Re: [SI-LIST] : Some Semiconductors are Unnecessarily Fast

Mike Degerstrom (degerstrom.michael@mayo.edu)
Thu, 15 Jul 1999 09:07:15 -0500

Raymond,

I'd also like to know more about the edge-rate control with use of
a series resistor as has been mentioned is used with the Analog
Devices 21020. I have seen mention of parts that use a reference
resistor to control output impedance, i.e., drive strengths such
as in the HP article by Esch, and Manley. I also noticed that
IBM sells SRAMs with programmable impedance control drivers.

Does anyone have a link to the AD 21020?

Mike

On Jul 15, 1:39pm, raymondl@qsa.qualitysemi.com wrote:
> Subject: Re: [SI-LIST] : Some Semiconductors are Unnecessarily Fast
> I have been a little bit curies about using a series resistor to control the
> edge
> rate. In my past projects, I usually used the resistor to control the
> equivalent
> output resistance, and left the edge rate controlled by some other internal
> circuits in the IC. Especially in point-to-point connection, well controlled
> output
> resistance at the driver already means termination. I have tried to reach to
> the web site of Analog Devices but was unsuccessful to get any information
> about 21020.
>
> About the series resistor, it is of course more cost effective by putting it
in
> the IC. However, the process and temperature variations can easily change
> the diffusion resistor by more than 25% (some foundry spec. even looser).
> Together with the PCB trace tolerance, it is very difficult to use diffusion
> resistor to provide a good matching of the driver resistance to the PCB
trace.
>
> Regards,
> Raymond
>
>
>
>
>
> Steve Weir <weirsp@a.crl.com> on 15/07/99 10:04:03
>
> Please respond to si-list@silab.eng.sun.com
>
> To: si-list@silab.eng.sun.com
> cc: (bcc: Raymond Leung/QSA/AU)
>
> Subject: Re: [SI-LIST] : Some Semiconductors are Unnecessarily Fast
>
>
>
>
> I have been looking for a fabricator who supports this, but have not found
> one. Does anyone know of one?
>
> Regards,
>
> Steve.
> At 02:20 PM 7/14/99 -0500, you wrote:
> >
> >
> >All,
> >
> >I'm also hearing that the microwave guys are using thick film printed
> >resistors,
> >as part of the etch, for termination elements - even on internal signal
> layers.
> >Anybody out there willing to share experiences?
> >
> >
> >Regards,
> >
> >Roy
> >
> >
> >
> >
> >
> >
> >Philip Gantt <pgantt@cisco.com> on 07/14/99 12:42:09 PM
> >
> >Please respond to si-list@silab.eng.sun.com
> >
> >Sent by: Philip Gantt <pgantt@cisco.com>
> >
> >
> >To: si-list@silab.eng.sun.com
> >cc: (Roy Leventhal/MW/US/3Com)
> >Subject: Re: [SI-LIST] : Some Semiconductors are Unnecessarily Fast
> >
> >
> >
> >
> >Has anyone considered that if a more resistive material (other than
copper)
> >were
> >used for PCB laminates, the inherent resistance of the conductors would
slow
> >the
> >edge rates and also reduce EMI? This might eliminate the need for
> >external resistors as well. Just a thought...
> >
> >Jon Keeble wrote:
> >
> >> I've seen one or two interesting approaches to controlling edge speeds.
> >>
> >> The first was a method of controlling the edge speed:
> >> The Analog Devices 21020 DSP chip has an external resistor for setting
> >> edge speeds (all outputs). I've no idea how this works internally.
> >>
> >> The other method reduced the production spread of edge speeds:
> >> It seems that the parts that run faster also have faster edges.
> >> The method involved
> >> - an onboard oscillator that ran at a speed determined by the speed of
> >> the device
> >> - internal control on the output drivers to vary the edge speed
> >> - a circuit that translated the oscillator speed to more or less drive
> >> on the outputs
> >>
> >> As a designer, it is obvious tpo me that there is a real need for
> >> control over edge speeds.
> >>
> >> I find it really exhausting to have to add hundreds of external
> >> resistors, not to mention the additional costs, numbers of parts,
> >> numbers of nodes to test etc etc.
> >>
> >> I'd encourage the IC designers to be a bit more inventive.
> >>
> >> Jon Keeble.
> >>
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>-- End of excerpt from raymondl@qsa.qualitysemi.com

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Mike Degerstrom         Email:    degerstrom.michael@mayo.edu	
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