8.5. Reset

The boundary-scan interface includes a state-machine controller (the TAP controller). In order to force the TAP controller into the correct state after power-up of the device, a reset pulse must be applied to the nTRST signal.

If the boundary scan interface is to be used, nTRST must be driven LOW, and then HIGH again. If the boundary scan interface is not to be used, the nTRST input may be tied permanently LOW.


A clock on TCK is not necessary to reset the device.

The action of reset is as follows:

  1. System mode is selected (the boundary scan chain cells do not intercept any of the signals passing between the external system and the core).

  2. The IDCODE instruction is selected. If the TAP controller is put into the Shift‑DR state and TCK is pulsed, the contents of the ID register is clocked out of TDO.

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