ARM ARM7TDMI - Page 225

Debug in Depth
ARM DDI 0210C Copyright © 2001, 2004 ARM Limited. All rights reserved. B-15
Figure B-3 on page B-14 shows the default version and part number. If necessary,
contact your supplier for the correct device identification code.
Operating mode When the IDCODE instruction is current, the ID register is
selected as the serial path between TDI and TDO. There is no
parallel output from the ID register.
The 32-bit device identification code is loaded into the ID register
from its parallel inputs during the CAPTURE-DR state.
The least significant bit of the register is scanned out first.
B.6.3 Instruction register
Purpose Changes the current TAP instruction.
Length 4 bits.
Operating mode In the SHIFT-IR state, the instruction register is selected as the
serial path between TDI and TDO.
During the UPDATE-IR state, the value in the instruction register
becomes the current instruction.
During the CAPTURE-IR state, b0001 is loaded into this register.
This value is shifted out during SHIFT-IR. On reset, IDCODE
becomes the current instruction.
The least significant bit of the register is scanned in and out first.
B.6.4 Scan path select register
Purpose Changes the current active scan chain.
Length 4 bits.
Operating mode SCAN_N as the current instruction in the SHIFT-DR state selects
the scan path select register as the serial path between TDI and
TDO.
During the CAPTURE-DR state, b1000 is loaded into this
register. This value is loaded out during SHIFT-DR, while a new
value is loaded in.
During the UPDATE-DR state, the value in the register selects a
scan chain to become the currently active scan chain. All further
instructions, such as INTEST, then apply to that scan chain. The
currently selected scan chain changes only when a SCAN_N
instruction is executed, or when a reset occurs. On reset, scan
chain 0 is selected as the active scan chain.