Potential homemade XenoGC Fix

Alright so 8 months later now and I have an update...

http://www.gc-forever.com/wiki/index.php?title=XenoGC_Clone

The Wiki states to use C4 for the low fuse and D9 for the high fuse but after some testing C4 can't be the right fuse and the chip often doesn't start or work properly, after trying 84 for the low fuse as suggested it worked perfectly without the need for the Reset wire fix posted above. I also tested with E4 like some other poster wrote on another site, and although it did seem to work a little better than C4, 84 definitely worked the best and now it works every time. For reference, this is with Atmega8L-8PU.

Thanks guys. Maybe someone should update that Wiki?

 

Maybe it's because they are being silly and added two resistors which doesn't exist on the original thing. If you omit the leds, you really only need the 1K resistor for the reset pin.

You only need the 1K resistor to hold the AVR chip reset high (so it can run).

You want it running from the internal oscillator and you need to make sure the EEPROM got programmed properly as well. (on my setup at the time I was using pony prog and SPI parallel (SP12 type on the wiki) to program them.

Only fuse bits that really matter are the oscillator configuration and the ones which allow the chip to write to the eeprom by itself (so it can store configuration changes).

I'd suggest you try the chip without the 1K and 100 ohm resistors first before making experiments with the chip programming.

 

Honestly, it's working perfectly fine after changing the fuses to 84 so I'm not too concerned about changing anything else.

I'm aware you can run it without the resistors + LEDs but I choose to leave them because they're great for diagnostic purposes. Also, if you look at a pic of an original XenoGC it has a 1K ohm resistor for the reset, 2x 120 ohm resistors (which I believe are for the LEDs), and 2 other SMD components that I'm not 100% about, I think one is the decoupling cap and the other I'm not certain about...

 

Those two are 100 nanofarad (104) ceramic capacitors to stabilize (decouple) the power for the ATMEGA chip.

Therefore there's no resistors on the path between the modchip and the drive circuits. That's what I was talking about. Two extra resistors which don't belong there.

 

Unusual that the "Brown Out Detection" bit would improve stability. (Used this AVR fuse calculator to determine the difference between C4 and 84.) It really shouldn't affect the behavior of the chip at all. (Unless a bad wire to vcc?)

This instructable uses the wiki to make the modchip and the fuse settings seemed fine. I'll be trying to make my own XenoGC this Summer. (Though my actual goal is to try porting XenoGC to ATMega88/168/328. This would give a wider selection of chips and allow more storage to add new features in the future.) Which means I'll be using completely different fuse settings anyway.

I think the extra two resistors were only if one was going to leave SPI connection in place, but yeah I'd leave them out.

 

The main thing is that it was using BOD disabled in combination with the shortest oscillator startup time - and the datasheet specifically says that using the shortest startup time is only recommended with the BOD enabled. My guess is that the VCC rise time was slow enough that the oscillator startup time had expired before VCC had hit the correct operating level, and this was causing the chip to malfunction.

 

Do'h... Guess I should have read all of the previous posts. Well then, I'll keep those fuses in mind when I get around to building my XenoGC.