B.2. Resetting the TAP controller

The boundary-scan interface includes a state machine controller called the TAP controller. To force the TAP controller into the correct state after power-up, you must apply a reset pulse to the DBGnTRST signal:

Note

A clock on CLK with DBGTCKEN HIGH is not necessary to reset the device.

The action of reset is as follows:

  1. System mode is selected. This means that the boundary-scan cells do not intercept any of the signals passing between the external system and the core.

  2. The IDCODE instruction is selected. When the TAP controller is put into the SHIFT-DR state, and CLK is pulsed while enabled by DBGTCKEN, the contents of the ID register are clocked out of DBGTDO.

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