JTAG ICE MKIII Not Reading Voltage Correctly via JTAG

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I have a custom board designed with a Mega16U that I am trying to bring up. Using the squid cable on the ICE, I wired up TCK/TMS/TDI/TDO and VCC and GND (two grounds). When I try to read the device voltage in Studio, it's jumping between ~2.5V and 3V. But my DMM says it's a steady 5.1V. I then noticed that if I disconnect TDO, the voltage reading drops to 0V. Does the ICE use the data lines somehow to try and measure voltage? I'd expect that with just VCC and both grounds hooked up I should be able to read the voltage, but no. Is my ICE hosed? 

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nobbyv wrote:
Does the ICE use the data lines somehow to try and measure voltage?

No:  there is a specific target voltage sense line - see the User Guide.

 

Perhaps you have some dodgy connections - leading to strange "leakage" effects ... ?

 

A schematic & some photos might help - see Tip #1 in my signature, below, for how to post pictures:

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nobbyv wrote:
Is my ICE hosed? 

Suggest to start with a thorough cable-check...

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Here's the micro portion of the schematic. Please note two things:

1) It says Mega32U, but for whatever reason the 32U was hard to find so I am using the 16U

2) This is Rev 2 of this board. We switched to the 16U from another Mega device, and due to a miscommunication between me and the guy that designed the board, we missed that the 16U doesn't offer debugWire. So the TCK/TMS/TDI/TDO signals were not routed to the J11 header as they should have been. He did leave me testpoints for them though which I soldered hookup wire to

 

I have swapped out the USB cable running to the ICE. No change. Unfortunately, I only have the one squid cable, but I was able to check continuity of the VCC (pin 4) and GND (2 & 10) wires. All good. Just to confirm: with just those three wires connected, I should be able to measure voltage, correct?  

 

 

EDIT: Tried another board; same result: with just pins 2, 4 and 10 hooked up, Studio reports a voltage of 0V. 

Last Edited: Tue. Sep 14, 2021 - 12:23 PM
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Well, I am suitably embarrassed. The squid cable is keyed where it connects to the ICE3. It looked like the "lock" was on the top of the ICE3...it is not. It is on the bottom. I had it upside down.