why switch case is required?

[ashish]

Example 9: connect key with P3.2. Using interrupt count no of key press and display it on common anode seven segment display connected to P0.

#include<reg51.h>
unsigned int c=0,x=0; //initialize key counter and key flag
void extint0(void) interrupt 0
   {
      x=1;                       // set key flag
      c++;                      // increment key count
      if(c==9)                 // if its 9
         {
           c=0;                // reset it to 0 and
           P0=0xC0;         // display 0
         } 
     } 
void main(void)
    {
           IE=0x81;         // enable ext int 0
           P0=0xC0;        // display 0
    loop:while(!x);      // check key flag
           switch(c)       // if it is set get the count
                  {
                    case 1: 
                        P0=0xF9; // display digit from 1 to 9
                        break;
                    case 2: 
                        P0=0xA4;
                        break;
                   case 3: 
                       P0=0xB0;
                       break;
                   case 4: 
                       P0=0x99;
                       break;
                 case 5: 
                      P0=0x92;
                      break;
                 case 6: 
                      P0=0x82;
                      break;
                 case 7: 
                      P0=0xF8;
                      break;
                 case 8: 
                      P0=0x80;
                      break;
                 case 9: 
                      P0=0x90;
                      break;
                } 
       x=0; // clear key flag
       goto loop; // loop continue
}

in this program, why switch case is required?

[Ashutosh Bhatt]

this programs displays key count on single 7-segment display

so when key is pressed – interrupt is generated – counter increments (c++)

the counter c takes values from 0 to 9 only

so in switch case all values from 0 to 9 are given.

the HEX value given to PORT0 coresponds to 7-segment code that will display that digit on 7-segment

[AJISH ALFRED]

Hi Ashish,

There are 9 cases and the current value of ‘c’ decides which case should get executed. Inside each case there is a code to display a digit, like P0=0xB0;. The first case is for the digit ‘1’, second case for the digit ‘2’ and so on.

The value of c gets counted up in the external interrupts ISR.

[ashish]

that is ok.

but why to print that long hex number on lcd.

it can simply print digit 1 to 9. right?

[AJISH ALFRED]

The Seven Segment has 8 or more pinouts and is connected to port0 with 8 pins. The pins should be connected to 5V or 0V in a predefined pattern to display each number.

When you write the value  P0=0xF9 

to port P0, the following values appears on the port pins, 11111001. 1 indicates 5V and 0 indicates 0 volt. Since the pins of seven segment are connected to this port pins, for a value of 0xF9 it will display “1”.

Similarly writing 0xA4 displays “2” and so on.

[ashish]

ok

but why 0xF9 will display 1

why not any other like 0xF8?

[Ashutosh Bhatt]

I think you should read basic construction of 7-segment display

this is common anode type display

that means common pin is given Vcc supply

now to lit on any segment we have to give 0

so F9 means 11111001. so only two segements will lit all others are off.

if you read construction then you will find these two segments are b and c that to gather makes ‘1’

[Author]

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