RE: [SI-LIST] : Definition of Hi-Speed (UC)

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From: Eric Bogatin (eric@bogent.com)
Date: Mon Nov 01 1999 - 10:52:14 PST


The definition of High Speed I use in the classes I teach is that high speed
is when the interconnects are no longer transparent to the signals. In this
regime, the electrical effects of the interconnects will affect signal
quality and if you don't take the interconnects into account, system
performance will be degraded.

As has been pointed out by many on this list, this criteria depends more on
the rise time of signals than on the clock frequency. In my last article on
ChipCenter (click on this link if you want to read it:
http://www.chipcenter.com/signalintegrity/ ), I wrote about the coming
crisis when even jelly bean parts soon will have sub 0.5 nsec rise times,
even if they are used in circuits with only 10 MHz clocks.

However, sometimes managers don't really understand about rise time, and
just think in terms of clock frequency. They just want a number and want you
to fill in the blank with your best guess.

So, given the caveat of the importance of rise time over clock frequency,
here's the rule of thumb I sometimes use to pick a number:

The starting clock frequency for high speed is F > 200 MHz/L, where L is the
longest length of the board, in inches.

If your product is a 10 inch board, like a PC mother board, high frequency
is any clock frequency above about 20 MHz. If your product is a PC card
(PCMCIA format), 4 inches in a side, high frequency is when the clock is 50
MHz and above.

Here are the assumptions behind this "rule of thumb":

1. the rise times of signals are about 1/10 the clock period

2. transmission line effects dominate (ie, the interconnect is not
transparent) when TD > 1/3 x rise time

3. the substrate is FR4 and the speed of a signal is about 6 inches/nsec.

Note, as with all rules of thumb, if you are trying to make a distinction
between is 25 MHZ or 35 MHz really the "onset" of high speed, you don't want
to use a rule of thumb. But, if you want to know is 10 MHz or 50 MHz high
speed, this rule of thumb is a pretty quick way of getting an answer.

For those interested, I have added a paper I wrote on "Rules of Thumb I Have
Known and Loved" to my collection of downloads for November. You are welcome
to download a pfd version of this by visiting my web site at:
<http://www.bogatinenterprises.com/>

--eric

Eric Bogatin
BOGATIN ENTERPRISES
Training for Signal Integrity and Interconnect Design
26235 W. 110th Terr.
Olathe, KS 66061
v: 913-393-1305
f: 913-393-1306
pager: 888-775-1138
e: eric@bogent.com
web: <http://www.bogatinenterprises.com/>

> -----Original Message-----
> From: owner-si-list@silab.eng.sun.com
> [mailto:owner-si-list@silab.eng.sun.com]On Behalf Of Beal, Weston
> Sent: Monday, November 01, 1999 11:22 AM
> To: 'si-list@silab.eng.sun.com'
> Subject: RE: [SI-LIST] : Definition of Hi-Speed (UC)
>
>
> The best definition I've ever heard of High-Speed was from someone on this
> list. Speak up or I'll take the credit.
>
> High-Speed is any design that has faster edge rates or clock rates than
> you've designed before.
>
> It's new to you, so you need to study and analyze it.
>
> Regards,
> Weston
>
>
> -----Original Message-----
> From: Lum Wee Mei [mailto:lweemei@dso.org.sg]
> Sent: Sunday, 31 October, 1999 10:54 PM
> To: 'si-list@silab.eng.sun.com'
> Subject: [SI-LIST] : Definition of Hi-Speed (UC)
>
> Pardon me for asking this stupid question because I am at a
> loss of how
> to explain hi-speed to my boss. He thinks that hi-speed is
> as simple and
> straightforward as resistance = V/I and nothing else.
> Hi-speed should be
> some circuits that need to operate at xxMHz or more.
> Anything less than
> xxMhz is not hi-speed.
>
> I would appreciate anyone of you experts out there who can
> enlighten me
> in a simple and easy to understand definition so that my
> boss can
> understand.
>
> Regards.
>
>
>
>
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