[IBIS] Comment on BIRD113.1, Weak Tie-up or Tie-down Resistance and Voltage

All

BIRD113.1 is up for discussion of Friday, October 1.

Some Weak tie-up or tie-down components look resistive. Others
formed from internal FET devices look more like constant
current sources over most of the range.

This distinction makes a difference if one is to use the Weak_*
information to determine the overall state of a particular
net.

We suggest adding Weak_I as an optional alternative to Weak_R.

The rules would be:

Weak_I and Weak_V (for supply reference information)

or

Weak_R and Weak_V

The current conventions for Weak_I are:

   Positive for tie-down (similar to [Gnd Clamp]
   Negative for tie-up (similar to [Power Clamp]

-----

Example 1.

Vdd is 5 V. Net has one device with 100 k to GND, and the other with
100 k to Vdd.

The resistor divider DC voltage is estimated to be 2.5 V and
state of the net is ambigous.

Example 2.

Same situation, except the device with a FET tie-up device is better
looks more like a -50 uA current source over most of its
range (instead of a 100 k resistor)

The DC output state is held High at 5 V.

-----

So the approximate shape has an impact on the calculated
DC state.

Bob

-- 
Bob Ross
Teraspeed Consulting Group LLC     Teraspeed Labs
121 North River Drive              13610 SW Harness Lane
Narragansett, RI 02882             Beaverton, OR 97008
401-284-1827                       503-430-1065
http://www.teraspeed.com           503-246-8048 Direct
bob@teraspeed.com
Teraspeed is a registered service mark of Teraspeed Consulting Group LLC
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