Resetting The Tap Controller - ARM ARM7TDMI Technical Reference Manual

Debug in Depth
B.2

Resetting the TAP controller

B-6
The boundary-scan (JTAG) interface includes a state machine controller named the TAP
controller. To force the TAP controller into the correct state after power-up, you must
apply a reset pulse to the nTRST signal:
• when the boundary-scan interface or EmbeddedICE-RT is to be used, nTRST
must be driven LOW and then HIGH again
• when the boundary-scan interface or EmbeddedICE-RT is not to be used, the
nTRST input can be tied permanently LOW.
Note
A clock on TCK is not necessary to reset the device.
The nTRST signal:
1. Selects system mode. This means that the boundary-scan cells do not intercept
any of the signals passing between the external system and the core.
2. Selects the IDCODE instruction.
When the TAP controller is put into the SHIFT-DR state and TCK is pulsed, the
contents of the ID register are clocked out of TDO.
3. Sets the TAP controller state machine to the TEST-LOGIC RESET state.
4. Sets the scan chain select register to
chain, if present.
Note
You must use nTRST to reset the boundary-scan interface at least once after power up.
After this the TAP controller state machine can be put into the TEST-LOGIC RESET
state to subsequently reset the boundary-scan interface.
Copyright © 2001, 2004 ARM Limited. All rights reserved.
0x3 , which selects the external boundary-scan
ARM DDI 0210C