[SI-LIST] Re: Termination of un-used clocks

Mike,

Consider that in normal operation, you would probably employ a series=20
termination to match the line which ideally would be open at the end.  In=20
reality, the load presents a small capacitance.  Now, if you plan to have=20
your board operate normally like this, then removing the load capacitance=20
of the receiving IC is not going to impact your EMC.  But since you have a=
=20
source termination resistor, assuming that it is not part of a pack, if you=
=20
are really concerned, you can open the line by not populating the resistor.

If you place a parallel resistive load at the far end of the line, as Tom=20
says, you will have more current flow, and it is the magnetic induction of=
=20
high speed currents that causes the most trouble.

Regards,


Steve.
At 02:18 PM 2/25/2003 +0800, =3D?gb2312?q?Bi=3D20Han?=3D wrote:

>if you do not terminate them, "open" is also a kind of termination and=20
>will cause reflection. If there are sensitive circuit, such as sense=20
>amplifier, the SI may be corrupted.
>However, if you terminate them to ground, the "short" matching condition=20
>will also cause reflection and even draw great current.
>Will a termination resistor work in mix-mode circuit?
>Can any expert give more detail analysis?
>Thanks!
>BH
>
>  Tom Dagostino <tom@xxxxxxxxxxxxx> wrote:
>If you terminate them you will have more current flowing in the part's
>supply pins causing more power dissipation, more Ldi/dt voltage drops in
>both the ground and power supply leads and more noise generated in the=
 clock
>lines themselves, the power supply and as potential EMI sources.
>
>I cannot think of any argument in favor of terminating them.
>
>Tom Dagostino
>Teraspeed Consulting Group LLC Teraspeed Consulting Group LLC
>2926 SE Yamhill St. Device Modeling Division
>Portland, OR 97214 13610 SW Harness Lane
>Beaverton, OR 97008
>http://www.teraspeed.com 503-430-1065
>tom@xxxxxxxxxxxxx
>
>
>-----Original Message-----
>From: si-list-bounce@xxxxxxxxxxxxx
>[mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of Kuriakose, Anand
>Sent: Monday, February 24, 2003 9:02 PM
>To: 'si-list@xxxxxxxxxxxxx'
>Subject: [SI-LIST] Termination of un-used clocks
>
>
>
>Hi,
>
>Whats the better way of dealing with un-used high speed ( 66Mhz to about
>133Mhz) clocks? Should they be terminated near the source to ground or to=
 be
>left unconnected.
>
>I happen to check out a similar discussion in the si-list archive, but
>coudnt come to any conclusion.
>
>Regrds,
>Anand.
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