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I have an ATtiny85 which had a simple program PWMing its output pins to pulse some LEDs.

While messing round trying to wire up Raspberry Pi SPI to program it with avrdude, it's gone into a state where when powered up it pulses PB0,PB1,PB2,PB3,PB4 at maybe 5 Hz (guessing by eye), even if I hold reset to ground (which I would expect to make it an SPI slave). I never got as far as actually feeding in any program code - just using avrdude to check that it can see the ATtiny85, which it could till this happened.

Is this broken or is it some state I can fix?

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    \$\begingroup\$ Have you checked the power rail? How is the MCU wired up? How are your LEDs wired up? The MCU when reset is held low will do nothing - all its IO pins will go high-z. So if you are seeing LEDs toggle when reset is low, you have got something wired up wrong. \$\endgroup\$
    – Tom Carpenter
    Apr 19, 2020 at 18:21

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Reset to default
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I'm not an Attiny expert, but this sounds like the firmware that's running on the attiny is going haywire, possibly resetting the thing after a while or something.

Doesn't sound like any "diagnostic" mode at all: MCUs don't tend to do such things – there might be dangerous things attached to their outputs, so wildly pulsing one's outputs is among the last things one would want.

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