pc I/O interface card

Hi,

I`m trying to build a card to interface with the pc. I have found several tutorials explaining how to connect a PIC with the computer by different methods, that part is partially solved.

The problem: i have to control 3 groups of 20 pushbuttons and 1 group of 30 LEDS. I planned to interface with the pc using a PIC16F877A with 40 pis, so i have 33 IO pins. I dont know how to connect the pushbuttons and the leds...

-Is there any IC that allows me to expand the I/O ports?
-I thought that i can use a PIC to control each group (3 pics for the buttons, 1 pic for the leds, and 1 pic to interface with the pc). Can I connect 4 PICs to the one that drives the interface with the pc by I2C protocol? If so, could anyone post the scheme, please?

Thx in advance

I suggest you try looking up 'multiplexing', you only need 16 pins (8x8 = 64 keys) to read 64 keys. Likewise you can multiplex the outputs as well, 12 pins (5x7 = 35 LED's).

Check my tutorials for some examples.

There's a more complex scheme called 'charlieplexing', and if you look at these forums it's mentioned a lot.

Thx for answering,

So you say I can connect the leds as if they were a matrix of pushbuttons? That would be great. The problem is that maybe i need to expand the circuit later with mre IOs, so i need to know if I can connect more PICs by I2C in case i run out of pins in the original circuit

raus said:

Thx for answering,

So you say I can connect the leds as if they were a matrix of pushbuttons? That would be great. The problem is that maybe i need to expand the circuit later with mre IOs, so i need to know if I can connect more PICs by I2C in case i run out of pins in the original circuit

Click to expand...

You 'could', but I2C isn't a very good choice, it's easier to use a simple serial connection - and it only requires one I/O pin instead of two.

for expandibility of output, I'd go with the 74HC595 shift reg. You'll only need 3 I/O pins for any number of outputs since you can daisy-chain them. 8 outputs per chip. They are cheap ($.50 or less) and it's very easy to implement. You could drive them with SPI hardware on the 877, if you want, but bit bang works quite well.

I've also seen a 3 to 8 decoder used to row select a switch matrix so 11 pins can drive an 8x8 array. you could even use the 595 SR to do it.

so 14 pins could drive all your buttons and LEDs.