Another tinyISP lives, quick question...

USB AVR Programmer and SPI interface. Adafruit's USBtinyISP.

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jbsnyder
 
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Joined: Sat Dec 08, 2007 2:39 pm

Another tinyISP lives, quick question...

Post by jbsnyder »

So, I've built one of these from scratch using parts I had laying around and a few additional ones from digikey. Seems to be working happily with one of my targets, which is nice because I need to fix both my STK500 & AVR Dragon (both died mysteriously while working on the same project, I suspect ESD, but who knows), and I feel a little more comfortable having something a little better than a dasa available.

My quick question is this: the guide says not to turn off the external power jumper unless really needed. I'm wondering why this is? Is this just a case of "if it ain't broke..." or is there another reason to leave it on even while mostly working with 5V targets? Is this because the buffer is technically getting powered by the target at that point?

Otherwise, fabulous little design, thanks ladyada! I'll be buying a x0x when my number comes up :-)

Also, I'm highly appreciative of knowing that this thing is simple, and there is a buffer between the target and the avr micro, and that both are in sockets. I'm not looking forward to further fixing/damaging my atmel development tools while trying to fix them for burned out soldered on chips that apparently couldn't handle something that was thrown at them.

Also, I would say that buying the PCB would have been worth it for the extra time it takes to do all the wiring on a little radioshack board :P

adafruit
 
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Post by adafruit »

even with both 5V targets its bad bad practice to connect two VCCs!

jbsnyder
 
Posts: 6
Joined: Sat Dec 08, 2007 2:39 pm

Post by jbsnyder »

Hmm.. I think I'm still slightly confused.

The idea on V2 is to have the micro and the buffer all possibly powered from the target instead of from USB, correct? If PWR is traced by default, and the BUS power jumper is connected, then you would have VCC going straight to VTG. This certainly wouldn't be good if your target is running at 3.3V or similar and suddenly it gets flooded with 5V.

My original assumption had been that you might want to have external power off (no connection from USB power -> VTG), so that the buffer gets powered from the target, and have the attiny powered from usb. This way if the target goes nuts one has to go through the buffer first before toasting something further up the line. Is there anything wrong with this approach? It sounds to me like this might be a decent default state for both 5V and 3.3V targets, so long as the level shifter can handle the over-VTG voltages from the micro.

The assumptions I've made are based on the usbspi v2.0.sch schematic. It looks to me like Vcc for the level shifter is not the bus vcc, but one that could be either from VTG or BUS VCC.

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