[SI-LIST] 答复: about PCI clock length

  • From: <myan@xxxxxxxxxxxxxx>
  • To: <weirsp@xxxxxxxxxx>
  • Date: Sun, 11 Apr 2004 20:05:23 +0800

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          1, If a typical figure for PCB propagation delay of 150ps per =
inch is used and PCI clock is 33MHz , the trace length of data signal is =
shorter than 5/0.15 =3D33.33 inch ,  is it right ?
        2. how  many inchs  is the clock trace length  longer than the =
data signal trace length ?
        3. In our  PCB :
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PCIdevice
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PCI device
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Motorola 8245
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10/100PCI device
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 10/100 PCI device
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       =20
             =20
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                      !                                 !                =
                        !                                  !             =
                            !
                                         PCI BUS
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                             =
=A3=A1=A1=AA=A1=AA=A1=AA=A1=AA=A1=AA=A1=AA=A1=AA=A1=AA=A1=AA=A1=AA=A1=AA=A1=
=AA=A1=AA=A1=AA=A1=AA=A1=AA=A1=AA=A1=AA      signal  length   30 inch    =
=A1=AA=A1=AA=A1=AA=A1=AA=A1=AA=A1=AA=A1=AA=A1=AA=A1=AA=A1=AA=A1=AA=A1=AA=A1=
=AA=A1=AA=A1=AA=A1=AA=A1=AA=A1=AA=A1=AA =A3=A1
                                    =20
          The clock length is 40inch and it is equal to each device=20
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          It can work ? ? ?            =20
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-----=D3=CA=BC=FE=D4=AD=BC=FE-----
=B7=A2=BC=FE=C8=CB: steve weir [mailto:weirsp@xxxxxxxxxx]=20
=B7=A2=CB=CD=CA=B1=BC=E4: 2004=C4=EA4=D4=C211=C8=D5 19:12
=CA=D5=BC=FE=C8=CB: si-list@xxxxxxxxxxxxx
=D6=F7=CC=E2: Re: [SI-LIST] about PCI clock length
=20
Myan,

1. PCI 66 MHz allows 5ns for propagation and settling, PCI 33 MHz allows =
10ns.  These are the real limitations that you need to be aware of.  =
Because PCI is not really terminated properly, the configuration of the =
drivers and the loads, and their locations will determine what you can =
actually get away with.

2. PCI 66 MHz allows 1ns skew.  A one inch difference is only about =
1/5th of that. =20

3. See (1).  PCI performs best when there are only driving devices at =
the far ends of the bus.  The more loads you place in the middle, the =
more your margins will erode.  All of that being said, there are plenty =
of PC's that run just fine with five PCI slots.

Steve.
At 05:18 PM 4/11/2004 +0800, myan@xxxxxxxxxxxxxx wrote:


I have some questions :
1. how long is the limit length of PCI clock trace length on PCB ?=20
2. Is there any skew between the PCI clock and data signals besides that =
PCI clock is 2.5inch and data signals is shorter than 1.5 inch on =
daughter card ?
3. is ther any problem that the PCI host is in the middle of the PCI bus =
and there are two PCI devices on the both sides of the PCI host=20
=20
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Steve Weir
Teraspeed Consulting Group LLC=20
2926 SE Yamhill St.=20
Portland, OR 97214=20
(503) 239-5536=20
http://www.teraspeed.com <http://www.teraspeed.com/>=20
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