PCI Clk Signal Track

In high speed design, it is sometimes necessary to match the length of signal traces so that the signal edges occur in sync. For example, it may be necessary to make ensure that the clock signal is in sync with the data bus and that all data bus line transitions reach the receiver at the same time. To accomplish this, many PCB tools will allow you to add 'delay' to a line by putting in the "trombone" sections like appear at the bottom of the picture you linked to. You may have also noticed that these traces are rounded in the curves. This has the effect of keeping the trace width constant to help control the characteristic impedance of the line.

It's part of the PCI spec (IIRC the clock trace i specified to be

2.5" from the edge tabs to the chip). I've had designs work without it, but considered myself lucky.

--
  Keith

i have a similar pci card, which when i measured the 'tromboned' track its about 1.919" almost 2", this trace works in 33Mhz, in spec it says that pci clk should be 2.5"...

Does pcad have this function somewhere?

SioL

I am using PCAD 2001 and it does not have this function that I know of. I looked briefly at the 2004 version but I didn't see much difference in that version from the 2001 copy. I haven't looked at the 2006 version. Altium, which is discontuing PCAD is pushing everyone towards their designer package, which probably has this feature but I can't say for certain.

I do know that Cadence / Allegro has this feature as I saw it demonstrated and was significantly impressed.

As an aside (this is semi off topic, but your question made me think of it), Cadence is probably the route that I will go when I finally do update my PCB tools. Having evaluated both Altium and Cadence, I felt that the Cadence tools were better. I say better in that I felt that the emphasis was more on the PCB design rather than attempting to be a "master control program" that implements the interface to all of the tools you use such as your software development package and FPGA design package.

check the utils directory and look for SIA.exe. This is in my tango PCAD from '96. there maybe better utils in your version

eg

Net Copper Manhattan Total Violation Length Length Length Length Difference

------------------------------------------------------------------------

Assignments

------------------------------------------------------------------------

12V 2793.23 775.00 3568.23 VIOLATION + 2568.23

martin

I have not used PCad in some years, but you will be looking for an attribute that sets a specific length on a net (assuming this is an addin card, the net will be point to point).

I suggest searching for that.

Note that if you are designing a PCI add-in card, *all* the signals have required lengths. Get the spec and read it

If you think you haven't enough time, I am sure you will consider you should have after the board doesn't work (highly likely if you don't pay attention to the relative lengths).

As noted elsewhere, I particularly like the Cadence toolset as it can import such rules directly from OrCad and other Cadence tools.

Cheers

PeteS

I have crossposted this to sci.electronics.cad as there are more likely to be current PCAD users there.

You're looking for an attribute or constraint that requires a net to be some specific length with a set tolerance (for the PCI CLK it's 2.5 +/-

0.1 inch according to my handy copy of the spec here)

Cheers

PeteS

I am at this stage, although not having gold plated edge connector is suspect. Although I've been reading the spec and only noticed the requirements for clock line, capacitors etc, not for all the signal lines.

SioL

It sure is a pain in the butt to measure a zig-zagged line from a printout.

SioL

Can't measure it in the program? Most tools have some sort of function for that, although some of them are certainly less than optimal. Some will give you the details if your query the net in a certain manner or with a certain tool/command.

--
Sincerely,
Brad Velander.

"SioL"  wrote in message 
news:w7Zqg.4161$oj5.1410300@news.siol.net...
>  wrote in message

> It sure is a pain in the butt to measure a zig-zagged line from a 
> printout.
>
> SioL

It's even better if the tool will permit you to set the constraint and DRC it if you are outside the constraint (total track length in this case). I do not know if PCad has that functionality.

Cheers

PeteS

PCAD will allow you to measure the trace length, and it is pretty easy to do in the PCB tool. You can also set a design constraint on the net and a DRC will tell you if you are within spec.

The feature that I saw in the Cadence tool that I liked is that it will automatically generate the Trombones (or bevels or squares depending on your preference) for you automatically to generate a given trace length. It also shows you the trace length as you route it and changes colors according to whether or not you are within tollerance of the length.

I'll give it a try, thanks.

SioL

SioL, I figured that it should but I haven't used PCAD since approx. 1991, back in the good ole DOS days. These days I am using P99SE and they have a tool that works very well for these types of measures but it is hidden in the software and you have to do your own customizations to access the feature, sort of an undocumented semi-hidden feature.

-- Sincerely, Brad Velander.