000000฀฀฀ ;฀Set฀all฀8฀bits฀(pins)฀to฀0

Input฀Mode

To฀make฀RB0฀an฀input฀and฀read฀its฀value,฀we฀have฀the฀choice฀of฀a฀similar฀set฀of฀options:

;Direct฀Pin฀Addressing

;฀Assume฀we฀have฀already฀declared:

;฀ BitVar฀฀Var฀฀ Bit

;฀ ByteVar฀Var฀ Byte

;฀to฀hold฀the฀value฀being฀read

;---Input฀B0฀ ฀ ;฀First฀make฀it฀an฀input.฀B0฀is฀a฀constant BitVar฀=฀PortB.Bit0฀฀ ;฀PortB.Bit0฀is฀a฀variable

Dir8฀=฀1฀ ฀ ;฀Special฀purpose฀function,฀DIR8฀is฀for฀pin฀B0 BitVar฀=฀In8฀ ฀ ;฀Likewise฀for฀IN8

Input฀8฀฀ ฀ ;฀B0฀is฀an฀alias฀for฀8฀so฀can฀use฀8฀directly BitVar฀=฀PortB.Bit0฀฀ ;฀Make฀the฀variable฀assignment

TRISB.Bit0฀=฀1฀฀ ;฀TRISB฀variable฀controls฀PortB฀I/O฀

฀฀฀ ฀ ฀ ;฀direction,0=output฀&฀1=input.฀

BitVar฀=฀PortB.Bit0฀฀ ;฀PortB.Bit0฀is฀a฀variable

;Byte฀at฀a฀time฀addressing฀to฀deal฀with฀multiple฀pins

;in฀one฀instruction

;---TRISB฀=฀%11111111฀ ;฀Sets฀all฀8฀pins฀to฀1,฀i.e.,฀input ByteVar฀=฀PortB฀฀฀฀ ;฀Read฀all฀8฀bits฀(pins)฀into฀ByteVar.฀

DIRH฀=฀%11111111฀฀฀ ;฀Make฀all฀8฀Pins฀in฀PortB฀input ByteVar฀=฀INH฀฀฀฀฀฀ ;฀Read฀all฀8฀bits฀(pins)฀into฀ByteVar

Pin฀Variables฀vs.฀Addresses

One฀common฀error฀by฀beginners฀is฀confusing฀pin฀variables฀with฀pin฀addresses.฀The฀functions฀Output,฀Low฀

and฀Input฀require฀a฀pin฀address฀as฀their฀argument.฀The฀pin฀address฀may฀be฀one฀of฀MBasic’s฀pre-deﬁned฀

constants,฀for฀example,฀B0,฀or฀its฀equivalent฀numerical฀value,฀8.฀The฀pin฀address฀may฀also฀be฀the฀value฀of฀a฀

variable,฀such฀as:

For฀I฀=฀B0฀to฀B7฀ ;฀I฀goes฀from฀8฀(B0)฀to฀15฀(B7)

฀ Low฀I฀ ฀ ;฀Makes฀B0฀low,฀then฀B1฀through฀B7฀sequentially Next

In฀the฀Low฀I฀statement,฀Low฀operates฀on฀the฀value฀of฀I,฀which฀it฀interprets฀as฀the฀address฀of฀a฀pin.฀When฀

I฀is฀8,฀for฀example,฀Low฀operates฀on฀pin฀RB0.฀Thus,฀the฀above฀code฀fragment฀is฀identical฀with:

For฀I฀=฀8฀to฀15฀

฀ Low฀I฀

Next

Pin฀variables฀are฀used฀to฀read฀the฀value฀of฀a฀pin฀or฀of฀a฀port฀and฀to฀write฀to฀a฀pin฀or฀port฀via฀an฀assignment฀

(the฀“=”฀sign).฀Thus฀we฀have฀ByteVar฀=฀PortB,฀or฀PortB฀=฀$FF.฀We฀also฀may฀use฀PortB฀like฀any฀other฀

byte฀variable,฀such฀as฀x=2*PortB.

If฀we฀try฀to฀read฀the฀value฀of฀pin฀B0฀as฀an฀input฀with฀the฀statement฀BitVar฀=฀B0,฀the฀compiler฀will฀produce฀

no฀error,฀but฀BitVar฀will฀not฀hold฀the฀desired฀result.฀Rather,฀this฀statement฀is฀identical฀with฀BitVar฀=฀8.฀If฀

testing฀for฀a฀pin฀value฀condition฀in฀a฀loop฀statement,฀it’s฀important฀that฀the฀variable฀construct฀be฀used.

;To฀test฀Pin฀B0฀

;---If฀PortB.Bit0฀=฀1฀Then

;฀execute฀code฀goes฀here EndIf

;The฀following฀code฀will฀compile฀but฀won’t฀work

;since฀it’s฀the฀same฀as฀writing฀If฀(8฀=฀1),฀which฀is฀always฀false If฀B0฀=฀1฀Then

;execute฀code฀goes฀here EndIf

Finally฀it’s฀possible฀to฀read฀from฀a฀pin฀or฀port฀that฀is฀set฀for฀output฀in฀whole฀or฀in฀part,฀and฀to฀write฀to฀a฀pin฀

or฀port฀that฀is฀set฀for฀input฀in฀whole฀or฀in฀part.฀No฀error฀message฀will฀be฀generated.฀If฀you฀are฀experiencing฀

strange฀or฀unstable฀results฀reading฀or฀writing฀to฀pins฀or฀ports,฀check฀to฀ensure฀the฀correct฀direction฀is฀set฀and฀

that฀you฀are฀correctly฀using฀pin฀variables฀and฀pin฀constants.

PICs฀equipped฀with฀analog-to-digital฀converters฀apply฀the฀designators฀AN0,฀AN1…฀to฀pins฀that฀also฀have฀an฀

analog฀function.฀Thus,฀a฀16F876฀pin฀name฀RA0/AN0฀indicates฀that฀the฀pin฀has฀three฀possible฀uses:฀digital฀

output,฀digital฀input฀(RA0)฀and฀analog฀(AN0)฀input.฀The฀process฀of฀assigning฀a฀pin฀to฀be฀an฀analog฀input฀is฀

discussed฀in฀detail฀in฀Chapter฀11.

Run฀Time฀vs.฀Program฀Time฀Pin฀Assignments

All฀the฀pin฀assignments฀we฀have฀discussed฀to฀this฀point฀are฀run฀time฀alterable,฀i.e.,฀their฀status฀may฀be฀altered฀

by฀the฀program฀on฀the฀ﬂy.฀In฀one฀part฀of฀your฀program฀a฀pin฀may฀be฀an฀input฀and฀later฀in฀the฀program฀the฀

same฀pin฀may฀be฀an฀output.฀However,฀in฀some฀PICs—most฀often฀those฀in฀the฀8฀and฀18-pin฀packages—cer-tain฀pin฀conﬁgurations฀may฀only฀be฀established฀at฀program฀time,฀a฀task฀usually฀accomplished฀via฀an฀option฀

dialog฀box฀in฀MBasic฀before฀compiling฀your฀code.฀(This฀permits฀Microchip฀to฀make฀their฀smaller฀package฀

devices฀more฀ﬂexible,฀but฀at฀the฀cost฀of฀confusion฀to฀beginning฀programmers.)฀Then,฀depending฀upon฀the฀

program฀time฀conﬁguration,฀further฀run฀time฀changes฀may฀be฀possible.฀Program฀time฀pin฀setup฀is฀highly฀

device฀specific฀and฀reference฀to฀the฀data฀sheet฀for฀your฀specific฀PIC฀will฀be฀beneficial.

We’ll฀explore฀the฀difference฀between฀run฀time฀and฀program฀time฀alterable฀pins฀in฀the฀context฀of฀the฀12F629,฀

which฀has฀3฀pins฀that฀must฀be฀conﬁgured฀at฀program฀time:

12F629฀Example฀of฀Pins฀Conﬁgured฀at฀Program฀Time

Pin฀Name Program฀Time฀Conﬁguration Run฀Time฀Conﬁguration

GP3/MCLR/Vpp GP3฀(general-purpose฀I/O) GP3:฀Input

GP3:฀Output MCLR/Vpp฀(master฀clear฀/฀Vprogram) None

GP4/T1G/OSC2/CLKOUT

GP4฀(general-purpose฀I/O)

GP4:฀Input GP4:฀Output T1G฀(timer฀1฀gate) OSC2฀(second฀resonator฀connection) None

CLKOUT฀(clock฀out) None

Figure฀ 2-8:฀ Program฀ time฀ pin฀ options฀

for฀12F629.

12F629฀Example฀of฀Pins฀Conﬁgured฀at฀Program฀Time

Pin฀Name Program฀Time฀Conﬁguration Run฀Time฀Conﬁguration

GP5/T1CKI/OSC1/CLKIN

GP5฀(general-purpose฀I/O)

GP5:฀Input GP5:฀Output T1CKI฀(Timer฀1฀clock฀in)

CLKIN฀(external฀clock฀input) None

OSC1฀(ﬁrst฀resonator฀connection) None

If฀the฀GP3/MCLR/Vpp฀pin฀is฀deﬁned฀at฀program฀time฀to฀be฀a฀gener-al-purpose฀I/O฀pin,฀it฀may฀be฀used฀for฀input฀or฀output฀exactly฀as฀we฀

have฀earlier฀discussed,฀and฀changed฀from฀input฀to฀output฀under฀pro-gram฀control.฀However,฀if฀at฀program฀time฀it฀is฀deﬁned฀as฀MCLR,฀it฀

is฀unavailable฀for฀any฀other฀purpose.฀This฀selection฀is฀accomplished฀

with฀the฀MCLR฀check฀box฀found฀in฀MBasic’s฀Conﬁguration฀dialog฀

box,฀as฀shown฀in฀Figure฀2-8.฀

The฀two฀oscillator฀pins฀also฀must฀be฀deﬁned฀at฀program฀time,฀but฀

are฀linked.฀If฀you฀plan฀to฀use฀an฀external฀resonator฀or฀crystal,฀the฀

OSC1฀and฀OSC2฀pin฀conﬁguration฀must฀be฀active.฀If฀you฀plan฀to฀

use฀an฀external฀clock฀source,฀then฀the฀CLKIN฀option฀must฀be฀active.฀

If฀you฀wish฀to฀use฀the฀internal฀RC฀oscillator,฀then฀the฀CLKOUT฀pin฀

may฀either฀be฀GP5฀or฀OSCOUT.฀If฀an฀external฀RC฀oscillator฀is฀used,฀

the฀RC฀network฀must฀connect฀to฀the฀OSC1฀pin.฀Table฀2-3฀shows฀

how฀these฀options฀interact฀and฀how฀they฀are฀selected฀in฀the฀MBasic฀

conﬁguration฀box฀of฀Figure฀2-8.฀(The฀MBasic฀conﬁguration฀options฀

correspond฀to฀the฀ﬁrst฀column฀in฀Table฀2-3.)

Table฀2-3:฀Conﬁguration฀Dialog฀for฀12F629฀–฀Oscillator฀Conﬁguration.

Conﬁguration฀

Dialog฀Box

Oscillator฀

Conﬁguration฀ GP4/T1G/OSC2/CLKOUT฀Function GP5/T1CKI/OSC1/CLKIN฀Function

LowPower LP OSC2—crystal฀connection OSC1—crystal฀connection฀

External XT OSC2—crystal฀connection OSC1—crystal฀connection

HighSpeed HS OSC2—crystal฀connection OSC2—crystal฀connection

ExternalClk EC GP4฀(general-purpose฀I/O) CLKIN฀(external฀clock฀input) IntRC฀on฀GP4 INTOSC GP4฀(general-purpose฀I/O) GP5฀(general-purpose฀I/O) IntRC฀on฀ClkOut INTOSC CLKOUT฀(clock฀waveform฀output) GP5฀(general-purpose฀I/O)

ExtRC฀on฀GP4 RC GP4฀(general-purpose฀I/O) CLKIN-RC฀circuit฀on฀pin

ExtRC฀on฀ClkOut RC CLKOUT฀(clock฀waveform฀output) CLKIN-RC฀circuit฀on฀pin

Further฀complicating฀an฀already฀complex฀matter,฀an฀external฀clock฀source฀may฀be฀used฀in฀the฀LP,฀XT฀and฀

HS฀modes฀by฀feeding฀it฀into฀OSC1,฀in฀which฀case,฀OSC2฀is฀unused.฀The฀LP,฀XT฀and฀HS฀modes฀set฀internal฀

parameters฀in฀the฀oscillator฀section฀of฀the฀12F629฀and฀establish฀the฀maximum฀resonator฀or฀crystal฀frequency฀

and฀associated฀capacitor฀values.฀Section฀9฀of฀Microchip’s฀PIC12F629/675฀Data฀Sheet฀should฀be฀consulted฀

for฀speciﬁcs.

LVP฀Programming฀Pin฀Selection฀

One฀compile฀time฀feature฀shared฀by฀16F876/877฀chips฀(including฀the฀“A”฀versions),฀along฀with฀many฀other฀

ﬂash฀program฀memory฀PICs,฀is฀the฀low฀voltage฀programming฀(LVP)฀option.฀Historically,฀ﬂash฀memory฀

has฀required฀application฀of฀a฀programming฀voltage฀two฀or฀three฀times฀that฀of฀the฀normal฀operating฀voltage,฀

typically฀12V฀for฀a฀PIC฀operating฀with฀V_DD฀of฀5V,฀known฀as฀high฀voltage฀programming฀(HVP).฀Newer฀PICs,฀

such฀as฀the฀16F876/877/876A/877A฀may฀be฀optionally฀programmed฀in฀LVP฀mode,฀using฀only฀+5V.฀Whether฀

a฀PIC฀that฀supports฀LVP฀actually฀has฀LVP฀enabled฀is฀determined฀by฀a฀conﬁguration฀bit,฀the฀value฀of฀which฀is฀

keyed฀to฀the฀LVP฀checkbox฀in฀MBasic’s฀conﬁguration฀setup฀dialog฀box฀seen฀in฀Figure฀2-8.฀If฀the฀chip฀does฀

not฀support฀LVP,฀as฀is฀true฀in฀Figure฀2-8,฀the฀LVP฀check฀box฀is฀grayed฀out.฀

To฀ensure฀maximum฀ﬂexibility฀when฀programming฀both฀older฀and฀newer฀model฀PICs,฀Basic฀Micro’s฀ISP-PRO฀and฀its฀0818฀and฀2840฀Development฀Boards฀use฀high฀voltage฀programming฀and฀for฀the฀programs฀in฀this฀

book฀you฀should฀not฀select฀LVP฀mode฀in฀MBasic’s฀programming฀options.฀Microchip฀enables฀LVP฀as฀part฀of฀

the฀manufacturing฀process,฀so฀when฀programming฀a฀new฀PIC฀for฀the฀ﬁrst฀time฀you฀will฀ﬁnd฀it฀necessary฀to฀

clear฀the฀LVP฀selection฀box฀in฀MBasic’s฀conﬁguration฀menu,฀if฀that฀model฀PIC฀has฀LVP฀functionality.฀

Basic฀Micro’s฀ISP-PRO฀does฀not฀support฀LVP฀and฀programs฀only฀using฀HVP฀mode.฀But,฀since฀a฀PIC฀with฀

LVP฀enabled฀is฀still฀programmable฀via฀HVP,฀you฀can,฀nonetheless,฀select฀LVP฀and฀program฀16F876/877/

876A/877A฀chips฀with฀MBasic฀and฀the฀ISP-PRO.฀However,฀if฀you฀do฀so,฀pin฀RB3฀becomes฀the฀LVP฀control฀

pin฀and฀is฀no฀longer฀available฀as฀a฀general-purpose฀I/O฀pin.฀(The฀speciﬁc฀pin฀used฀for฀LVP฀control฀varies;฀for฀

example,฀a฀16F628฀uses฀RB4.)฀Not฀every฀LVP-capable฀model฀PIC฀behaves฀so฀nicely฀and฀you฀may฀ﬁnd฀some฀

model฀devices฀refuse฀to฀program฀if฀you฀inadvertently฀leave฀the฀LVP฀option฀selected.฀I’ve฀even฀seen฀different฀

samples฀of฀the฀same฀model฀PIC฀behave฀differently฀with฀the฀LVP฀option฀selected.฀In฀this฀case,฀clearing฀the฀

LVP฀check฀box,฀followed฀by฀several฀cycles฀of฀MBasic’s฀“erase”฀function฀usually฀restores฀programmability.

Weak฀Pull-Up

One฀last฀remark฀and฀we฀may฀leave฀this฀overly฀long฀discussion฀of฀pins.฀Many฀PICs฀have฀built-in฀“weak”฀pull-up฀

resistors฀for฀Port฀B,฀usable฀when฀set฀to฀be฀an฀input.฀We’ll฀deal฀with฀ﬂoating฀input฀gates฀and฀the฀need฀for฀pull-up฀resistors฀in฀Chapter฀4,฀but฀MBasic’s฀procedure฀for฀controlling฀Port฀B’s฀internal฀pull฀ups฀is฀SetPullUps฀

<mode>฀where฀mode฀is฀one฀of฀two฀pre-deﬁned฀constants,฀Pu_Off฀or฀Pu_On฀for฀de-activating฀or฀activating,฀

respectively,฀internal฀pull-up฀resistors.฀Pull-up฀resistors฀for฀all฀eight฀pins฀of฀Port฀B฀are฀activated฀or฀deactivated฀

by฀this฀command,฀not฀individual฀pins.฀For฀Port฀B฀pins฀that฀are฀set฀to฀be฀outputs,฀SetPullUps฀has฀no฀effect.

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