[SI-LIST] Re: trace impedance difference in same signal group of Intel guide
- From: "Ken Cantrell" <Ken.Cantrell@xxxxxxxxxxxxxxxx>
- To: "fei xue" <harrison_cls@xxxxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Wed, 5 May 2010 07:35:44 -0600
Harrison,
Are the memory loads the same? One reason to lower the trace impedance is
to load match. If you have a heavier capacitive load, then you need to
lower the trace impedance.
Ken
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx
[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of fei xue
Sent: Wednesday, May 05, 2010 1:12 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] trace impedance difference in same signal group of
Intel guide
Hello all,
May I raise a question to all of you, as you know, Intel does more
constraints restrictions than any other semiconductor vendor especially in
MCH platform design guide (PDG). For example, in DDR2 control group signal
constraints (CKE, CS and ODT signals) of Intel Bigby 3210 MCH chipset, the
characteristic trace impedance of the DDR2 control group signals from MCH to
Channel A dimms is 37Ohm+/-15%, however the characteristic trace impedance
from MCH to Channel B dimms is 28Ohm+/-10%. The traces of Channel B dimms
are longer than Channel A dimms. My question are,
1, why is the trace characteristic trace impedance of Channel A dimms
different from impedance of Channel B dimms.
2, I checked the IBIS model, the output buffer model of Channel A dimms is
the same as model of Channel B dimms, they are 38.9Ohm (I got this from IBIS
VI curves), so it is not the reason of buffer output impedance.
3, I do not think the trace impedance's difference is because of trace
distance difference between Channnel A and Channel B, 37Ohm and 28Ohm are
big gaps.
4, PCB manufactory gave us feedback that they can not control trace
impedance within 28Ohm+/-10%, what they can do is controlling impedance
within 28Ohm+/-5Ohm, how to evaluate this impact? can we confirm with
manufactory that controlling impedance within 28Ohm+/-5Ohm is ok or not.
I appreciate if you have any hints on this.
Harrison
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