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TMDSEMU110-U: XDS110 DEBUG PROBE NOISE GROUND ISSUE

Shai Berman
Mastermind 7400 points
Part Number: TMDSEMU110-U

Hello Team,

My customer is facing a problem of noised ground while connecting XDS110 to his PCB and the PC.

Brief description of the problem below:

All the voltages in the PCB are the same polluted during connection of XDS110 to the PCB. There is some random noise appears on the power.

The amplitude of such noise is approximately about 50mv peak. Actually, XDS110 gets its power from my PCB (please see attached adapter card)

Ones I have several references on my PCB they are all affected by this fluctuation resulting in incorrect analog measurements (in my case it is current).

Then the XDS110 is connected only to power (via phone adapter for example) this problem does not exist, only if XDS110 is connected to the Laptop (even no communication presents from the DSP side)

I use Matlab and Simulink and communicate to the target via External Mode for the debugging, so in this situation, the XDS110 is connected to the target and communicate instantly, but the reading of the analog measurements is wrong and noisy ( I verified it by scope measurement in the hardware) in this situation.  The distance  (harness length) between the target and the XDS110 is about 30 cm. I use buffer to compensate the length (see attached)

Does all inputs/outputs from the target side of the debugger and target power are optically isolated (like in XDS100)? Can I get the internal Elec. Schematic of the debugger to see that I can do in order to solve this problem or please provide your solution.

ADAPTER FOR JTAG COMM _ JTAG_.pdf

Please advise the above, thank you very much.

Best regards,

Shai

  • 0
    TI__Guru**** 168367 points

    Shai,

    Shai Berman said:
    The amplitude of such noise is approximately about 50mv peak. Actually, XDS110 gets its power from my PCB (please see attached adapter card)

    I am not entirely sure how or where you are measuting this noise, but I would guess that 50mV of noise is not enough to affect the normal noise margin of the XDS110 output (typically 3.3V TTL).

    Also, the schematic shows the adapter and I don't necessarily see how the XDS110 is getting its power through the PCB - the TMDSEMU110-U is powered via the USB connection. If you mean that pin 5 of the cTI 20-pin connector is tied to the target 3.3V, that is the correct connection. 

    Shai Berman said:

    Ones I have several references on my PCB they are all affected by this fluctuation resulting in incorrect analog measurements (in my case it is current).

    Then the XDS110 is connected only to power (via phone adapter for example) this problem does not exist, only if XDS110 is connected to the Laptop (even no communication presents from the DSP side)

    Ah, now I think I understand the scenario; the 50mV is being injected into the analog section - the XDS110 communications are fine. This is caused by the ground loop created between your board, the XDS110, the laptop, its power supply and the power supply of your board.

    When using the phone adapter, which is probably a 2-pin power supply (thus without a GND reference), will make the XDS110 to be "floating" in relatinship with the GND. 

    This is a common scenario that requires cutting the connection in one of the nodes - in other words, isolating (floating) either the laptop or the PCB from GND (the third pin on the three pin power supply). Just be careful if you float the laptop and the PCB has voltages in excess of 30V - the entire setup may be raised to high voltages and any exposed metal becomes an electrical shock hazard. 

    The other alternative is to purchase a JTAG isolator for the XDS110 - the most suitable one is the Blackhawk ISO110

    https://www.blackhawk-dsp.com/products/jtag-isolation/iso110 

    Shai Berman said:
    Does all inputs/outputs from the target side of the debugger and target power are optically isolated (like in XDS100)?

    The XDS100 reference design does not have isolation - this was added to the C2000 development kits as a convenience. 

    Hope this helps,

    Rafael

    P.S. a video from a famous video blogger (Dave Jones) that explains ground loops is here.

  • 0 in reply to desouza
    Mastermind 7400 points

    Rafael,

    Thank you so much for details answer, Actually, I shall make several modifications on it in order to connect to my adapter.

    Could you please recommend me which ISO parts at TI I can use in order to achieve an isolated environment such as Blackhawk ISO110.

    Can you send a elec. Schematic of such a card?

    Thanks in advance.and regards,

    Shai 

  • 0 in reply to Shai Berman
    TI__Guru**** 168367 points

    Shai,

    Unfortunately Blackhawk's ISO adapter is their own design and we don't have it.

    If you are interested in adding such functionality, you could always check one of the isolated C2000 Launchpads and see if you can design something that is suitable. 

    Regards,

    Rafael 

  • 0 in reply to desouza
    Mastermind 7400 points

    Rafael,

    I have found http://www.ti.com/tool/LAUNCHXL-F28027 has isolated XDS interface, I'll try to implement this design (based on ISO72xx).

    any comment from your side?

    Thanks and regards,

    Shai

  • 0 in reply to Shai Berman
    TI__Guru**** 168367 points

    Shai,

    Just beware of the maximum JTAG TCLK speed - isolator interfaces tend to reduce the maximum data throughput of the JTAG interface.

    Also, check the signaling and design at:

    http://software-dl.ti.com/ccs/esd/documents/xdsdebugprobes/emu_xds_target_connection_guide.html

    Despite it covers 4-wire JTAG, its design requirements are quite similar. 

    Hope this helps,

    Rafael