[SOLVED] Cortex-M JTAG connector interface. Differences between Texas Instruments and SEGGER guides

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  • [SOLVED] Cortex-M JTAG connector interface. Differences between Texas Instruments and SEGGER guides

    Hi,
    I am designing a board with Texas Instrument's TM4C123GH6PM microcontroller and I have found contradictory information between the TI's TM4C12x JTAG guide and SEGGER J-Link user guide.

    In the first one, "Using TM4C12x Devices Over JTAG Interface", it uses a series resistor in pin1(VTref), a pull down resistor in TDO line, no pull-up nor pull-down on TDI line and series resistors in TMS, TCK, TDO and TDI (screen shot attached)
    On the other hand, the SEGGER J-Link user guide says that VTref (Pin1) MUST NOT HAVE a series resistor, TDO line has no pull-up or down, TDI has to be pulled to a defined state and it says nothing about lines' series resistors (screen shot attached).

    In summary, what interface should I use in my design?

    [img]https://forum.segger.com/index.php?page=Attachment&attachmentID=2573&h=b82f4444b09a75cc6889bf6eb2279c9adeb4ce07[/img]

    [img]https://forum.segger.com/index.php?page=Attachment&attachmentID=2571&h=ea87e4908bbc09b4ae38dd883c5f59c7b094ab4a[/img]




    [img]https://forum.segger.com/index.php?page=Attachment&attachmentID=2572&h=dd744240f797a8e42761a190757cd84ee2d1a97f[/img]
    Images
    • JTAG_TM4C12x_guide.jpg

      162.12 kB, 1,398×819, viewed 116 times
    • SEGGER_UM08001.jpg

      200.56 kB, 838×862, viewed 76 times
    • differences.jpg

      23.04 kB, 466×144, viewed 50 times
  • Hello,

    Thank you for your inquiry.
    We suggest following the J-Link user manual instructions.
    For reference the following eval board design can be used: ti.com/tool/dk-tm4c123g

    Best regards,
    Nino
    Please read the forum rules before posting: Forum Rules

    Keep in mind, this is not a support forum. Its main purpose is user to user interaction.
    Our engineers will try to answer your questions between their projects if possible but this can be delayed by longer periods of time.
    Should you be entitled to support contact us per e-mail.
    Alternatively our support ticketing system can be used as well: segger.com/ticket/
  • Hi Nino,

    Thanks for the answer! The thing is that the schematics you suggest from TI don't follow SEGGER's guidelines (TDI line is not pulled to any defined state) nor their own TI's guidelines (they just pull up TMS and TCK and forget about the rest). I already noticed that in TI's Tiva Launchpad schematics and, when I asked TI about that, they answered me that the evaluation boards are made in a quick and cheap way, that they shouldn't be taken very seriously, and that I should follow the TI's TM4C12x JTAG guide... funny..

    I am surprised about the different opinions/implementations in guidelines between probe manufacturer, microcontroller manufacturer and schematics of working boards, about something that should be a standard. I guess that it works with different configurations as long as you pull-up TMS and TCK (the only thing everyone agrees! :)

    Best regards,
    Javi
  • Hello Javi,

    I have discussed this with our hardware engineer. He told me as follows. The guidelines in the J-Link documentation show what J-Link needs/expects for optimal connection to any target device from J-Link perspective. But there are some target devices that expect some other input states or need to be protected further.
    This falls under special requirements that we do not have information for. In case of TI it looks like such special handling is required (at least TI claims so).
    In that case we suggest following TI's guidelines in the TM4C12x JTAG guide. Our hardware engineer says that J-Link should be able to work with that setup as well without any issues.

    Best regards,
    Nino
    Please read the forum rules before posting: Forum Rules

    Keep in mind, this is not a support forum. Its main purpose is user to user interaction.
    Our engineers will try to answer your questions between their projects if possible but this can be delayed by longer periods of time.
    Should you be entitled to support contact us per e-mail.
    Alternatively our support ticketing system can be used as well: segger.com/ticket/