Attiny85 question.

[brokenstarguitar]

Ive been doing some research on tap tempo and came across the Attiny85. Now i have a few laying around and was wondering if anyone knows the difference between the Attiny85V-10PU and the Attiny85-20PU? Now i know the MHz are different but i guess what I really want to know is which one would be better for a tap tempo or does it depend on the circuit its going into? And if thats the case then what specs should i look for? Any insight or advice would be greatly appreciated and helpful being im not familiar with this chip. Thanks in advance!

[anotherjim]

I think the V-10pu spec is 10Mhz max, but can run at lower power supply volts (<3v?) at it's full speed. The 20pu needs close to 5v to run it at it's full 20Mhz. Since you probably have 9volt available so can use 5volt, it makes no difference really.
As it happens, I found I could buy the 85-20pu  cheaper than any of the ATtiny variants.
I haven't used one for tap-tempo (yet) myself.

One thing with the 85's - no 16 bit timer/counters - that might matter for your design.

Are you planning to use the internal RC clock? I suspect it would be good enough for tap tempo. It would drift of course, but the tempo it calculates would surely be true to whatever it's clock happened to be, and close enough for rock&roll? I think I'd rather go with internal clock than lose the use of 2 I/O pins.

[brokenstarguitar]

No. Im thinking about using it just for the tap tempo. Nothing else. So the you think the 20pu with an outside clock like a 555 would be better? Is that what youre saying?

[anotherjim]

No, I didn't say the 20pu would be better -  so long as the V-10pu 10Mhz clock maximum at 5volt supply is fast enough for you - I'd say it depends on which you can get cheaper.
You have the choice of timing with the CPU clock or an external reference, whatever chip type. An external clock only needs one pin, a crystal or resonator needs 2 pins. The internal RC clock leaves all of the i/o pins free. If you want to keep ISP programming, you can't use the Reset as i/o. So you have to decide how much i/o you must have and see what's left for the clock option.

[slacker]

Jim raises a good point about the I/O, depending on what exactly you want to do is 6 I/O pins enough? A tap tempo controller probably wants at least a switch for you to tap the tempo with, an output to flash a LED at the tapped tempo and probably a pot so you can dial in the speed instead of tap it. Then you need some sort of output to drive what ever it is you're controlling with it, depending on what that is, it might just need one pin or it could need more. You need to think about that before you start designing anything.

If you've got both chips I would use the fastest to start with, if once you get on with the programming it turns out you don't need the extra speed you could switch to the slower one. For a DIY project there's nothing really in it though, they're both cheap, and you might find the 20U is cheaper than the 10U, it was where I looked.