Concept of Debouncing:
In 8051 PORT 1, PORT 2 & PORT 3 have internal 10k Pull-up resistors whereas this Pull-up resistor is absent in PORT 0. Hence PORT 1, 2 & 3 can be directly used to interface a switch whereas we have to use an external 10k pull-up resistor for PORT 0 to be used for switch interfacing or for any other input.Figure 1 shows switch interfacing for PORT 1,2, and 3 of microcontroller.
Figure 1: microcontroller port 1,2, or 3 interface with switches
For any pin to be used as an INPUT PIN a HIGH (1) should be written to the pin if you don’t do this the pin will always be read as LOW.In the above figure when the switch is not pressed the 10k resistor provides the current needed for LOGIC 1 closure of switch provides LOGIC 0 to the controller PIN.Let's write a small program where whenever a switch is pressed a LED is turned ON. Consider that switch is connected to P2.0 & an LED connected to P2.1.
First we initialize the Port Pins that we are using. Since we are using P2.0 as an Input Pin we write logic 1 to it . We are using PIN P2.1 for LED. Initially LED is turned OFF this is done by pulling the PIN HIGH.
Now we check if the Switch is pressed or not. So we wait till switch is pressed as soon as switch is pressed the LED is TURNED ON. Then we wait till the switch to be released & then we TURN OFF the LED).
You must have noticed that the status of the input pin is same as that of Output Pin i.e. if the switch is not PRESSED (HIGH) the Output pin is also HIGH (LED OFF) & when the switch is PRESSED (LOW) the Output pin is also LOW (LED ON) using this logic we can do in another way.
Basic concept of switch interfacing (debouncing concept)
There is a problem in practically interfacing these switches to the controller. In the above case you have considered the switches to be an IDEAL SWITCH where when the switch is pressed the controller directly gets LOGIC 0.
But practically when a switch is closed the contacts open & close rapidly for about 30ms. This is called as SWITCH BOUNCING. Figure 2 shows its waveform.
As you can see the switch release is clean without any bouncing. When a switch is pressed the contacts open & close for about 20ms. In the above procedure the LED will flicker initially when the switch is pressed because of the SWITCH BOUNCING but since the flickering will be very fast & will not be detected by human eye.Even though 20ms is very short time in human terms for a microcontroller it is a very long time. Without SWITCH DEBOUNCING the controller will think that the switch was pressed many times.
Code:
Here, are the code of switches interfacing 8051 to help your concept related to this topic.#include"REG52.h"
sbit sw=P3^7;
void main()
{
int i,count=0;
while(1)
{
if(sw==0) // Switch is pressed
{
for(i=0;i<=1500;i++); //wait for switch to come back
if(sw==0) // Again check switch is pressed or not
{ P2=count;
while(sw==0);
count++;
}
}
}
}
Explanation of Code:
- I have commented the area which specifies the concept of Debouncing in embedded C. Check switch is pressed or not by using if(sw==0). Now, you release the switch it will bounce back and wait for sometime until it reaches the required state.
Now, you have to again check the switch is pressed or not. And when switch is pressed at once, it will start the counter. Keep on counting until another time it is pressed by making while(sw==0) loop.
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