About Jtag Port Buffering; Figure 2-7 Jtag Connection Without Buffers; Figure 2-8 Jtag Connection With Tdo Buffer - ARM DSTREAM-ST System And Interface Design Reference Manual

2.6

About JTAG port buffering

JTAG buffering is sometimes required on the target board to improve signal integrity and increase the
usable bandwidth of the interface. This can be achieved using common off-the-shelf parts at very little
cost.
In most cases, the JTAG connector of a target system connects to a single device, for example:
Pull-up and pull-down resistors are omitted for clarity.
A resistor must be placed close to the TDO pin of target device to act as a series terminator. This is the
simplest scenario, and it is easy to achieve good signal integrity since each signal is point-to-point.
But, if the TDO output of the target device has a very weak drive-strength (<4mA), this could
significantly limit the maximum frequency of the JTAG interface. You can resolve this by placing a
buffer close to the TDO pin of the target device with the appropriate series termination resistor:
Sometimes, two or more devices are daisy-chained together in the target system:
ARM 100893_0100_00_en
Note
JTAG
CONNECTOR
JTAG
CONNECTOR
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2 ARM

Figure 2-7 JTAG connection without buffers

Figure 2-8 JTAG connection with TDO buffer

Non-Confidential
DSTREAM-ST target interface connections
®
2.6 About JTAG port buffering
ARM
Processor/ASIC
TDI
TMS
TCK
TDO
ARM
Processor/ASIC
TDI
TMS
TCK
TDO
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