Easy solution is NOT to use an old obselete PIC, use a more modern one like the 16F819 or 16F88 - these are pin compatible and have analogue to digital converters built-in.
Oh my god... so you will have to use external ADC to have some decent results... mmm nice way to learn SPI , I2C and multiplexing in one shot... good luck!
If he's got large stocks of 16F84's then it makes sense to use them, what doesn't make sense is ordering them now (and it hasn't made sense for over six years). The actual code is not really any different to the 16F628, but there are a couple of slight changes required - and the 16F628 is a FAR more useful chip.
odering them only if you have to replace some and don't have anything else than the .HEX file and knowledge/time to redo the software... i hate using external Crystal when i don't really need. at least 2 extra i/o for free.. not bad.
For school, i don't know if they stock them... i guess it's much a matter of rewriting some books... lazy? Maybe!
Yeah i know how to, there's no big deal to port code from one to another... But Ask to most repair shop tech to modify the code this is why i said knowledge/time.
Anyways galdz03. Here's an example of how to connect your 7 Segments display.
mmm you'll be short in i/o soon if you have to use external ADC...
between the PIC and 7 segment, use a drive like 7447 will give you 3 extra i/o. Using a LED driver like MAX6958 is another choice