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TMS570LC4357: nTRST signal, pull up or pull down, and what happens if boards have the pull up installed?

Iceberg
Intellectual 565 points
Part Number: TMS570LC4357

Hi,

There's been a lot of confusion online about the nTRST signal.  Some reference boards have the signal weakly pulled up, while others have the signal weakly pulled down.  What happens to a board that has the signal pulled up and what are the implications of leaving that signal high all the time and having the debug system active?  Does it affect system performance or make the system more likely to hit random breakpoints?

Best,

Josh Karch

  • 0
    TI__Guru**** 196306 points
    Hello Josh,

    The nTRST resets the TAP and the debug logic on the chip. It has a weak internal pulldown resistor, so the nTRST is asserted as default, the device will not enter debug in error. You need to drive nTRST high through JTAG emulator when you want to debug. I recommend to use a pull-down or leave it without any pull on MCU PCB board, normally the external JTAG emulator has built-in pull-up to drive the nTRST high.

    If the external emulator drives the nTRST with an open drain output, the pull-up is required to make it work.
  • 0 in reply to QJ Wang
    Intellectual 565 points

    QJ, the issue we have is we currently have a board with this pullup resistor installed, so the signal is held high.  What negative impacts can happen if nTRST is held high while a debugger is not connected?  I know we can pull that resistor, but let's pretend we don't remove the pullup resistor.  What are the implications of not removing that pullup which is installed on a board performance wise, or does it enable any preset breakpoints to trigger even when the debug connector is not installed?

    Best

    Josh Karch

  • 0 in reply to Iceberg
    TI__Guru**** 196306 points

    Hello Josh,

    I recommend to pull-down nTRST signal. If you leave this signal at high, it may have a possibility to enter debug mode in error.