Repaired JTAGICE MKII with resistors

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Last weekend I blew up my target board and JTAGICE MKII while hot plugging another board into the target.
After repairing the target board, I discovered the emulator had been damaged.

The emulator was taken apart, and three level translators were found on the PCB that connect to the target cable. After some research, and checking around with a meter, I discovered that 3 of the ISP lines connect to a MAX3392. This was the likely cause of the failure. Problem is, I didn't have a MAX3392, and I needed my emulator in a bad way.

The MAX3392 level shifts 5V signals down to the logic level of the target DUT (say 5V to 3V). Output level translation from a higher voltage to a lower one is not usually much of a problem. So I removed the MAX3392 and replaced the level translator with four 1K resistors (one for each channel). This will force the clamping diodes in the DUT to conduct about 2mA on each channel (5V-3V)/1K. This works perfectly.

This repair is not for the faint of heart due to the fine pitch of the PCB pads (I had to use a microscope). Of course, with the repair, I've lost some of my ESD protection on these channels, but for miswire protection, the resistors are probably a superior solution. Sourcing capability of the 3392 was poor anyway (20uA). Even with the 1K's in place, the pin drive is stronger with the resistors.

In all my spare time, I may develop an interface board to provide better protection for the JTAGICE MKII target lines. This certainly looks like place for improvement.

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There is an ATMEL application note for debugWIRE:
AVR077: Opto Isolated Emulation for the DebugWIRE
http://www.atmel.com/dyn/resourc...