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A question about JTAG cable

Z-SEED
Intellectual 660 points


Hi,

Could anybody tell me the limit about the length of the emulator's JTAG cable? It is said that the length between CPU and JTAG could be less than 6 inchs, however I could not find that content in IEEE1149.1 standare. And in my opinion that 6 inchs is used between CPU and JTAG in target board, not between emulator and the target board's JTAG. Am I right?

Thank you very much.

  • 0
    Guru 192875 points

    Hi,

    The distance between CPU and emulator will be the length of USB data cable and can be quite long. But between emulator and the kit/custom board should be around 6inches or as small as possible.

    Regards,

    Gautam

  • 0
    Mastermind 7925 points

    The maximum length will depend on voltage and signal frequency. It should be no longer than 50[cm] and sometimes working maximum will be 10[cm].

    In practice you should use as short one as possible. If the shortest does not work you can try to reduce JTAG frequency. 

    Some more info: http://infocenter.arm.com/help/index.jsp?topic=/com.arm.doc.dui0155k/Chdbgcdj.html

    Regards,
    Maciej 

  • 0
    TI__Guru**** 171309 points

    Hi,

    Adding to Gautam's and Maciej's answers, the 6 inches are a recommendation from TI and not part of the IEEE1149 standard. This is empirically determined to meet the JTAG data rates that correspond up to 50MHz of unbuffered TCLK without compromising data integrity - check section 6 of this page. There it also says that speeds would increase to 100MHz (with probably a reduction in the JTAG length), but that references a technology that was discontinued (high speed RTDX).

    Keep in mind the distance is between buffers, and in the case of an emulator it is measured between the (edit)  output drivers inside the emulator board connector and the device pins (not the board connector), therefore several emulators have cables shorter than 6 inches to compensate any length in PCB traces.

    Hope this helps,

    Rafael