Keypad 3 Wire

If you don't know exactly how a keypad works I suggest you look at the Keypad article first

The standard way of using a 3 x 4 keypad is to use 7 pic pins. This method uses 3 pic pins, saving 4 pic pins. This enables one to use smaller pics while still have enough pins left for other I/O's. The trade-off in this case is the pic must have 3 A/D inputs.



How it works is really simple. The 5 x 1k resistors between 5V and GND forms a volatge devider that gives about 1V, 2V, 3V and 4V voltage drop over the resistors measured from ground. If a 10-bit A/D is used, you have a count of 1024 for the full 5V.

This means that about 205 counts per devider resistor is going to be counted. I say about, because it is easy to compensate for variations of counts and component tolerances in the software as you will see.

Looking at the circuit you can see that if no key is pressed, 0V will go to the A/D input.

Row 1, 2 and 3 gets pulled to GND by the 100k resistors.
If the * key is pressed, row 1 will output about 1V (or about 205 counts in AN1)
The 7 key will give about 2V (or about 410 counts in AN1)
The4 key will give about 3V (or about 615 counts in AN1)
The1 key will give about 4V (or about 820 counts in AN1)
The same will values will be read by AN2 if you press keys 0, 8, 5 and 2
and again the same values will be read by AN0 if you press keys #, 9, 6 and 3
So by reading the values with the different A/D's, you know which key is pressed.

Here's a neat trick. The three A/D inputs can be used as 5V logic inputs as well, but you will have to compensate in the software for that. The code below only provides for reading the keys voltages, you will have to add -

to the bottom of KeyIn1, KeyIn2 and KeyIn3 (i.e. before the 'GoTo KeyIn4')

The code compiles to about 407 program words. It is advisable that you make sure the rest of the code will also fit. I use this in quite a few 16F684's that has 2k of program memory. The code above will return with the following values in Disp1 -

Key 1 Returns a 1
Key 2 Returns a 2
Key 3 Returns a 3
Key 4 Returns a 4
Key 5 Returns a 5
Key 6 Returns a 6
Key 7 Returns a 7
Key 8 Returns a 8
Key 9 Returns a 9
Key 0 Returns a 0
Key * Returns a 10
Key # Returns a 11
Logic on ADIn1 (if implemented) Returns a 12
Logic on ADIn2 (if implemented) Returns a 13
Logic on ADIn0 (if implemented) Returns a 14
No key pressed Returns a 15
All you need to do when the code returns with a value in Disp1 is decide what to do with it.

Note – the same can be done with a 16 key keypad using 4 A/D pic pins. Not bad saving 4 pic pins in using either a 12 key or 16 key keypad.

In low power apps the 5V supply for the keypad resistors can be supplied by a pic pin. The keypad will consume no power when that pin is made low or 0

Should work as well with 3V3 pics, keypad also gets supplied with 3v3 then.

contributed by Fanie.