J-Link / J-Trace User Guide (UM08001) - Microcontrollers

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 target
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 connected
to the JTAG port.
Examples
The following table shows a few sample configurations with 1,2 and 3 devices in different
configurations.
Device 0
Chip(IR len)
TDI Device 1 TDO TDI Device 0 TDO
TCK
TMS
TRST
Device 1
Chip(IR len)
TDI TDO
Device 2
Chip(IR len)
JTAG
Position IR len
ARM(4) - - 0 0
ARM(4) Xilinx(8) - 0 0
Xilinx(8) ARM(4) - 1 8
Xilinx(8) Xilinx(8) ARM(4) 2 16
Table 5.5: Example scan chain configurations
TCK
J-Link / J-Trace (UM08001) © 2004-2010 SEGGER Microcontroller GmbH & Co. KG
TMS
TRST
TCK
TMS
TRST
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