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PCI Clk Signal Track

N

Noway2

Jan 1, 1970
0
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:
http://www.dearhoney.idv.tw/MUSEUM/CMEDIA_CMI8738.jpg

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.
 
K

Keith

Jan 1, 1970
0
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?


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.
 
Ayon kay Noway2:
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.

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"...
 
S

SioL

Jan 1, 1970
0
Noway2 said:
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
 
N

Noway2

Jan 1, 1970
0
SioL said:
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.
 
M

martin griffith

Jan 1, 1970
0
Does pcad have this function somewhere?

SioL
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
 
SioL said:
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
(http://www.pcisig.org/)

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 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
(http://www.pcisig.org/)

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
 
S

SioL

Jan 1, 1970
0
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
(http://www.pcisig.org/)

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).

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
 
S

SioL

Jan 1, 1970
0
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)

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

SioL
 
B

Brad Velander

Jan 1, 1970
0
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.
 
Brad said:
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.

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
 
N

Noway2

Jan 1, 1970
0
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.
 
S

SioL

Jan 1, 1970
0
Noway2 said:
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.

I'll give it a try, thanks.

SioL
 
B

Brad Velander

Jan 1, 1970
0
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.
 
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