[SI-LIST] Re: min/max trace length
- From: "Peterson, James F (EHCOE)" <james.f.peterson@xxxxxxxxxxxxx>
- To: "steve weir" <weirsi@xxxxxxxxxx>, "bernd schuster" <bernd.schuster12@xxxxxxxxxxxxxx>
- Date: Wed, 13 Aug 2008 07:59:03 -0400
One thing worth noting in the below question and comments. When it comes
to pcb trace timing, traces can be too long (setup times at the receiver
are violated, and traces can be too short also (hold times at the
receiver are violated).
Keep in mind, you can have a clock period of 1Hz and still violate your
hold time requirements in a conventionally clock circuit.
That is why you have to do the timing analysis. Do it for hold times
(answer will tell you the minimum trace length), and do it for setup
times (tells you the maximum trace length).
Regards,
Jim Peterson
Honeywell
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of steve weir
Sent: Wednesday, August 13, 2008 5:30 AM
To: bernd schuster
Cc: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: min/max trace length
Always solve for timing requirements. Flight time depends on physical
distance and effective eR. Stripline traces see the material eR,
microstrip see a mix of the material and air. Once you know the fixed
timng of your chips then you can start to build a budget for flight
time, and SI effects like loading, ringback, crosstalk induced push-out,
pull-in etc. At 100MHz SDR you can likely safely tolerate at least 3ns
flight time which would be at least 17.7".
From a practical standpoint flash memory timing is so slow that you
would have to have enormous trace runs before flight time becomes a
significant issue.
Steve
bernd schuster wrote:
> Hi,
> I often read minimum and maximum trace lengths in datasheets of sdram
chips
> (for example). As I figured out the length depends on the setup margin
and
> hold margin.
>
> If I have a setup margin of 2ns and hold margin of 0.8ns (e.g.) -
which
> formulas will tell me the min. and max. trace length? The system is
working
> at 100MHz with a fall time / rise tim of 2ns.
>
> How do I have to modify the formula when I want to calculate the
timing for
> an external flash memory (without clock traces)?
>
> best regards
> Bernd
>
>
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