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David Brownell <david-b@pacbell.net> Mention how parallel clock voting implementations of RTCK work, 

and reference TI's free VHDL code.

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oharboe 2009-07-12 14:08:16 +00:00
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@ -5661,6 +5661,18 @@ held device example'' - the adaptiveness works perfectly all the
time. One can set a break point or halt the system in the deep power
down code, slow step out until the system speeds up.
Note that adaptive clocking may also need to work at the board level,
when a board-level scan chain has multiple chips.
Parallel clock voting schemes are good way to implement this,
both within and between chips, and can easily be implemented
with a CPLD.
It's not difficult to have logic fan a module's input TCK signal out
to each TAP in the scan chain, and then wait until each TAP's RTCK comes
back with the right polarity before changing the output RTCK signal.
Texas Instruments makes some clock voting logic available
for free (with no support) in VHDL form; see
@url{http://tiexpressdsp.com/index.php/Adaptive_Clocking}
@b{Solution #2 - Always works - but may be slower}
Often this is a perfectly acceptable solution.