Determining Values For Scan Chain Configuration - Segger J-Link User Manual

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)
ARM(4)
ARM(4)
Xilinx(8)
Xilinx(8)
Table 5.5: Example scan chain configurations
J-Link / J-Trace (UM08001)
Device 1
TDI TDO
TDI
Device 1 Device 2
Chip(IR len) Chip(IR len)
- -
Xilinx(8) -
-
ARM(4)
Xilinx(8)
ARM(4)
Device 0
TDI
TDO
JTAG
Position IR len
0
0
1
2
© 2004-2011 SEGGER Microcontroller GmbH & Co. KG
TDO
0
0
8
16
103