[SI-LIST] Re: Why SSTL_2 does not need 1_25V Pull Up in AGP card ?
- From: Mike Brown <bmgman@xxxxxxxxxx>
- To: chris.mcgrath@xxxxxxxx, si-list@xxxxxxxxxxxxx
- Date: Sun, 13 Oct 2002 21:04:36 -0500
Chris's remarks about reduced swing being an EMI benefit are certainly
true. The reduced swing and fast signalling are a result of the
termination of the net accomplished by the "pull-up" resistor.
The SSTL2 driver is a "totem pole" structure (A P-channel and N-channel
FET in series between VDDQ and VSS.) As such, it needs no "pull-up" to
establish a valid voltage level in either state. The driver swings
rail-to-rail under light loading, and beyond the rails with unterminated
transmission line loads. The so-called "pull-up" to VDDQ/2 is a
termination resistor, of 50 ohms more or less, presumably at one end
(Class 1, 8 ma driver) or both ends (Class 2, 16 ma driver) of the
interconnection trace. The resistor consumes DC power in both states
and is considerably more power-hungry than a series-terminated net would
be. There is a minimum of 8 (or 16) ma of current flowing all the
time, rather than just for the round-trip time of the interconnect.
Note that a 1.25v termination supply must both source and sink current.
Thevenin equivalent termination (100 ohms to VDDQ, 100 ohms to VSS)
does not have this issue, at the expense of higher power consumption
from the 2.5 volt supply.
Series termination also works with SSTL2, if you are concerned with the
power and can stand the time delay imposed by full-swing transitions and
the mid-swing porch on multi-drop nets. (Don't do this with a clock
net!) Other termination topologies are possible with SSTL2, within the
8 (or 16) ma Ioh and Iol specs of the driver. SPICE is probably needed
to prove the viability of anything other than a conventional topology.
The Vref in SSTL2 is a receiver threshold control and has almost nothing
to do with the 1.25 volt termination voltage other than VDDQ/2 being
near the "ideal" threshold value to minimize duty cycle distortion. DCD
is of great concern with DDR-clocked devices.
Regards
Mike
chris.mcgrath@xxxxxxxx wrote:
>SSTL2 isn't really "pulled up" to 1.25V. More accurately, this is the
>reference voltage (often referred to in data sheets as VREF) that the
>signals swing around. This bias allows for less voltage swing and this
>translates into a better EMI picture since the edge rates are slower yet can
>still achieve the same data transfer speeds. One would also expect that
>less power is required since the signals are not banging between the rails.
>
>-Chris
>
>
>
>
>>-----Original Message-----
>>From: Jack W.C. Lin [mailto:JackWCLin@xxxxxxxxxxxx]
>>Sent: Thursday, October 10, 2002 10:54 PM
>>To: si-list@xxxxxxxxxxxxx
>>Subject: [SI-LIST] Why SSTL_2 does not need 1_25V Pull Up in
>>AGP card?
>>
>>
>>Hi All SI friends:
>>We all know that in SSTL_2 spec, DDR signals need 1_25V pull
>>up. But I
>>found one thing in recently job for AGP 8X card simulation,
>>all DDR signals
>>need not 1_25V pull up! SSTL_2 input receiver is differential input
>>receiver, the signal should swing above/below on 1_25V. Why
>>does it still
>>can work properly?
>>Anyone provide reasonable explanation? Thanks
>>Jack
>>
>>
>>------------------------------------------------------------------
>>To unsubscribe from si-list:
>>si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>>
>>or to administer your membership from a web page, go to:
>>//www.freelists.org/webpage/si-list
>>
>>For help:
>>si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>>
>>List archives are viewable at:
>> //www.freelists.org/archives/si-list
>>or at our remote archives:
>> http://groups.yahoo.com/group/si-list/messages
>>Old (prior to June 6, 2001) list archives are viewable at:
>> http://www.qsl.net/wb6tpu
>>
>>
>>
>>
>------------------------------------------------------------------
>To unsubscribe from si-list:
>si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
>
>or to administer your membership from a web page, go to:
>//www.freelists.org/webpage/si-list
>
>For help:
>si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
>
>List archives are viewable at:
> //www.freelists.org/archives/si-list
>or at our remote archives:
> http://groups.yahoo.com/group/si-list/messages
>Old (prior to June 6, 2001) list archives are viewable at:
> http://www.qsl.net/wb6tpu
>
>
>
>
>
------------------------------------------------------------------
To unsubscribe from si-list:
si-list-request@xxxxxxxxxxxxx with 'unsubscribe' in the Subject field
or to administer your membership from a web page, go to:
//www.freelists.org/webpage/si-list
For help:
si-list-request@xxxxxxxxxxxxx with 'help' in the Subject field
List archives are viewable at:
//www.freelists.org/archives/si-list
or at our remote archives:
http://groups.yahoo.com/group/si-list/messages
Old (prior to June 6, 2001) list archives are viewable at:
http://www.qsl.net/wb6tpu
Other related posts:
