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I have a couple Attiny85 based USB development boards (or rather, cheap Digispark clones from Ebay). The pin 5 seems to be always at high, no matter what I do. I am suspecting the manufacturer didn't disabled the reset. And hence it is pulled up internally.

Can someone please confirm this? I am tempted to lock the reset bit, but this means I can no longer program with over ISP. So I am hesitent.

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  • 1
    Here is the complete answer, whith photos – user116396 Jul 11 '16 at 8:11
  • If anyone is still reading this, despite the above. Pin 5 will happily drive a LED or similar opto device without a resistor. Measure the current, 15-18mA is quite OK. (3.6mA with 3mm garden variety red LED) It cannot provide 40mA, even with reset disabled. So buffer, or try and use it for input. Test it for yourself. Search the datasheets for "weak". – mckenzm Oct 25 '17 at 7:50
up vote 2 down vote accepted

nRESET does indeed source approximately Vcc when the RSTDISBL fuse is unprogrammed. Since this is meant to be a USB development platform, you should burn (and test) Micronucleus to the chip before programming the fuse.

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  • awesome. thanks again for your concise and accurate answer Ignacio. – Adam Lee Sep 19 '15 at 5:16
up vote 2 down vote

By default RSTDISBL fuse bit is not programmed (default=1), then you can program chip through ISP. it means you cannot use this pin as IO. Because of the nature of RESET pin, it is Active Low in Atmel MCUs, then it's normally pulled up.

If you wanna test that it is Reset pin or a IO, connect it through a 220R resistor to GND and see what will occur to chip.

In most cases manufactures skip this pin to Reset pin, but it's not general. I insist that you don't change RSTDISBL, or you will be in a big trouble to reprogram your chip, e.g. using High Voltage Programming that is not so pleasant as ISP is.

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  • great, it makes a lot of sense now. What are the outcomes when having 220R to GND? – Adam Lee Sep 19 '15 at 5:18
  • if its in IO configuration, connecting pin directly to GND will consume huge current and will damage pin. Then it's a current limiter for safety. – HOPE Sep 19 '15 at 5:56

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