RE: [SI-LIST] : Critical Length

=?Big5?B?u6+kaLPHIChzY2NoYW8p?= (scchao@cri.tatung.com.tw)
Tue, 30 Mar 1999 15:35:14 +0800

Abe Riazi:
I think there is something wrong with your calculation about =
propagation
delay.
The propagation delay for the light in vacuum is about 85 ps/in which =
is
longer then your trace delay. It=A1=A6s impossible!

Regards,
Shih-Chieh Chao
E-mail: scchao@cri.tatung.com.tw <mailto:scchao@cri.tatung.com.tw>=20

-----Original Message-----
From: Abe Riazi [mailto:ariazi@anigma.com]
Sent: Tuesday, March 30, 1999 3:03 PM
To: 'si-list@silab.eng.sun.com'
Subject: RE: [SI-LIST] : Critical Length

D. C. Sessions:

Thank you for your response. I like to explain how the
values used in
the Critical length examples had been selected.

In regard to the PCI Bus example, the Rise Time of 1.5
ns was chosen
because many models provided for simulation of PCI Bus
environment have
edge rates near that value. Furthermore, 1.5 ns meets the
PCI Slew
rate requirements (i.e. 1- 4 V/ns for 3.3 V and 1- 5 V/ns
for 5.0 V
signaling). =20

As for the Rambus channel example, the propagation Delay
of 81.2
ps/in resulted form the product:
Delay =3D ( 2.9 pF/in )( 28 Ohms)
The 28 Ohms was obtained from a Rambus document and
should be valid.
The 2.9 pF/in was extracted from High-Speed Digital Design A
Handbook of
Black Magic, and now I feel that it is too low for Rambus
channel
computations. A recent publication states 200ps to 400 ps
edge rates
for Rambus bus. I have seen slower rise times in driver
models furnished
for Rambus simulations. Therefore, value of 400 ps was
employed in this
example. =20

Regards,

Abe Riazi
ariazi@anigma.com

>----------
>From: D. C. Sessions[SMTP:dc.sessions@vlsi.com]
>Sent: Monday, March 29, 1999 12:54 PM
>To: si-list@silab.eng.sun.com
>Subject: Re: [SI-LIST] : Critical Length
>
>Abe Riazi wrote:
>
>> Presented below is a numerical example, which
utilizes Rise Time and
>> Delay values within the PCI Bus specifications:
>> Delay =3D 2.0 ns/ft
>> Risetime =3D 1.5 ns
>> Let k =3D 1/6, then:
>> Critical Length =3D (1/6)(1.5 ns)/(2 ns/ft) =3D 1.5
Inches
>
>You're going to have a VERY hard time (outside of edge-rate
controlled
>stuff like USB) finding anything with an edge that slow.
>
>> Let us consider another example, selecting values
applicable to the
>> Rambus Channel:
>> Delay =3D ( C )( Zo) =3D ( 2.9 pF/in)( 28 Ohms ) =3D 81.2
ps/in
>> Rise Time =3D 400 ps
>> Again letting k =3D 1/6, it follows:
>> Critical Length =3D 0.821 Inches
>> This result implies that trace segments and stubs
smaller than 0.824
>> inches are lumped, whereas those larger than 0.824 inches
are classified
>> as distributed. Such short critical lengths are often
accompanied by
>> numerous design and simulation challenges.
>
>Your numbers are off. 800ps/in is the speed of light in
vacuum.
>Actual PWB speeds range from about 160ps/in for microstrips
to
>almost 200ps/in for striplines. For RAMBUS, because of the
noise
>constraints and low impedance, you have to figure on
stripline
>with the consequent 190+ ps/in. Also, the edge rates are
not
>_minimum_ 400ps; IIRC that's the receiver-guaranteed-max.
>
>So figure on 250ps edges and 190 ps/in and you end up with
an edge
>in about 1.2 inches. That's a critical length at 1/6 of
200 mils,
>not over 800.
>
>
>--=20
>D. C. Sessions
>dc.sessions@vlsi.com
>
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