Re: IBIS - I/O Buffer Modeling Cookbook

Hi ,

There are another few questions about ibischk3 results:

1. I've got the next warning:
 WARNING - [Model] iovb has no description of the buffer's high state
                    DC drive characteristics (no [Pullup] table). This
warning
                    can be silenced by changing the Model_type or by adding
a
                    [Pullup] table.
This cell ('iovb') is 'Open_drain' type which according to Ibis maker cook
book do not
suppose to have [Pullup] table , only the [pulldown],[gnd_clamp] and
[power_clamp] tables ,
What do you think might be the reason of this error ?.

2. I've got the next warning:
WARNING (line 427) - Pulldown Typical data is non-monotonic
WARNING (line 427) - Pulldown Maximum data is non-monotonic
WARNING (line 428) - Pulldown Minimum data is non-monotonic
according to pulldn subtraction gnd_clamp cookbook and standard instructions
,
it is impossible to get monotonic data.
It is even mentioned in the standard that the [pulldn] or [pullup] tables
might be non-monotonic.
Is it all right to ignore this warning ?

With regards

Yoked.
"Ross, Bob" wrote:

> Hello Yoked:
>
> We are temporarily having reflector e-mail problems,
> so you message did not get sent to the general list.
>
> However, I am responding on the ibis-users reflector
> and copying Stephen Peters, who authored the Cookbook.
> My responses are in your text noted by "*" marks. These
> are brief responses without looking at the Cookbook
> document. I am trying to convey what was intended,
> not any editorial issues regarding how it is documented.
>
> Bob Ross
> Mentor Graphics
>
> ---------------------
>
> Subject: I/O buffer modeling cookbook
> From: Yoked Erlich (r58559) (r58559@email.sps.mot.com)
> Date: Sun Aug 12 2001 - 04:18:21 PDT
>
> Hi ,
>
> I would like to ask some questions about the 'I/O buffer modeling
> cookbook'
> paper (revision 2.0X) , Which I've got from 'www.eig.org' web site.
>
> 1. Is there anywhere I can find revision 3.2 cookbook?
>
> * No, a Version 3.2 Cookbook has not been written. Cookbook 2 was
> intended to cover Version 3.2 features, but we never got that far.
>
> 2. I've an I/O buffer model which working in some chip pads as I/O ,
> some pads as steady input and some pads as steady output.
> I would like to know if the buffer grouping is made by the buffer
> type or by the pad direction?
> If buffer type is the answer so one model with I/O type in enough
> for this buffer,
> If pad direction is the answer so three model is needed for this
> buffer , I/O model , Input model and Output model
> ( in this case Input and Output model tests will be exactly the same
> as I/O model ) .
>
> * If the buffer is truely an I/O, then it should be provided and
> documented as I/O. A user might have additional knowledge that
> certain I/O buffers are configured only as Input or Output buffers.
> It would be permissible to modify that instance of the model and
> just change the Model_type. That would help the EDA tool do
> only the simulations that are relevant for the design.
>
> 3-6 questions are about 'Extracting Data for the [Ramp] keyword'
> paragraph in the cookbook :
> 3. It is written that 'if the device does not have enough
> drive capability to make a significant output transition than a
> higher load may be used' ,
> How much is significant output transition ?
>
> * I do not have any hard guideline. I prefer impedances in the
> order of magnitude of transmission line loads since that is a
> typical high-speed design application. That ususally captures
> a fast edge that would be seen with a PCB interconnect.
>
> * This makes no sense for certain weak buffers which would
> require multiple reflections to change from one state to the
> other for an open line. If the application is not high-speed,
> then it does not manner. I would guess that a higher value
> load might be OK for a driver source impedance > 300 ohms.
> Others might have different ideas.
>
> 4. it is written that ' For an open drain , measure the rise and fall
> time into the load resistor and voltage used
> by the manufacture when specifying propagation delays' , This
> sentence is not clear
> what is the test structure in open drain buffer case?.
>
> * This is ambiguous. I prefer low impedance (50 ohm) pullups to
> capture a faster edge. This may be in the order of magnitude of
> the load seen during the intial part of the transition when the
> device is connected through a PCB interconnect.
>
> 5. it is written that ' Falling edge ramp data is captured with the load
> resistor tied to VCC'
> , Is VCC mean the pmos Driver source voltage (pad voltage) or low
> voltage?
>
> * This means that one end of the load resistor is connected to
> Vcc, and the other end is connected to the output pad.
>
> 6. It is written that 'simulations are performed with C_comp included in
> the circuit' , This sentence is not clear.
> As I understood C_comp is the capacitance seen when looking from the
> pad back into the buffer ,
> It is written that C_comp should be added to the simulation , but
> C_comp is not a capacitor it is capacitance measurement.
>
> * It is difficult to de-embed the die capacitance information from a
> Spice net list (since some of the contribution is due to paramaters
> within the transistor/diode models) or from a real physical measurements.
>
> * So the intent is to clarify that the die capacitance is already
> included in any waveform extraction. Since C_comp is a first order
> approximation of the die capacitance, its effects should be included
> in an EDA tool simulation. I have not looked at the specific
> wording, but I assume this is confusing.
>
> 7. In 'Ground Clamp' paragraph it is written that 'The data in table
> must cover the range of -Vcc to Vcc ' ,
> Isn't it suppose to be -Vcc to 2*Vcc ?
>
> * This response also covers 9.
>
> * No, the minimum range is intended to be -vcc to vcc. The Power
> Clamp covers from vcc to 2vcc. When this minimum range was
> established, most Inputs were at negligle current at Vcc and the
> slope (current vs voltage) was also nearly 0. So concatenation
> of the tables produced the same effect as extrapolation.
>
> * It is allowed to extend the clamp ranges beyond the specified
> range. In some cases you should do so, especially if you are
> including the effect of an internal terminator. In such cases,
> the conditions above are not met, and I prefer -vcc to 2vcc for
> both clamp tables. Care must be taken so that
> currents are not double counted. However, for many situations,
> the "minimal" IBIS guidelines are sufficient.
>
> 8. In 'pulldn' paragraph it is written 'If the buffer is a 3-state
> device then first subtract the ground clamp current
> from the pulldown current then enter the result into the [pulldown]
> table' ,
> first , it is written above the graph 'Pullup I/V curve after
> subtraction of ground clamp currents' Which is probably mistake.
> second , Should I replace the pulldn results in Ibis model file
> after the subtraction?
>
> * What is entered in the [Pulldown] table is the result after
> subtraction.
>
> 9. In 'Power Clamp' It is written that 'the data in the table must cover
> the range of vcc to vcc*2' ,
> Isn't the range suppose to be -Vcc to 2*Vcc ?
>
> * See 7 above.
>
> --
> With regards,
> Yoked.
> _____________________________________
> E-mail: yokede@msil.sps.mot.com
> Tel: 972-9-9522482 Fax: 972-9-9522888
> WIRELESS CIRCUIT - IO GROUP
> MOTOROLA SEMICONDUCTOR ISRAEL

--
With regards,
Yoked.
_____________________________________
E-mail:           yokede@motorola.com
Tel: 972-9-9522482 Fax: 972-9-9522888
WIRELESS     CIRCUIT  - IO      GROUP
MOTOROLA     SEMICONDUCTOR     ISRAEL