Jtag Port Buffering; Figure 3-5 Jtag Connection Without Buffers; Figure 3-6 Jtag Connection With Tdo Buffer; Figure 3-7 Daisy-Chained Jtag Connection Without Buffers - ARM DSTREAM-PT Reference Manual

3.2

JTAG port buffering

JTAG buffering is sometimes required on the target board to improve signal integrity and increase the
usable bandwidth of the interface. You can implement JTAG buffering using common off-the-shelf parts,
at little cost.
Usually, the JTAG connector of a target system connects to a single device, for example:
Pull-up and pull-down resistors are omitted for clarity.
To act as a series terminator, you must place a resistor close to the TDO pin of target device. Placing a
resistor close to the TDO pin is the simplest option, and achieves good signal integrity because each
signal is point-to-point.
However, if the TDO output of the target device has a weak drive-strength (<4mA), the TDO output
could significantly limit the maximum frequency of the JTAG interface. To resolve this, place a buffer
close to the TDO pin of the target device with the appropriate series termination resistor:
Sometimes, two or more devices are chained together in the target system:
101714_0100_02_en
Note
Copyright © 2019 Arm Limited or its affiliates. All rights reserved.
Debug
Connector
TMS
TCK
TDO

Figure 3-5 JTAG connection without buffers

Debug
Connector
TMS
TCK
TDO

Figure 3-6 JTAG connection with TDO buffer

Debug
Connector
TMS
TCK
TDO

Figure 3-7 Daisy-chained JTAG connection without buffers

Non-Confidential
3 Target board design
3.2 JTAG port buffering
TDI
TDI
TDI
Target
Device
TDI
TMS
TCK
TDO
Target
Device
TDI
TMS
TCK
TDO
Target
Device
TDI
TMS
TCK
TDO
Target
Device
TDI
TMS
TCK
TDO
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