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JTAG-HS3 warm, locks up system


Question

I have a JTAG-HS3 programmer that is performing strangely.

When powered, it seems excessively warm.

I can use it to program my Zynq board if in JTAG boot mode, but it will not boot up unless I disconnect the programmer.  I checked the reset line, and it is not being asserted by the programmer.

I tried a different unit and it works fine.

Is this a common failure mode?

I will be embedding a JTAG-HS3 in my test fixture for the Zynq board.  If I remove power to the programmer, will the JTAG signals be high impedance?  Should I insert buffer gates on TMS, TCK, and TDI to isolate the unit under test from the programmer?

Thanks for your help.

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@Don Koch

Excessive heat suggests that there may be a damaged component on the board.

The reset line is active low and is driven by an open drain buffer that has a 100 ohm series resistor between it and pin 14 of the JTAG connector. There is no onboard pull-up on the reset line and it is assumed that the host board will pull the line to the inactive state (high). When you say reset is not being asserted, do you mean it's always high or always low? Does your board have any push-pull buffers that drive the reset line?

When you say remove power do you mean remove the USB cable or remove VREF from pin 2 of the JTAG header? The onboard buffers will tri-state when they aren't powered. However, the output buffers are powered from the VREF pin of the JTAG header, while the USB controller, which drives the inputs of the output buffers is powered from USB VBUS. If VREF is present but USB power isn't then the buffers will be powered but they should still be held in tri-state because the output enable pin is pulled to ground through a resistor. I don't think you will need to insert extra buffers between TMS, TCK, and TDI.

Thanks,
Michael

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Michael --

Thanks for responding.  Above where I asked what would happen if I removed power, I was referring to removing USB-side power.  In our fixture, connections are always present -- everything is under program control and we cannot require the personnel performing the test to plug and unplug cables during the test.

How would the JTAG-HS3 behave if I never gave it USB-side power, and only have it power from the target?   Would the USB interface still work, or does it need 5V to drive the USB port?

From my black-box testing, when JTAG target power is present the HS3 stops drawing current from the USB port.

For my test fixture use, I am thinking that I should have the test fixture power the HS3 with a controllable (on/off) supply (3.3V on the JTAG side).  We can do this because the HS3 is not directly connected to the UUT.  The USB power pin would always be disconnected.

When we want to program the UUT we will turn on the HS3, download the JTAG data, and turn it off.  Power cycle the UUT, and we are going.

Does this sound like it would work?

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@Don Koch

If I understand what you are asking is if you plug in a USB cable that only provides GND, D+, and D-, but not VBUS, then would the module still work? The answer is no because the USB controller is powered by VBUS, not VREF. The tri-state buffers between the USB controller and the actual I/O pins prevents any current from flowing into the USB controller when it's not powered. The TDO pin has an open drain buffer between it and the USB controllers input pin so no current should flow in that way either.

 

If you find that the module works without powering VBUS then there must be a leakage path that I'm not seeing and that will warrant further investigation on our end.

Thanks,
Michael

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