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keypad connected to 8051


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nismo

Thu Feb 04 2010, 09:24AM

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Joined: Thu Jul 31 2008, 09:03AM

Posts: 92
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I am now interfacing 8051 with a 4x3 keypad. may i know what is the function of this symbol "~" in the code with i have highlighted below in red?

#include <AT89X51.H> //Include file for 8051
#define keyport P2 //keypad connected to P2
#define col1 P2_0 //column 1
#define col2 P2_1 //column 2
#define col3 P2_2 //column 3
#define col4 P2_3 //column 4
#define TRUE 1 //some defines
#define FALSE 0

/*
+---------------------------------------+
| Prototype: void key_init(void); |
| Return Type: void |
| Arguments: None |
| Description: Initialize ports and |
| Keypad. |
+---------------------------------------+
*/
void key_init(){
keyport &=0x0F; //make Rows as o/p and cols are i/p
}

/*
+-----------------------------------------------+
| Prototype: unsigned char get_key(void); |
| Return Type: unsigned char |
| Arguments: None |
| Description: To read key from the keypad |
+-----------------------------------------------+
*/
unsigned char get_key(){
unsigned char i,k,key=0;
k=1;
for(i=0;i<4;i++){ //loop for 4 rows
keyport &=~(0x80>>i); //to make rows low 1 by 1
if(!col1){ //check if key1 is pressed
key = k+0; //set key number
while(!col1); //wait for release
return key; //return key number
}
if(!col2){ //check if key2 is pressed
key = k+1; //set key number
while(!col2); //wait for release
return key; //return key number
}
if(!col3){ //check if key3 is pressed
key = k+2; //set key number
while(!col3); //wait for release
return key; //return key number
}
if(!col4){ //check if key4 is pressed
key = k+3; //set key number
while(!col4); //wait for release
return key; //return key number
}
k+=4; //next row key number
keyport |= 0x80>>i; //make the row high again
}
return FALSE; //return false if no key pressed
}

Ajay Bhargav

Thu Feb 04 2010, 01:32PM

Rickey's World Admin

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Joined: Fri Feb 24 2006, 04:56AM

Posts: 7542
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~ means complement.

keyport &=~(0x80>>i);

keyport = keyport AND (complement of (0x80 rightshifted i times)

i goes from 0 to 3 for 4 rows. rows are at higher nibble of port. so this line will clear every row one by one. put value of i from anywhere between 0 to 3 keyport will be 0xFF by default so see what happens when you execute that above statement.

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nismo

nismo

Thu Feb 04 2010, 06:50PM

Registered Member #9479

Joined: Thu Jul 31 2008, 09:03AM

Posts: 92
Thanked 1 time in 1 posts

How about the one i have highlighted now. Why do we have to use "&=" instead of "="?

#include <AT89X51.H> //Include file for 8051
#define keyport P2 //keypad connected to P2
#define col1 P2_0 //column 1
#define col2 P2_1 //column 2
#define col3 P2_2 //column 3
#define col4 P2_3 //column 4
#define TRUE 1 //some defines
#define FALSE 0

/*
+---------------------------------------+
| Prototype: void key_init(void); |
| Return Type: void |
| Arguments: None |
| Description: Initialize ports and |
| Keypad. |
+---------------------------------------+
*/
void key_init(){
keyport &=0x0F; //make Rows as o/p and cols are i/p
}

/*
+-----------------------------------------------+
| Prototype: unsigned char get_key(void); |
| Return Type: unsigned char |
| Arguments: None |
| Description: To read key from the keypad |
+-----------------------------------------------+
*/
unsigned char get_key(){
unsigned char i,k,key=0;
k=1;
for(i=0;i<4;i++){ //loop for 4 rows
keyport &=~(0x80>>i); //to make rows low 1 by 1
if(!col1){ //check if key1 is pressed
key = k+0; //set key number
while(!col1); //wait for release
return key; //return key number
}
if(!col2){ //check if key2 is pressed
key = k+1; //set key number
while(!col2); //wait for release
return key; //return key number
}
if(!col3){ //check if key3 is pressed
key = k+2; //set key number
while(!col3); //wait for release
return key; //return key number
}
if(!col4){ //check if key4 is pressed
key = k+3; //set key number
while(!col4); //wait for release
return key; //return key number
}
k+=4; //next row key number
keyport |= 0x80>>i; //make the row high again
}
return FALSE; //return false if no key pressed
}

Ajay Bhargav

Sun Feb 07 2010, 10:48AM

Rickey's World Admin

Registered Member #1

Joined: Fri Feb 24 2006, 04:56AM

Posts: 7542
Thanked 1330 times in 1254 posts

keyport &=0x0F

means keyport = keyport & 0x0F
i am clearing the only high bits of port.

now the question why &= why not just =? right?

lets say that lower bits of port are used for LEDs so when you are manipulating higher bits of the port you dont want LEDs to be disturbed..
if my port condition is currently 0x4 i.e. third LED is on, then after
keyport = 0x0F all LEDs will get on which we do not want..
so if we do keyport &= 0x0F then
first 0x0F is ANDed with keyport value then stored back..
so keyport & 0x0F = 0x04 & 0x0F = 0x04
no keyport = 0x04
this only clears the high bits and leave the lower bits intact. so you get the point of using &=?

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