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TM4C1294NCPDT: JTAG interfacing

Varun Rege
Prodigy 70 points
Part Number: TM4C1294NCPDT
Other Parts Discussed in Thread: EK-TM4C1294XL

I am preparing a custom microcontroller board with the TM4C1294NCPDT microcontroller. I have included a ARM 10-pin JTAG connector as follows:

The TM4C1294NCPDT datasheet states that each of the TCK, TMS,TDO and TDI inputs on the microcontroller chips have internal pull-up resistors. But in the Using TM4C12x Devices Over JTAG Interface document, it is stated that external pull-up resistors are in fact required for the JTAG connector as shown below:

I do realize the microcontroller shown above is not the TM4C1294NCPDT but it does belong to the TM4C129x family.

And also, the Tiva™ C Series TM4C1294 Connected LaunchPad (EK-TM4C1294XL) which hosts the TM4C1294NCPDT chip also has external pull-up resistors for the JTAG pins on the launchpad (although of 0 ohms) as shown:

So my question is, do I have to include external pull-up resistors for the JTAG pins on my custom design despite the datasheet stating that the TM4C1294NCPDT chip has internal pull-up resistors on the same pins? If yes then why? 

  • 0
    TI__Guru*** 128265 points

    Hello Varun,

    This detail is discussed in our System Design Guidelines in Section 3.7: https://www.ti.com/lit/pdf/spma056

    The answer is going to be system specific regarding the trace lengths of the JTAG signals. I would say the comment for TCK applies for all JTAG pins, that is something we may want to better clarify on that Design Guidelines.

    Also has TDO is recommended to have external pull-down, that should be added since internal it is a pull-up.

  • 0 in reply to Ralph Jacobi
    Prodigy 70 points

    Hello, Ralph.

    Thanks for your response. All my JTAG trace lengths are under 32 mm and not near any areas of noise so I guess I should be fine without adding external pull-ups. But I didn't exactly understand your comment about TDO; are you saying I should add an external pull-down for TDO regardless of trace length or other factors? Considering TDO has both internal pull-up AND pull-down resistors.

    Best,

    Varun Rege

  • 0 in reply to Varun Rege
    TI__Guru*** 128265 points

    Hello Varun,

    Sorry, I didn't have as much time to look into this yesterday but I clearly should have done more thorough researching before I commented.

    First of, the LaunchPad isn't using pull-ups. The diagram you showed is just to show a connection with zero ohms where you can disconnect the JTAG from the device to use the LaunchPad to program other boards.

    Now with that cleared up, I had looked at the documentation and it implied that TDO needed a pull down while Table 4-2. JTAG Port Pins State after Power-On Reset or RST assertion of the D/S stated that it was a pull-up. That made me think that you'd possibly need the pull down.

    However, further looking into this indicates that the pull-down isn't actually accurate, since TDO is a push-pull. The pull-up resistors are to keep the logic level stable when the JTAG pins are not in use, so there is no need for a pull-down to be used here.

    Based on your system description, you do not need any external resistors on the JTAG lines.

    Further details can be seen in this post: https://e2e.ti.com/support/microcontrollers/other/f/908/p/771730/2854588#2854588