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Isolating JTAG with ADUM3400

Hi everybody,

actually I'm trying to isolate the JTAG-Ports of teh ADSP-BF537 with 2 ADUM3400.

But there are several problems.

If I bypass TCK and TDO , so just TMS, EMU, TRST and TDI are isolated, everything works fine.

If I isolate any of the other two signals I get an error in VDSP: "Visual DSP has detected 2 devices in the JTAG chain".

Has anyone an idea? I use the ADDS-HPUSB-ICE emulator.


  • Hi Michl,

    Clock speed through the isolators may be the problem.  The ICE can be set manually for speeds between 10 and 50 MHz data rate.  The default is 50MHz.  Even at 10 MHz, the pulse width of TCK would be only about 50ns.  Even the higher speed BRW version of the ADuM3400 appears to have a minimum pulse width of 100ns.

    Because JTAG is a development tool, you may not need to worry about the specified speed at worst case temperature.  Slowing down the ICE to 10MHz may give you the performance at room temperature that you need.  This operation would not be guaranteed.


  • Temperature isn't a problem, but the speed as you mentioned.

    Well, perhaps I should you use the CRWZ-version with 11.1ns pulse width?

    Any other ideas?

    Thanks so far.

  • Hi, now I tried the ADUM3400CRWZ but I still have the same problem.

    As I said in my first post it works by bridging TDO and TCK.

    But I recognized the following:

    When I'm connected to the target board and then remove the bridges it works. So it seems to be a problem when the ICE connects the first time to the board.

    Any ideas?



    Ok, now it's running nearly bug-free .

    I changed the platform config in VDSP and added an unknown device. Now I can debug, program flash and so on through the ADUMs.

    But there is still one (big) bug: It just works by bridging the two grounds of the ADUMs, so isolation is no longer existent.

    Thanks for helping!

  • Hi Michl,

    I was incorrect in my previous post.  The default speed is 10MHz not 50MHz.

    The behavior you report is very puzzling. Have you found a solution?  Can you post a schematic?

    Maybe the problem is elsewhere.  Our:

    Describes the hardware needed for JTAG.

    The ADUM3400CRWZ takes a fair amount of power.  Could this be a power supply problem?

    EMU should be an open drain (pull down only).  It therefore requires a pullup resistor on the input side of the isolation.

    If we can not solve the problem here, you may wish to contact:


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    Thank you,
    EZ Admin