RE: [SI-LIST] : Critical Length

Abe Riazi ([email protected])
Mon, 29 Mar 1999 23:02:34 -0800

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

As for the Rambus channel example, the propagation Delay of 81.2
ps/in resulted form the product:
Delay = ( 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


Abe Riazi
[email protected]

>From: D. C. Sessions[SMTP:[email protected]]
>Sent: Monday, March 29, 1999 12:54 PM
>To: [email protected]
>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 = 2.0 ns/ft
>> Risetime = 1.5 ns
>> Let k = 1/6, then:
>> Critical Length = (1/6)(1.5 ns)/(2 ns/ft) = 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 = ( C )( Zo) = ( 2.9 pF/in)( 28 Ohms ) = 81.2 ps/in
>> Rise Time = 400 ps
>> Again letting k = 1/6, it follows:
>> Critical Length = 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.
>D. C. Sessions
>[email protected]
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