# Do I need ISP isolation?

I'm going to use MISO/MOSI/SCK pins of 8 bit AVR with ISP and they should connect to shift register. They are not going to work simultaneously, but the question is how to isolate register from ISP?

Update:

U7 is ISP header. U1 is ATTiny45/85. U2 is LED display with shift register M512RD, but I'm mostly sure that the question is more conceptual and doesn't require exact parts.

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Could you please provide a bit more detail, and perhaps a schematic and links to the specific parts you are referring to? – Anindo Ghosh Jan 17 '13 at 12:53
@Anindo Ghosh: done. – Pablo Jan 17 '13 at 13:04
I see only SR inputs, no outputs. I assume you don't care what the display shows while programing, so why would you want to isolate the SR inputs from being driven by your ISP? – Wouter van Ooijen Jan 17 '13 at 13:39
@Woutervan Ooijen: Yes, I don't care about diplay while programming, but my concern is can display somehow disrupt programming? – Pablo Jan 17 '13 at 13:45
If you are going to be actively driving against them, I'd try to avoid a resistor below 1K. Unless you data rate when talking to the display is quite fast or your routing long and intricate, that's not likely to cause a problem in operation. One thing you can do is test it with a resistor several times larger than the one you plan to use - if it still operates reliably, you should generally have some safety margin when you cut back to the intended value (provided you are still on the same side of the characteristic impedance, which in the kilo ohm range you would be) – Chris Stratton Jan 17 '13 at 18:16