Ports 0-3 are both byte accessible and bit accessible. So
below are few examples to see how to use byte accessible property of ports.
#include<reg51.h>
/*
* program to get byte from P1
* and send it to P2
* after 1/2 second delay
*/
void msdelay(unsigned int x)
{
unsigned
int i,j; //loop variables
for(i
= 0; i<x; i++)
for(j
= 0;j<1275; j++);
}//end of msdelay()
void main()
{
unsigned
char mybyte;
P1
= 0xFF; //make P1 input
while(true)
{
mybyte
= P1; //get byte from P1
msdelay(500);
P2
= mybyte;
}//
end of while
}// end of main
// end of program
|
#include<reg51.h>
/*
* increment LED's connected on
* Port 2
*
*/
#define LED P2;
// we can also define P2 as shown
above
void main()
{
LED
= 0; //clear P2
while(true)
{
LED++;
}//
end of while
}// end of main
// end of program
|
Bit addressable I/O programming
As Port 0-3 are bit addressable too so here are few
example to see how we can use it.
There are few things to see. In reg51.h we can use ports
bit in format Px^y, where x is port number and y is bit number.
Also we need to use sbit data type to use bits of ports.
#include<reg51.h>
/*
* toggling bit P2.4 without
* disturbing other P2 bits
*
*/
void msdelay(unsigned int x)
{
unsigned
int i,j; //loop variables
for(i
= 0; i<x; i++)
for(j
= 0;j<1275; j++);
}//end of msdelay()
sbit mybit = P2^4;
//this is how we do it
void main()
{
while(true)
{
mybit
= 1;//turn on
msdelay(somedelay);
mybit
= 0;//turn off
}//
end of while
}// end of main
// end of program
|
#include<reg51.h>
/*
* door sensor connected to P1.1
* buzzer connected to P1.7
* when door sensor open sound buzzer
* by sending pulse of few hundered hz
*
*/
void msdelay(unsigned int x)
{
unsigned
int i,j; //loop variables
for(i
= 0; i<x; i++)
for(j
= 0;j<1275; j++);
}//end of msdelay()
sbit dsensor = P1^1;
sbit buzzer = P1^7;
// this is how we do it
void main()
{
dsensor
= 1; // make it input
while(dsensor
== 1)// check
{
buzzer
= 0;
msdelay(300);
buzzer
= 1;
msdelay(300);
}//
end of while
}// end of main
// end of program
|
Accessing SFR address 80 – FFh
Another way to access ports through bits or bytes is
through their RAM address. These RAM address can be accessed by using sfr datat
type of reg51.h. See table below for addresses.
Port 0
|
Address
|
P0.0
|
80h
|
P0.1
|
81h
|
P0.2
|
82h
|
P0.3
|
83h
|
P0.4
|
84h
|
P0.5
|
85h
|
P0.6
|
86h
|
P0.7
|
87h
|
Port 1
|
Address
|
P1.0
|
90h
|
P1.1
|
91h
|
P1.2
|
92h
|
P1.3
|
93h
|
P1.4
|
94h
|
P1.5
|
95h
|
P1.6
|
96h
|
P1.7
|
97h
|
P1.8
|
Okay there is no P1.8.
|
Port 2
|
Address
|
P2.0
|
A0h
|
P2.1
|
A1h
|
P2.2
|
A2h
|
P2.3
|
A3h
|
P2.4
|
A4h
|
P2.5
|
A5h
|
P2.6
|
A6h
|
P2.7
|
A7h
|
Port 3
|
Address
|
P3.0
|
B0h
|
P3.1
|
B1h
|
P3.2
|
B2h
|
P3.3
|
B3h
|
P3.4
|
B4h
|
P3.5
|
B5h
|
P3.6
|
B6h
|
P3.7
|
B7h
|
Few program demonstrating usage.
#include<reg51.h>
/*
* toggle ports P0,P1,P2
* continuously with 250ms delay
*
*/
sfr P0 = 0x80;
sfr P1 = 0x90;
sfr P2 = 0xA0;
// see how address are initailized
using sfr keyword
void msdelay(unsigned int x)
{
unsigned
int i,j; //loop variables
for(i
= 0; i<x; i++)
for(j
= 0;j<1275; j++);
}//end of msdelay()
void main()
{
while(true)
{
P0
= 0x55;
P1
= 0x55;
P2
= 0x55;
msdelay(250);
P0
= 0xAA;
P1
= 0xAA;
P2
= 0xAA;
msdelay(250);
}//
end of while
}// end of main
// end of program
|
#include<reg51.h>
/*
* program to toggle P2.1 on and off
* for some time
*
*/
sbit mybit = 0xA1;
one more way to decalare P2^1
void main()
{
unsigned
int z;
for(z
= 0;z< sometime; z++)
{
mybit
= 0;
mybit
= 1;
}//
end of for loop
}// end of main
// end of program
|
This is all I could think off. Rest I will explain as it
comes.
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