AVR One is dropping IO voltage

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I'm developing an ATTiny85 project. 9VDC is fed to the board where it gets regulated down to 5V. My test program just outputs a square wave on all 6 IO pins. Everything looks fine (i.e. low = 0, hi = 5V) with the emulator unplugged, but when I plug it into the ISP connector on the board the outputs drop to lo = 0, hi = 3V. This only happens when I'm in debug mode, though debugging works fine. Any idea why? Thanks.

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I'm not experienced with AVR one, but check your programmer's settings - does AVR one try to apply a supply voltage when connected? is that set to 3V? if it is, that could be fighting your onboard regulator. Look at VCC when the target is held in reset.

Otherwise is your AVR trying to drive a pin that the ISP connection pulls to GND?