The propagation speed in a stripline (internal) trace is equal to (speed
of light) / sqrt (dialectric const).  That works out to about 180 ps/
in.   Assuming your traces are a few inches long, propagation delay of
the traces accounts for only a very small fraction of the delay.  To
reduce propagation delay by 2ns, you would have to decrease trace length
by 12 inches.  

Expensive materials have a higher dielectric constant and therefore more
propagation delay.  Microstrip traces may propagate slightly faster but
still only about 140ps/in.  85ps/in is the speed of light, which is the
ultimate speed limit of the universe.

It seems to me that the software will have to be modified to wait an
additional 2 ns, or the device it's getting data from will have to
generate its own clock/interrupt.  

CJ
-----Original Message-----
From: DesignerCouncil [mailto:[log in to unmask]] On Behalf Of
Balasubramanian Krishnan
Sent: Thursday, August 31, 2006 8:21 AM
To: [log in to unmask]
Subject: [DC] How to minimise PCB trace propagation time

Dear Experts,
      In one of our design we see that signal round trip delay (clock
'high
to low' transition and data arrival) is beyond the timing requirement of
the microcontroller used. The microcontroller expects the data to arrive
within 14 nanoseconds after the clock pulse is activated but the data is
reaching only after 16 nanoseconds delay and hence the device fails. No
over shoots or undershoots are noticed in the clock signal, the clock
frequency is 24 MHz with rise time of about 4  to 5 nano seconds. The
PCB
trace length cannot be reduced. Costly PCB material (for low relative
permittivity) is not acceptable being a cost effective product.
      How to handle this issue? Your expert guidance will be of great
use.


Sincerely

K.Balasubramanian
Project Leader - Hardware.

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