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I have a working project built around ATTiny85 on a breadboard running at 5V. Now I want to move this prototype on a PCB where it will be powered from batteries. All components used are capable of running 3.3V so I want to run the ATTiny85 at 3.3V as well.

I don't have a programmer so I use ArduinoISP to program the ATTiny. If I change the fuses on Tiny to run it at 3.3V can I still use the 5V Arduino to program the tiny?

Since the ATTiny can easily run at 5V I don't see any problem with programming it at 5V levels and then run it at 3.3V. Is that correct?

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Since the ATTiny can easily run at 5V I don't see any problem with programming it at 5V levels and then run it at 3.3V. Is that correct?

Yes. But if there are any components on the board that can't run at 5V then you must provide some way of protecting them from the excessive voltage.

If I change the fuses on Tiny to run it at 3.3V can I still use the 5V Arduino to program the tiny?

There are no fuses required to change the actual operating voltage of the device. There is the BOD threshold, but you can still operate the device at a higher voltage without issue regardless.

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Most ideal would be to use a buffer chip (like there is on the AVR Pocket Programmer).

But in your case you could get away with just placing some resistors (1k or something) on the ISP lines. This is because if the chip is powered by 3v and you put a 5v signal on one of the pins it's clamping diodes will redirect the higher voltage to VCC, raising VCC to 5v. In your case the 5v is not necessarily a problem. But if you circuit is using e.g. 100mA, that means 100mA will go through the clamping diode. It's not designed for that.

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