ibis modeling understanding

[ankitgarg0312]

i read from some appnotes that ibis V-I curves contains tables of voltage and current
this voltage column for pullup is
vtable = vcc - vout

is above correct?
if yes, how does ibis specify when pullup or pulldown will work ( on what input voltage levels pullup/pulldown will be switched on)

V-T curves are extracted at which slew at input?
if slew changes how will output be calculated by simulator??

please help , i am breaking my head on understanding ibis files......

 

[gaurav1712]

i read from some appnotes that ibis V-I curves contains tables of voltage and current
this voltage column for pullup is
vtable = vcc - vout

is above correct?
if yes, how does ibis specify when pullup or pulldown will work ( on what input voltage levels pullup/pulldown will be switched on)

Yes, that is correct. The IBIS file contains the information I-V char from -Vdd to 2*Vdd. So, for a 3.3V device, the information will be from -3.3 to +6.6V.

Whats really happening is that for pulldown curve you turn off the pullup circuit and simulate the pulldown circuit with output voltages ranging from -Vdd to 2Vdd and measure the current.

The reason "vtable = vcc-vout" is used is to make the pullup and pulldown tables easy to compare.

V-T curves are extracted at which slew at input?
if slew changes how will output be calculated by simulator??

Does it matter what the slew of input is - as long as it fast enough! I do not see why you should worry about input slew for V-t curves

please help , i am breaking my head on understanding ibis files......

I hope this helps!

Success,
Gaurav

 

[ankitgarg0312]

i am trying to understand in terms of simulators ...
if i apply a pulse to a buffer, it will have its thresshold that above 2.5v pullup will work and below that pulldown will work .
where is this information in ibis?
how will a simulator understand when to switch on/off pullup/down circuit?
a input signal having swing of 1v or 5v should have different output , how is thing thing modelled in ibis?


regarding V-T curves
if my input signal slew changes then a cmos buffer will give different response for different slews..
how is this modeled in ibis?
what is meaning of fast enough signal??
signals can range from 1Mhz to 10GHZ , how will ibis buffer model slews for different input slews??

 

[gaurav1712]

in terms of simulator -

Both the pullup and pulldown are always to be used. Actually, all the I-V tables are always used. Any standard simulator with IBIS support will do that. So, for example, if there is a lot of leakage in pulldown, that effect will be show up in the result.

The V-t curves are there for only one simulation (and only one input). However, if u give a different input slew, the simulator should beable to handle that and give u relatively accurate result using the I-V tables. So, in a way I-V and V-t tables complement each other.

 

[ankitgarg0312]

thanks gaurav for your reply,

i have a ibis model, if i apply a 1v pulse i gets a 3v pulse at out and if i apply 0.5v pulse input i don't get any pulse at output.
as V-I curves are wrt output node voltage , how is input voltage controlling this behavior of buffer.
where is this information present in ibis models?