circuit to scan 4x4 keypad

[Build-A-Burger]

I want to build a circuit that scans my 4x4 keypad using a binary counter and a '138 to generate the row scan and then read the results with a '374 which is read by the processor. The counter is clocked with a 555 timer. The counter will have to reset after it reaches 3. The resistors are pull-down. What would I use as a counter? A '161? And how would I generate an interrupt for the processor? Also wondering if there's some way to incorporate a debounce.

 

[KlausST]

Hi,

Usually tjus all is done with a simple microcontroller. A one chip solution.

Maybe your processor could do the job.

Tell us more about the pfocessor. What is it for?

Klaus

 

[Build-A-Burger]

The processor is a TS-5500 SBC from Technologic Systems and I have a pc104 daughter card which gives me some extra io ports which I'm using to control relays and read switches. Since I have extra IO ports on the pc104 card I can have 4 bits send a scan and the other 4 bits poll for a keypress. Now that I think of it, I'm not really saving any io ports. I guess it would be worth the trouble (just barely) since I don't have to do a scan and check for a keypress every so many milliseonds. The SBC card is running uCOS-II on top of DOS. I thought one thing I could do is move the scan task up in priority only after an initial keypress to save on processor time.

 

[KlausST]

Hi,

I think a scan every 20ms should be enough not to miss even a very short keypress.

*****
Or you can improve:
It seems you want to activate a row by putting sending high to it.
Then a pressed key in that row gives a high at the relating column.
As long as no key is pressed every column is low.

When idle = waiting for a key press, you could activate all rows and set all columns to generate an interrupt when high.
So you don't need to poll all the time, but wait until a column interrupt is generated, Then scan the rows independently as usual.

Klaus

 

[betwixt]

Use a 74C922 if you can find one - all done in one IC, including the debounce.

Brian.