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TMS570LS3137: LS31x HDK schematic questions

Part Number: TMS570LS3137

Hi team,

We have a customer that has brought up a few questions about the LS31x HDK schematic and the absolutely required components. Please take a look at their observations and questions to it below:

-First, it shows the TCK input to the TMS570 with a 10Kohm pullup to 3.3V. But the datasheet says TCK has a fixed 100uA internal pulldown. And the Input current in the spec table in section 5.8 of the datasheet is 195mA max when the input is at VCCIO. So the internal pulldown can be as small as 3.3V/195uA = 16.9Kohm. So with this it looks like you have a 10K/16.9K voltage divider on the TCK input. And when not using JTAG, the TCK input could be sitting around 1.22V. Did the HDK designer really intend for this? If anything, shouldn’t they have put an external pulldown on TCK instead of a pullup so it wouldn’t be fighting the internal pulldown? Am I interpreting this correctly?

-Also, on TCK it shows AC termination with 100ohm and 22pF. Is this recommended for all design that use JTAG?

-It shows the nTRST pin connected to mosfet, gating the TRSTN from the emulator with POR_RESETn. Is this recommended for all designs? What would be the consequence on not doing this? Other chips don’t require this so wondering why it is like this.

Thanks in advance,

Billy

  • THanks Billy,

    I've forwarded to one of our HW experts. They should get back to you soon.
  • Hi Billy,

    It is better to pull-down the JTAG TCK signal externally. The AC termination on TCK is not required. AC termination is a good way for a clock signal with long transmission line. The TCK on HDK is routed to several headers and MCU.

    We recommended to use an external pull-down resistor only for nTRST.

    Regards,
    QJ