PIC Tutorial 2:

Switch debounce : What is it ?

When a switch is pressed it does not fully close the first time you press it because the metal contacts bounce off each other!  

Feeding the signal into a logic gate or a micro controller sends multiple key press signals which is not what you want so you have to ignore the bouncing signal - this is known as debouncing the switch.

The basic switch debounce microcontroller solution is: 

• Detect the 1st button press (or the 1st of many bounces!).
• Wait for a while.
• Check the switch again to see if it is still presed.

Note: Because each switch is constructed differently the characteristics of the key bounce will be different so you need to characterize different switches and assess how long the bounce lasts 400us, 800us or longer? (or just use a long delay).

The switch debounce solution 

The solution in all cases is time - you have to wait until the bouncing has stopped.  You either use a debouncing circuit or use software to wait for a period after the bouncing has stopped.  

The most common discrete switch debouncing circuit is a resistor and capacitor pair which slows the input signal feeding into a logic gate (charging the capacitor when the switch is closed) - in this case the gate must have hysteresis so that it reacts correctly otherwise it could oscillate anyway.  

When using a microcontroller the best way is to use software to debounce the switch as the microcontroller can easily wait a set time period before deciding that the switch value is valid.  You can also change the time period and do not need extra components.

Solderless breadboard

Add the switch and pull up resistor and then program the chip using the the hex file. 

Circuit diagram

Again the switch debounce circuit is easier to see on the schematic.

Software

The program simply turns on the LED when the button is pressed but the button is debounced. 

Note: Initially at power up the led is flashed on and off 6 times to show the microcontroller is active.

Input button (switch) port direction

Note how init_ports() sets up the port direction for bit five of the port as an input (setting bit 5 high) using the TRISIO register all the others remain as outputs.

Switch debounce

The code that does the debouncing is shown below get_key(). It returns zero for failure and 1 if the key was ok.

First of all it simply tests the key state to make sure that the key is pressed (low input in this circuit) and if not it returns 0.

Then it waits for 1 millisecond to let the bouncing stop to (this time delay can be changed depending on the characteristics of the switch i.e. to get the correct switch debounce action).

Then the key is checked again - if the key state has changed then it returns 0.  If it has not changed then the key has been held in a low state and is in the same state as first test - it is a valid key press so the routine returns 1.  

The following code is part of switch debounce source code:

////////////////////////////////////////////////////////////////////// void init_ports(void) {    TRISIO = (1<<5); // set as output except bit 5 GP5=i/p } ////////////////////////////////////////////////////////////////////// int get_key(void) {    // Is GP5 low - no so exit    if (GPIO & (1<<5)) return 0; // no key yet    delay_ms(1); // wait for key to settle    // Is GP5 high ? yes so exit = false key.    if ( (GPIO & (1<<5))>0 ) return 0; // was a false key so restart    return 1; // key ok so return valid }

Basically for the switch debounce procedure you test a key, wait a while and re-test - if the result is the same then it was a valid key.