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5.3.3 Determining values for scan chain configuration
When do I need to configure the scan chain?
If only one device is connected to the scan chain, the default configuration can be
used. In other cases, J-Link / J-Trace may succeed in automatically recognizing the
devices on the scan chain, but whether this is possible depends on the devices
present on the scan chain.
How do I configure the scan chain?
2 values need to be known:
• The position of the target device in the scan chain.
• The total number of bits in the instruction registers of the devices before the tar-
get device (IR len).
The position can usually be seen in the schematic; the IR len can be found in the
manual supplied by the manufacturers of the others devices.
ARM7/ARM9 have an IR len of four.
Sample configurations
The diagram below shows a scan chain configuration sample with 2 devices con-
nected to the JTAG port.
Examples
The following table shows a few sample configurations with 1,2 and 3 devices in dif-
ferent configurations.
Device 0
Chip(IR len)
Device 1
Chip(IR len)
Device 2
Chip(IR len)
Position IR len
ARM(4) --00
ARM(4) Xilinx(8) - 0 0
Xilinx(8) ARM(4) -18
Xilinx(8) Xilinx(8) ARM(4) 216
Table 5.5: Example scan chain configurations
Device 1 Device 0
TDI TDITDO TDO
TDI
TDO
JTAG
TRST
TCK
TMS
TCK
TMS
TRST
TCK
TMS
TRST
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