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66AK2G02: Pullup vs pulldown on ball L5 (TDI)

Debora
Intellectual 645 points

Part Number: 66AK2G02

Table 4-29 requires a pulldown on ball L5 (TDI) if unused. However according to table 4-1, this ball has a pullup during reset. Won't the pullup and the pulldown conflict?  Shouldn't it have a pullup like L3 and K4?

If the pulldown is required, what value is recommended to avoid conflict?

I am assuming that this ball counts as "unused" because though the net is brought to a JTAG header, the JTAG pod is not normally plugged in. Thus it is normally connected to a trace and connector with nothing driving it.  Inferring from a note later in section 4.5, this could result in a high current state which could damage the IO cell.

  • 0
    TI__Guru 65405 points
    Hi Debora,

    I've forwarded this to the HW design experts. Their feedback should be posted here.

    BR
    Tsvetolin Shulev
  • 0
    TI__Guru**** 161600 points
    Hi,

    Are you planning NOT to use JTAG on your board? If so, then go ahead and follow the Data Manual, Table 4-29. Unused Balls Specific Connection Requirements.

    If the pulldown is required, what value is recommended to avoid conflict?

    The most common value is 10k, you should be fine with it. The following guide may be useful to you, if you have future questions like this:
    www.ti.com/.../slva485.pdf

    If you are using the JTAG, then have a look at Section 2.19 JTAG and EMU of the Schematic Checklist (www.ti.com/.../sprac54.pdf).

    Best Regards,
    Yordan
  • 0 in reply to Yordan Kovachev
    Intellectual 645 points

    Hi Yordan,

    Thank you for the recommendation about 10K. I had thought there was something vulnerable about the I/O structure of these pins (based on the datasheet comments about the internal pullups/pulldowns being inadequate when traces were attached) so had thought that perhaps the value calculation would be different from usual and also that the traces on the JTAG lines would affect these pins whenever a pod wasn't plugged in even though we are using the JTAG.

    Can you confirm that the line about L5 in Table 4-1 Pin Attributes in the datasheet is correct and that L5 (TDI) has a pullup during reset?

    Regards,

    Debora

  • 0 in reply to Debora
    TI__Guru**** 161600 points

    Hi Debora,

    Yes, the ball reset state is Pull-Up.
    It is good to have a pulldown resistor on the TDI pin, if you don't use the JTAG. The recommended values are 1k to 10k, the lower the value the stronger the pull is. As I said typical values are 10k, also 4.7k, but in fact anything between 1k and 10k should do.

    Best Regards,
    Yordan