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[FAQ] TMS570LC4357: Why should the nTRST signal on Hercules devices (TMS570LC43x, TMS570LSx, RM4x and RM57) be pulled LOW?

Part Number: TMS570LC4357

Should the nTRST JTAG signal of Hercules devices be pulled low on PCB board? 

  • nTRST is JTAG “Test Reset”. It is low active. The nTRST is used for an asynchronous reset of the state machine of the JTAG Test Access Port (TAP). The debugger drives it by a push-pull driver. It resets the TAP by a certain JTAG sequence. The target device requires that the nTRST is first held low for a few TCK cycles and then raised after the TCK signal has started running, so the device can detect a rising edge on nTRST.

    A pull-down resistor should be added onto nTRST on target side. It ensures the on-chip debug logic is inactive when the debugger is not connected.

    From the debugger point of view, nTRST is optional.