16-key Keypad Decoding with an AVR MCU

This instructable will show you how to interface a 16-key keypad to your AVR microcontroller and read the key when a key is pressed. I'll introduce the keypad first, then the 74HC922 16-key decoder IC as a pin-saving mechanism, then finally how to take the data and massage it so that you get the correct number for the keypress.

A 16-key keypad can be a useful addition to any embedded project, possibly acting as a code input device for opening a door or as a general input to any project requiring the input of multiple values, like alarm circuits, games, puzzles, DTMF generators.

Step 1 Equipment List

This instructable doesn't have (m)any expensive components if you already have an AVR setup. You can buy everything online, although I've found if you check your local electronic warehouse/shop (if you have one) you will be able to pick up everything considerably cheaper, though.

You'll need the following:

• AVR microcontroller and a programmer. Arduino, Bare Bones Kit, Freeduino, Boarduino, and all the other clones work just fine. Of course, your custom own ghetto setup will suffice too. Just for the record, the firmware was written for an ATmega328p, so it should run well on that class of AVR's and probably many others with little to no modification.
• Something to compile your code.
• A 16-key keypad. The leading outlets carry them but they can tend to be spendy, so maybe try Jameco.com or do a search for "16-key keypad" on google. It shouldn't run you more than $6-7
• Solderless breadboard
• hookupwire, soldering iron, wire cutters, etc
• 4 10k resistors

Optional

These parts aren't necessary for you to figure out how it works, but this instructable shows several different ways to connect this keypad and read data from it, so depending on the parts you want to do, you may or may not need the following.

• 74C922 IC. This is a 16-key encoder. You can pick them up from mouser.com or digikey.com for around $5. I got a handful at my local electronics shop for $0.95 each.
• 2 x 0.1 uF tantalum capacitors. You can probably get away with ceramic if you have them already.
• DLO7135 Dot matrix LED. This is one SWEEET component. They sell for over $10-15 each online, but, again, I picked up 10 of these at my local electronics shop for $1.50 each.
• 8 pin right-angle header and matching female header for the keypad

I think that's about it. Let's get building!

Comments

[huubje]

Hello, Nice project and code I think : was just looking for something like this for making a code lock. My question: for what type of AVR have you written the C-code ? Can I use it without changes for my ATMega88 ?

[nevdull]

Hi,
Thanks. I've written it for an ATmega328P. You will need to edit the Makefile and change MCU to atmega88 and F_CPU to whatever speed at which you have your '88 running. You may need to change the AVRDUDE settings in the Makefile to reflect your programmer and port. Further, note which pins I have things on and either match those in your setup or change them in the code to match yours.

Also, if you're going to read the key value, you will need to make some sort of translation table or compare how the values are sent when checking a keycode. That's because the actual value of the key (ie 1 or 3 or B) doesn't get sent, but instead a number (hex) starting from 0 at key 1 and ending at F at key D. Make sense? The keypad values vs the value sent from a keypress is like this:

[ 1   2   3   A ]      [ 0   1   2   3 ][ 4   5   6   B ]      [ 4   5   6   7 ][ 7   8   9   C ]      [ 8   9   A   B ][ *   0   #   D ]      [ C   D   E   F ]

So you'll need to make sure you're translating/comparing the right maps.
Good luck!

[arturmariojr]

Please, do you have some of these IC 74C922 yet?
Could sell some to me?

[nevdull]

Sorry, I wish I could. I found just a couple at a local electronics shop. They've gotten pretty scarce recently. I don't know of a replacement part for it.

Good luck!

[mbroesby-olsen]

If anyone else is interested in using the MM74C922 IC, I have to warn you:

The diagram shown above confuses more than it helps, as the IC is setup differently - at least the datasheet tells me this: Column X1 + X2 is NOT on the left side. Or in general: the layout of the IC is not comparable to the layout above.

Just a friendly reminder to others :)

[mbroesby-olsen]

Hey Nevdull
- cool name by the way! :)

I am starting out a project tomorrow, using the 74C922 IC.
Would it be possible to use this IC on a 12 keypad-setup?
And is it difficult to translate your code to arduino-readable code?

I really could like to use this IC, as I need as many I/O ports as possible.