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The boundary-scan interface includes a state machine controller, 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:
When the boundary-scan interface is to be used, DBGnTRST must be driven LOW and then HIGH again.
When the boundary‑scan interface is not to be used, the DBGnTRST input can be tied permanently LOW.
A clock on CLK with DBGTCKEN HIGH is not necessary to reset the device.
The action of reset is as follows:
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.
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.
| Copyright © ARM Limited 1998-2000. All rights reserved. | ARM DDI 0084F |
| Non-Confidential | |
| PDF version | |
| Home > Debug in Depth > Resetting the TAP controller |