Matrix Keypads

You can figure this out with an ohmmeter and a bit of patience. The buttons close little switches. Each row is connected to one pin, and each column to another. Often, the bottom buttons have their own pins for some reason.

Thus, plug it into a solderless breadboard, and start trying to find sets of pins that are 0 resistance when a button is pushed. When you figure one out, write it down, move only one wire, and find the button that makes it low resistance, etc.

Once you have all the buttons figured out, you can set one of the rows and scan the columns one at a time to figure out if a button is pressed. You can also use the A/D, like this article:

Chinmay Pendharkar wrote:

Hi, Chinmay. I would guess you've got a ribbon cable coming from your matrix keypad, terminating with an IDC connector. If that's the case, you've got 10 lines simply because that's the smallest practical IDC connector. It's not weird -- three of the lines are just there for form.

Take the advice of Mr. Monsen and tease out the pinout using an ohmmeter. It's not the elegant solution, but there are many times when "just find out" is the fastest way to get where you want to go. And no, there isn't a standard matrix keypad pinout -- they vary all over the place.

Once you get to that point, you've asked a classic newbie question -- "How can I interface a matrix keypad with a microcontroller?" The answer depends on the type of uC you have.

In days of yore, microcontrollers had byte- or nybble-wide ports, which were programmable as either all inputs or all outputs. If you had one of those (or you wanted to do this with standard I/O port logic) you would do something like this (view in fixed font or M$ Notepad):

` Old Tyme Matrix Keypad Interface 2 X 3 ` ` .------------o-----------. ` | | | ` | 1 | 2 | 3 ` | T | T | T ` D | --- | --- | --- ` Ao-->|----o--o o--. o--o o--. '--o o--. ` | | | ` | | | ` .-------|----o-------|---. | ` | | | | | | ` | 4 | | 5 | | 6 | ` | T | | T | | T | ` D | --- | | --- | | --- | ` Bo-->|----o--o o--o o--o o--o '--o o--o ` | | | ` | | | ` Co----------------o | | ` | | | ` | | | ` Do-----------------------------o | ` | | | ` | | | ` Eo-----------------------------------------o ` | | | ` .-. .-. .-. ` R | | R | | R | | ` | | | | | | ` '-' '-' '-' ` | | | ` =3D=3D=3D =3D=3D=3D =3D=3D=3D ` GND GND GND created by Andy=B4s ASCII-Circuit v1.24.140803 Beta

For a 2 X 3 matrix (you can make this arbitrarily large -- I've made a small one here due to space constraints) you have two rows and three columns. The left side of the switches are the rows and the right side the columns. A and B are uC output pins, and C, D, and E are input pins. Initially, keep A and B low. To read the first row, make A high and B low. Read C, D, and E. This will give you the status of switches 1, 2, and 3. Then make B high and A low. Read C, D and E to get the status of switches 4, 5, and 6.

(If you were using TTL logic in days of yore, you would make the resistors pullup instead of pulldown, and reverse the diodes. A and B would be normally high instead of normally low. You would then read a logic "0" for an actice switch rather than a logic "1".)

Fast forward from those thrilling byte/nybble programmable port days of yesteryear to more recent times, where each individual pin is programmable as input or output on the fly. When a uC pin is an input, it is effectively removed from the circuit, acting like a tri-state output (or like a reverse biased 1N914 diode in the circuit above). You can then just replace the diodes with software (always a good trade

-- software is free) like this:

` Bit Selectable Tristate Matrix Keypad Interface` ` ` .------------o-----------. ` | | | ` | 1 | 2 | 3 ` | T | T | T ` | --- | --- | --- ` Ao--------o--o o--. o--o o--. '--o o--. ` | | | ` | | | ` .-------|----o-------|---. | ` | | | | | | ` | 4 | | 5 | | 6 | ` | T | | T | | T | ` | --- | | --- | | --- | ` Bo--------o--o o--o o--o o--o '--o o--o ` | | | ` | | | ` Co----------------o | | ` | | | ` | | | ` Do-----------------------------o | ` | | | ` | | | ` Eo-----------------------------------------o ` | | | ` .-. .-. .-. ` R | | R | | R | | ` | | | | | | ` '-' '-' '-' ` | | | ` =3D=3D=3D =3D=3D=3D =3D=3D=3D ` GND GND GND created by Andy=B4s ASCII-Circuit v1.24.140803 Beta

Now, in order to read the keypad, you normally have A - E as inputs. You then program A as an output bit, output a logic 1, and read C thru E=2E This gives you the status of SW 1-3. You then program A as input, and B as output, output a logic 1 on B, and read C-E to get status of switches 4-6. You are using the bit-programmable Input/output select to replace the diodes. Neat.

Of course, there is another improvement in more recent microcontrollers. Many now have pin-programmable weak pulldown resistors. This allows you to dispose of the external resistors entirely, like this:

` Bit Selectable Tristate Matrix Keypad Interface ` with Internal Pulldowns ` .------------o-----------. ` | | | ` | 1 | 2 | 3 ` | T | T | T ` | --- | --- | --- ` Ao--------o--o o--. o--o o--. '--o o--. ` | | | ` | | | ` .-------|----o-------|---. | ` | | | | | | ` | 4 | | 5 | | 6 | ` | T | | T | | T | ` | --- | | --- | | --- | ` Bo--------o--o o--o o--o o--o '--o o--o ` | | | ` | | | ` Co----------------' | | ` | | ` | | ` Do-----------------------------' | ` | ` | ` Eo-----------------------------------------' created by Andy=B4s ASCII-Circuit v1.24.140803 Beta

Programming is accomplished the same way as above, except you initialize the pins C-E with the weak pulldown resistors.

This scheme works well for any size keypad matrix, if you have enough I/O pins. Actually, you've got one right under your fingers now. The keyboard you're using right now almost certainly has the same setup -- a dedicated keyboard microcontroller reading a switch matrix and communicating with your computer, either through the keyboard port or USB.

If your microcontroller has 7 spare pins you can dedicate to your 4 X 3 matrix keypad, you're home free. If you don't have enough I/O (a common problem -- feature creep always uses up existing I/O in the real world), post again and describe your problem in more detail. Usually a combination of hardware tricks and crafty software can solve the problem. Mr. Monsen's link is one of the more creative possible solutions if you are out of pins.

Please be aware that, in the real world, you would also have to make some provision for ESD protection to protect the I/O pins of your microcontroller. But this should be enough to get you through your project.

Good luck Chris

it has 10 "outputs" to simplify the internal wiring of the unit.

it's probably wired in rows and columns with some of the rows (or columns) split across two pins.

it'd take only a few minutes with a continutiy tester to figure out the wiring of it. less if it's easily dissasembled or passes sunlight.

Bye. Jasen