Using ADC for button arrays?

[thecritic]

I am working on a Project with PIC16F676. I need several input buttons but I am falling short of input pins. I have one plan.
**broken link removed**

I think I should be able to use the PIC's ADC channel to read several buttons by utilizing only 1 pin. Do you think this is plausible? What are the troubles I may run into doing this?
Is this a general practice or I am trying to do something out of the box?
Also, can you think of better options?

 

[ronsimpson]

Last time I used the same value resistor for every resistor. (1k). I would change "64R through R" to all 1k. Then make the switches short to ground. The right switch shorts out 1k. The next switch shorts out 2k (1+1). Then next switch takes out 3k.............
If you want to stay with the idea of no switches = 1/2 supply then 128R should be 7k.

 

[thecritic]

Last time I used the same value resistor for every resistor. (1k). I would change "64R through R" to all 1k. Then make the switches short to ground. The right switch shorts out 1k. The next switch shorts out 2k (1+1). Then next switch takes out 3k.............
If you want to stay with the idea of no switches = 1/2 supply then 128R should be 7k.

Oh yeah. That is of course a better technique, no need of R,2R,4R... . Single value would do.
Provided that I have 10-bit ADC, how many buttons do you think I can use in this way?
2^10 = 1024? Don't think so!

 

[upand_at_them]

Another technique is to add a capacitor in there and time the response instead of using the ADC.

 

[ronsimpson]

The resistors are +/-5% or maybe 1%. The DAC is not good all the way to 10 bits. There are some errors in the DAC. I have used 10, 5% resistors in high volume products.

 

[thecritic]

Ok, the hardware part seems fine using ronsimpson's technique. (Couln't undersant upand_at-thems's). Any tips for the software part? Will denouncing/averaging be necessary or since only 10 discrete levels are involved quick reading will be sufficient.? I need Poll the buttons really fast, the PIC has lot other things to do.