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USBtinyISP not able to talk to AVR at 3.3V
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USBtinyISP not able to talk to AVR at 3.3V

by mattkrass on Fri Apr 15, 2011 11:25 pm

I have an AVR on a custom PCB running at 3.3V, with a 7.3728MHz crystal and appropriate load capacitors, and I'm trying to use my USBtinyISP to flash it and I'm not having much luck.

With the board powered by its own power supply (3.3V SMPS, verified with multimeter and scope, plenty of bypass capacitors, it seems fine) I have the USBtinyISP connected to the board by the 6 pin header, with the target power jumper disconnected. I believe this should cause the device to use the sensed VCC to balance its voltage levels. In this situation I am very intemittently able to communicate with the chip, but about 95% of the time I get a rc=-1 error from avrdude. If I disconnect the boards SMPS and jumper the USBtinyISP to provide 5V, it works perfectly, however the board cannot run at 5V without removing a lot of devices and this is not optimal at all :( I have tried this with my clock crystal, and the onboard oscillator at 8MHz and 1MHz and it rarely works at all. I've had this unit for a while, and I haven't had a single problem with it until now, so I'm ready to admit I've probably bungled something up, but I haven't a clue what that might be.

Suggestions?

Thanks,
Matt
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Re: USBtinyISP not able to talk to AVR at 3.3V

by adafruit on Sat Apr 16, 2011 8:17 pm

you're using it correctly so its unclear why it isnt working, perhaps the power pin on the ICSP is not providing 3.3v properly?

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Re: USBtinyISP not able to talk to AVR at 3.3V

by mattkrass on Sat Apr 16, 2011 10:22 pm

Figured it out, I had my AVR configured for a Full-Swing oscillator by fuse, and that worked perfectly fine at 5V, but at 3.3V the clock was not quite up to the task I suppose. I switched it to Ext. Oscillator 3.0 - 8.0 MHz and it seems to be working now. That and I cleaned up some questionable soldering. As I suspected, I bungled something up.

That said, I love this thing, its quite effective (and very robust, you wouldn't believe the kind of things I've done to this guy)

Matt
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Please be positive and constructive with your questions and comments.