# Help, designing a keypad matrix decoder

Discussion in 'Homework Help' started by fyanardi, Aug 15, 2005.

1. ### fyanardi Thread Starter New Member

Aug 15, 2005
2
0
Hi all,

I'm asked to design a 4x4 matrix keypad decoder. As I understand it has 8 inputs, R1-R4 from keypad matrix and C1-C4 from a 4 bit ring counter. But if I want to get the output expression, I have to work for 8 variable inputs, aren't I? It really gives me a headache till now. Are there any simpler methods that I can use to obtain the expression? Or any other method to create the matrix decoder?

Fredy Yanardi

2. ### pebe AAC Fanatic!

Oct 11, 2004
628
3
Hi Fredy,
A 4 x 4 keypad uses 4 input and 4 output lines. These would correspond to your R1 to R4 as inputs and C1 to C4 as outputs from the ring counter.

Imagine 4 wires placed vertically, and 4 wires placed horizontally crossing them. Call the vertical wires C1 to C4, and the horizontal wires A1 to A4. A push button is placed at each point where wires cross. You will have 16 pushbuttons with connections

C1 to R1 C2 to R1 C3 to R1 C4 to R1
C1 - R2 C2  R2 C3  R2 C4  R2
C1  R3 C2  R3 C3  R3 C4  R3
C1  R4 C2  R4 C3  R4 C4  R4

To establish which key is being pressed, each key is checked like this. The ring counter is clocked to set C1 wire high, leaving C2 to C4 low.

The 4 wires R1 to R4 are now checked in turn. If one of them, eg. R2, is high then the key C1/R2 was pressed.

If none of the R wires is high, the ring counter is advanced to put C2 high and other Cs low. R1 to R4 are checked again. The sequence is repeated with C3 high, then C4 high until all 16 crosspoints have been checked.

This is known as polling and this individual checking of the 16 keys would be repeated in a continuous sequence.

I hope this explanation will help you arrive at an expression for the action.

3. ### fyanardi Thread Starter New Member

Aug 15, 2005
2
0
Hi pebe,

Thanks for your explanation, its very helpful. Finally I got the expressions by observing that for each digit there is only one digit is high in R1-R2 and also one digit is C1-C4, So I express them in Sum Of Product. Every expression in ABCD contains 8 products. Well, it requires many and gates, but I can't get any other methods. This afternoon I have successfully tested it in my Lab.

Thanks and regards,

Fredy Yanardi

4. ### yvonnezoe New Member

Oct 30, 2008
8
0
thank you so much for your explanation! it does help me to understand my lecture boundnotes =D