Re: IBIS OD and ECL

Greetings,

I just want to make a short comment on the last paragraph of Steven's EMAIL
below. I agree with Steven's comment regarding the change in the shape of
the CMOS V/I curves when the power supply is not stable.

I already collected families of curves of a few devices to study the effects
of the Vcc voltage change. It seems to me that this change in the V/I curve
is proportional for CMOS devices and I am hoping to be able to derive a scaling
factor which would allow for using the Vcc as a scaling variable to change the
V/I curve on the fly. I also must mention that bipolar devices behave quite
differently, which needs some more thought.

I would also like to use a similar scaling factor to make adjustments on the
V/I curve due to the effects of temperature. This way we could eliminate the
need for best and worst case V/I tables and do all of that just by using two
scaling factors for Temp. and Vcc variations. Any comments?

Arpad Muranyi
Intel Corporation

Greetings Gentleman--

     I'm back from vacation and have had a chance to read and think about
the mail from Siuki Chan and others. I have a few comments and
clarifications.

    1. The [Voltage HIGH Range] and [Voltage LOW Range] key words that
        Kellee C. proposed were for a device whose output swings both
        positive and negative around ground, i.e. a device with 3 supplies:
        gnd, +V and -V. Siuki Chan is correct in stating that these key
        words are not strictly necessary for a device with only two supply
        rails and whose output only swings one way with respect to ground.

    2. I agree with Siuki C. that one can theoretically determine, from the
        point at which current = 0, which supply rail the data in the pullup or
        pulldown tables is referenced to. However, remember that IBIS
        requires that the data in the pullup table be 'VCC relative'.
        This means that for BOTH cases (pullup and pulldown), current = 0
        at V = 0 (or there abouts). The unspoken assumption was
        that in the pullup case V = 0 really meant V = VCC, and in the
        pulldown case V = 0 really was V = 0. In the case of ECL, the
        assumption about the pulldown case is violated. That is why I
        believe we need to make it explicitly known to the simulator that
        the pulldown curves are referenced to the most positive supply.

    3. Their is one more point that Siuki Chan raised earlier that I am
        now beginning to understand. It may not effect ECL or OD type parts
        but it is fundamental to complementary pair CMOS type
        outputs.
             The rational behind making the data in the pullup table VCC
        relative was to allow the simulators to be able to handle shifts
        in VCC. For example, suppose a simulation in run with VCC set
        to +5.2V, using data from a part whose curve was taken at VCC =
        5.0V. The simulator should be able to just shift the data point
        on the curve by 0.2V, IF THE SIGNIFICANT OPERATING PARAMETERS OF THE
        PULLUP XSISTOR HAVE NOT CHANGED. Now for ECL, this assumptions
        holds true. Both the base and collector of the output emitter
        follower are tied to VCC so neither Vcb or Ib changes. For an
        open drain or open collector type part again, changes in VCC do
        not effect the output xsistor. However, for a standard CMOS
        output, changing VCC changes Vgs on the pullup xsistor. This
        means that the SHAPE of the V-I curve has changed. The question
        is whether this change is enough to make a difference. Do
        any of the simulation vendors care to comment?

        I hope to hear from everyone at the meeting Friday.

        Thanks,
        Stephen Peters
        Intel Corp.
Received on Thu Aug 19 08:48:38 1993