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TDA3LA: EMI/other impacts when JTAG TMS ball is floating

Thorsten Lorenzen
Genius 17101 points
Part Number: TDA3LA

Hi Champs,

in a single camera design the JTAG port used to be routed to a connector as its is meant to be. However, for production the connector will not be populated. The TMS ball J3 from TDA3LA ABF package is not pulled but kept floating. 

Can you please state whether the ball J3 is going to float or whether the ball will cause EMI issues?

The ball offers internal pull up/down options. May those help to avoid possible risk?

Signal name:        Description:

tms                       JTAG test port mode select. An external pull-up resistor should be used on this ball. J3

Many thanks

Thorsten

  • 0
    TI__Genius 13175 points

    Hello, ,

    Please check in the datasheet for details, specifically Section "4.1.1 Unused Balls Connection Requirements". For this particular ball (J3), there is this requirement: 

    "

    These balls must be connected to the corresponding power supply
    through an external pull resistor if unused

    "

    So, judging by the terms in the datasheet, yes, to avoid risks, design should follow the above guidelines. 

    See this e2e also that asks a related question but for non-GPIO balls: https://e2e.ti.com/support/processors/f/791/t/838309 

    Hope it helps,

    thanks,

    Alex

  • 0 in reply to Alex Bashkov
    TI__Genius 17101 points

    Hi Alex,

    many thanks for your response.

    As the nTRST ball, in the design, is pulled down the complete JTAG logic should be in reset state. Otherwise I could imagine TMS (J3) could enforce another state in JTAG's state machine. 

    Can you please double check with the team the case nTRST is pulled down externally (3.3K)?

    I will ping Kyle as well

    Many thanks

    Thorsten

  • 0
    TI__Genius 13175 points

    Hello, Thorsten,

    Just saw your other e2e thread stating that the HW design cannot be changed anymore (and like you said it's in production)... I don't know the risks since they haven't followed the guidelines. Let us try to bring in more experts here who can give you perhaps a better answer. 

    Thanks,

    Alex

  • 0 in reply to Alex Bashkov
    TI__Mastermind 46612 points

    Thorsten,

    There should be no functional impact since TRSTn is asserted. However, to keep the IO in a defined state we strongly recommend enabling the internal pull-up early during device boot.

    Regards,

    Kyle