ARM ARM7TDMI - Page 220

Debug in Depth
B-10 Copyright © 2001, 2004 ARM Limited. All rights reserved. ARM DDI 0210C
The EXTEST instruction connects the selected scan chain between TDI and TDO.
When the instruction register is loaded with the EXTEST instruction, all of the scan
cells are placed in their test mode of operation:
• In the CAPTURE-DR state, inputs from the system logic and outputs from the
output scan cells to the system are captured by the scan cells.
• In the SHIFT-DR state, the previously captured test data is shifted out of the scan
chain using TDO, while new test data is shifted in using the TDI input. This data
is applied immediately to the system logic and system pins.
B.5.2 SCAN_N (b0010)
The SCAN_N instruction connects the scan path select register between TDI and TDO:
• In the CAPTURE-DR state, the fixed value b1000 is loaded into the register.
• In the SHIFT-DR state, the ID number of the desired scan path is shifted into the
scan path select register.
• In the UPDATE-DR state, the scan register of the selected scan chain is connected
between TDI and TDO and remains connected until a subsequent SCAN_N
instruction is issued.
• On reset, scan chain 3 is selected by default.
The scan path select register is 4 bits long in this implementation, although no finite
length is specified. The least significant bit of the scan path select register is shifted in
and out first.
B.5.3 SAMPLE/PRELOAD (b0011)
This instruction is included for production test only and must never be used on the scan
chains provided by the ARM7TDMI core. It can be used on user-added scan chains such
as boundary-scan chains.
B.5.4 RESTART (b0100)
The RESTART instruction restarts the processor on exit from debug state. The
RESTART instruction connects the bypass register between TDI and TDO. The TAP
controller behaves as if the BYPASS instruction had been loaded.
The processor exits debug state when the RUN-TEST-IDLE state is entered.