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Pullups on 4-wire JTAG signals?

David M. Alter
Guru 61235 points

Champs,

Need a sanity check.  My customer is using F5418 device with 4-wire JTAG mode.  His design shows 4.7K pullups to Vcc on TDI, TDO, TMS, and TCK signals.  I have never seen this on MSP430 before, and our EVM boards don't show pullups.  There is also no mention about using (or not using) pullups on these signals in the datasheet.  On TMS320 device datasheets, we typically have specific instructions about the use of pullups on JTAG signals.  I suspect the customer motivation for using these pullups is noise immunity.

QUESTION: Can anyone advise on whether these pullups are OK, or not recommended?

I think they are OK, but would like confirmation.  My biggest concern is TMS signal, as I'm not sure what effect having TMS high at reset will have.

Regards,

David

  • 0
    Prodigy 30 points

    Hi David,

    i think its no problem to use pullups for the JTAG lines. The programming tools as well have no problem working with external pullups. But i dont know if its reducing the risk of catching some errors, at least when connected to the programmer its not necessary. The JTAG lines have a tiny internal pullup and should not float without external hardware. Maybe on long signal lines, but i never experiences a problem without pullups.

  • 0
    TI__Expert 8105 points

    Pullups on these pins are not required as they are terminated as needed internally.

    When not used in-system, it is recommended that these pins are left open (section 1.6 of the 5xx User's Guide has additional info on this topic). Given that these signal pins in the 5xx family devices are also configurable as GPIO, it is not unreasonable that externally resistors might be used. In this case, the SBW 2-wire interface is the best bet for tool connectivity.

    But when the 4-wire JTAG pins need to be used for JTAG, it is recommended that no externals are used to avoid any threshold/timing issues with a given toolchain. In many cases it may be ok, for instance this specific case of the 4.7k pullups, but any deviation from the recommendations would need to be tested with the tools of choice prior to moving forward in any design.

    Regards, Zack

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