Silly Invention: Hardware-only ATmega defuser

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HardlyUnique
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Joined: Tue Dec 08, 2009 7:44 pm

Silly Invention: Hardware-only ATmega defuser

Post by HardlyUnique »

While building my Uzebox, I ran into some trouble. When I tested the sound it was *terrible*. I tried changing some Oscillator fuse bits and it seemed to improve a little. Then I got too ambitious changing fuse bits and bricked my chip. :oops:

I probably could have just attached an oscillator but this was more fun! I wanted to build something that used HVP but all the plans I found relied on another chip to work. I wanted my chip fixed without buying another one! So, I followed the datasheet and invented this very specific device that allowed me to reset the fuse bits, without running any software. I thought some of you might appreciate it.

The process is difficult; it took me three tries to get right. I wasn't sure if the LED (that indicates the write is happening) would be on long enough for me to see it, so I got my eye right up next to it and almost blinded myself when it worked! Still, it was SO rewarding to see that little light. :D This is actually one of the first circuits I've designed.

Here's the device:
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Oh yeah - what turned out to be wrong with my Uzebox? I didn't realize all the files I was flashing were from a buggy kernel I'd hacked. :oops:
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uze6666
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Re: Silly Invention: Hardware-only ATmega defuser

Post by uze6666 »

An odd looking contraption, but if it works! ;) Do you have the schematic, I'm curious to see how it works.

-Uze
HardlyUnique
Posts: 65
Joined: Tue Dec 08, 2009 7:44 pm

Re: Silly Invention: Hardware-only ATmega defuser

Post by HardlyUnique »

Eh, I never made a schematic, which might account for why it took me three days to build it. :oops:

I'll have to make one some day - I'm still starting out with electronics so I haven't made many.

I can tell you that it's mostly pull-up resistors and some pull-down resistors and switches, jumpers or buttons that short the logic to the non-pulled state. There's also a current-limiter, bypass capacitor and LED / resistor.

For instance, XTAL1 is normally pulled low, but when you press a button it pulses high. You hook up power, pulse XTAL1 6 times and remove a jumper that was preventing 12V from going to reset. this is how the device first enters HVP mode.

...Yeah, it's crazy; I'm going to have to make a schematic... :D
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