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ARM DDI0145B
Scan chain 1
Purpose
Primarily for debugging, although it can be used for EXTEST on
the data data bus DD[31:0] and the instruction data bus ID[31:0].
Scan chain 1 is selected via the SCAN_N TAP controller
instruction.
Length
67 bits.
This scan chain is 67 bits long, 32 bits for data values, 32 bits for instruction data, and
3 control bits, SYSSPEED, WPTANDBKPT, and DDEN. The three control bits serve
four different purposes:
•
Under normal INTEST test conditions, the DDEN signal can be captured and
examined.
•
During EXTEST test conditions, a known value can be scanned into DDEN to be
driven into the rest of the system. If a logic 1 is scanned into DDEN, the data data
bus DD[31:0] will drive out the values stored in its scan cells. If a logic 0 is
scanned into DDEN, DD[31:0] will capture the current input values.
•
While debugging, the value placed in the SYSSPEED control bit determines
whether the ARM9TDMI synchronizes back to system speed before executing
the instruction.
•
After the ARM9TDMI has entered debug state, the first time SYSSPEED is
captured and scanned out, its value tells the debugger whether the core has entered
debug state due to a breakpoint (SYSSPEED LOW), or a watchpoint
(SYSSPEED HIGH). If the instruction directly following one which causes a
watchpoint has a breakpoint set on it, then the WPT and BKPT bit will be set. This
situation does not affect how to restart the code.
Copyright © 1998, 1999 ARM Limited. All rights reserved.
Debug Support
5-23
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