I have an XPLAIN board connected to a STK600 via its JTAG header. (the STK600 supplies 3V via VTG on the JTAG header).
I programmed the AT90USB1287 via the JTAG in the normal way.
Then I connected the USB cable to the XPLAIN board which supplies its own 3V3 to the XPLAIN.
Now the STK600 is dead. One red LED next to the on-off switch. No VTG LED of any colour. No recognition of the STK600 as an USB device. And hence no communication at all.
I hope that I have not done too much damage. Before I try to look at the STK600 schematic, has anyone any suggestions.
I am a little surprised to have done anything so catastrophic when the two voltage supplies are similar. I could understand 5V fighting 3V3 would entail a winner and loser. (or possibly two dead losers)