Hello i have a question about the PCI-33 Mhz Clk signal trace in a pcb, i have found boards with somewhat zig-zag pci trace(CLK), what is the reason for this?
here is a good picture to show what i mean:
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.
Does pcad have this function somewhere?
SioL
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
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.
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here. All logos and trade names are the property of their respective owners.