JTAG pin states

[Andrew Greensted]

Hi All,
I'm building a JTAG chain demultiplexer so I can control 30 identical
JTAG chains from a single point.
Can anyone confirm the quiescent (not doing anything) states of the JTAG
pins; so I know what to hold the pin levels at when a chain is not being
used.
I see that TMS, TCK & TDI have internal pull-ups, so would I place a
logical 1 on these when they're not in use? TDO is easy enough to sort
out as it just goes high Z.

Many Thanks
Andy

--
Andrew Greensted Department of Electronics
Bio-Inspired Engineering University of York, UK

Tel: +44(0)1904 432379 Mailto: ajg112@ohm.york.ac.uk
Fax: +44(0)1904 433224 Web: www.bioinspired.com

 

[Andrew Greensted]

Just for anyone who searches in the future.
It seems that TDI and TMS are usually pulled high within target devices,
however externally pulling them high (4.7k) is probably a good idea. TCK
and TDO can be left floating.

This may help:
http://support.xilinx.com/xlnx/xil_ans_display.jsp?getPagePath=11433

Andrew Greensted wrote:

Hi All,
I'm building a JTAG chain demultiplexer so I can control 30 identical
JTAG chains from a single point.
Can anyone confirm the quiescent (not doing anything) states of the JTAG
pins; so I know what to hold the pin levels at when a chain is not being
used.
I see that TMS, TCK & TDI have internal pull-ups, so would I place a
logical 1 on these when they're not in use? TDO is easy enough to sort
out as it just goes high Z.

Many Thanks
Andy

--
Andrew Greensted Department of Electronics
Bio-Inspired Engineering University of York, UK

Tel: +44(0)1904 432379 Mailto: ajg112@ohm.york.ac.uk
Fax: +44(0)1904 433224 Web: www.bioinspired.com

 

[Lasse Langwadt Christense]

Andrew Greensted wrote:

Just for anyone who searches in the future.
It seems that TDI and TMS are usually pulled high within target devices,
however externally pulling them high (4.7k) is probably a good idea. TCK
and TDO can be left floating.

This may help:
http://support.xilinx.com/xlnx/xil_ans_display.jsp?getPagePath=11433

Andrew Greensted wrote:

Hi All,
I'm building a JTAG chain demultiplexer so I can control 30 identical
JTAG chains from a single point.
Can anyone confirm the quiescent (not doing anything) states of the
JTAG pins; so I know what to hold the pin levels at when a chain is
not being used.
I see that TMS, TCK & TDI have internal pull-ups, so would I place a
logical 1 on these when they're not in use? TDO is easy enough to sort
out as it just goes high Z.

Many Thanks
Andy

Making sure TMS stays high is a good idea, because them any glitches on
TCK will just keep (or get) the TAP controller in reset state

-Lasse