Forums

JTAG pin states

Started by Unknown February 2, 2004
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
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
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