( ESNUG 327 Item 12 ) ---------------------------------------------- [9/2/99]
Subject: Looking For Trouble -- "How Do I Synthesize A Fixed Time Delay?"
> I simply want to add a single buffer to delay a clock for driving a latch
> for a small amount, say, 20ns. Is it possible to synthesize a fixed time
> delay by some VHDL coding?
>
> - Chi Fung
From: Dave Pears <dpears01@harris.com>
Be very careful here. There really is no such thing as a "fixed delay"
in an actual device. Numbers given in the data sheets are most often
_worst_case_ meaning that the vast majority of devices will actually be
faster, hence less delay. Minimum delays are rarely given, so one really
has no way to control the delay created when a buffer is inserted.
Reminds me of that Mark Twain quote: "A man who carries a cat by the tail
learns something he can learn in no other way."
- Dave Pears
Harris Semiconductor
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From: Loc Mai <loc.mai@dreo.dnd.ca>
Add more buffers then observe the waveform on scope. I did this way in
my design. Good luck.
- Loc Mai
DND
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From: "Wade D. Peterson" <peter299@maroon.tc.umn.edu>
To the best of my knowledge, I've never seen a delay element like this in a
standard FPGA or ASIC device. If there was one, then you could use VHDL to
synthesize it.
Most people use one of the two following techniques to solve this problem:
1) Design your circuit assuming that all gates and nets have a very small,
but non-zero delay time. For example, you can design a reliable
synchronous circuits w/o the use of delay elements of any kind. For
examples of this, you can refer to our 'WISHBONE' interconnection
system on our website at http://www.silicore.net
2) If you really need a delay element, then you can add one external to
the part. There are quite a few companies that make delay line
devices, which give very precise timing delays. Kappa, Delay Devices
and Dallas Semiconductor are a few companies.
Also, you can make your own very predictable fixed delay elements with
inductor/capacitor parts.
- Wade D. Peterson
Silicore Corporation Minneapolis, MN
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