All,
The following is from IBIS 5.0, since IBIS 5.1 has some editorial errors
converting GND to the schematic ground symbol erroneously.
Page 34
| C_comp_pullup, C_comp_pulldown, C_comp_power_clamp, and
| C_comp_gnd_clamp are intended to represent the parasitic
| capacitances of those structures whose I-V characteristics
| are described by the [Pullup], [Pulldown], [POWER Clamp]
and
| [GND Clamp] I-V tables. For this reason, the simulator
| should generate a circuit netlist so that, if defined,
each of
| the C_comp_* capacitors are connected in parallel with
their
| corresponding I-V tables, whether or not the I-V table
exists.
| That is, the C_comp_* capacitors are positioned between
the
| signal pad and the nodes defined by the [Pullup
Reference],
| [Pulldown Reference], [POWER Clamp Reference] and [GND
Clamp
| Reference] keywords, or the [Voltage Range] keyword and
GND.
|
| The C_comp and C_comp_* subparameters define die
capacitance.
| These values should not include the capacitance of the
| package. C_comp and C_comp_* are allowed to use "NA" for
the
| min and max values only.
Page 35
| The C_comp and C_comp_* subparameters define die
capacitance.
| These values should not include the capacitance of the
| package. C_comp and C_comp_* are allowed to use "NA" for
the
| min and max values only.
Page 36:
C_comp 7.0pF 5.0pF 9.0pF
C_comp_pullup 3.0pF 2.5pF 3.5pF | These four can
be
C_comp_pulldown 2.0pF 1.5pF 2.5pF | used instead of
C_comp_power_clamp 1.0pF 0.5pF 1.5pF | C_comp
C_comp_gnd_clamp 1.0pF 0.5pF 1.5pF
Page 64
| |<-------------TERMINATOR Model--------------->|
|
| [Voltage Range] or
| [POWER Clamp Reference]
| o
| |
| POWER_ o---o---o
| clamp | |
| |--o--| \
| | | /
| | I-V | \ Rpower [Package] Keyword
| | | / Subparameters *
| |--o--| | |<----------------->|
| | |
| | | PIN
| o-----o-------o-----o-----/\/\/\--@@@@@@---o--o
| | |GND_ | | R_pkg L_pkg |
| | |clamp | | |
| | |--o--| | | |
| | | | \ | |
| | | I-V | /Rgnd | |
| | | | \ \ |
| | |--o--| / / Rac |
| | | | \ |
| | o---o---o / |
| | | | |
| C_comp === o === Cac C_pkg ===
| | GND or | |
| | [GND Clamp | |
| | Reference] | |
| o-------------------o----------------------o
| |
| o
| GND
Page 73
| All tables assume that the die capacitance is included.
| Potential numerical problems associated with processing
the
| data using the effective C_comp (or C_comp_* values as
| appropriate) for effective die capacitance may be handled
| differently among simulators.
Page 74
| The diagram below illustrates a general configuration from
| which a [Rising Waveform] or [Falling Waveform] is
extracted.
| The DUT die shows all of the available power and ground
pin
| reference voltage terminals. For many buffers, only one
| power pin and one common ground pin terminal are used.
The
| absolute GND is the reference for the V_fixture voltage
and
| the package model equivalent network. It can also serve
as a
| reference for C_comp, unless C_comp is optionally split
into
| component attached to the other reference voltages.
Page 75 (Presumably the "diagram below"):
| [External Reference] - (used only for non-driver modes)
|
| | [POWER Clamp Reference]
| |
| | | [Pullup Reference] - (the power reference terminal)
| | | ____
| | | | | [Composite Current]
| | | | V
| | | | PACKAGE | TEST FIXTURE
| _|__|__|_ |
| | DUT | L_dut R_dut | L_fixture R_fixture
| | die |---@@@@@--/\/\/\--o-----|--@@@@---o---/\/\/\---
V_fixture
| |_________| | | |
| | | | | |
| | | | | |
| | | C_dut === | === C_fixture
| | | | | |
| | | |_____|_________|____________
| | | | GND
| |
| | [Pulldown Reference]
|
| [GND Clamp Reference]
There are several realities here
1. The physics
a. C_comp must be connected as up to four capacitors to the up to
four voltage rails at the buffer.
b. Voltage rails are defined at the terminals of Pullup, Power Clamp,
Pulldown and GND Clamp supplied by the chip at the buffer.
2. The simulator
a. C_comp can be connected to either the Pullup, Power Clamp,
Pulldown, GND Clamp, any simulator reference node including Node 0.
b. C_comp can be distributed as two or more capacitors connected to
the Pullup, Power Clamp, Pulldown, GND Clamp, any simulator reference node
including Node 0.
3. IBIS says
a. "may be handled differently among simulators"
There is no place in IBIS that says C_comp must be connected to a
different reference voltage then Pullup, Power Clamp, Pulldown, GND Clamp.
All IBIS says is that it "may be handled differently among simulators".
One should not put too much importance on the following statement on page
74:
The absolute GND is the reference for the V_fixture voltage and the
package model equivalent network. It can also serve as a reference for
C_comp, unless C_comp is optionally split into component attached to the
other reference voltages.
1. The term "absolute GND" cannot be interpreted as Node 0.
2. "absolute GND" can be interpreted as the reference voltage for the
V_fixture, which is most likely (although not necessarily) the reference
voltage that is used to physically connect to the Pulldown (or GND Clamp)
buffer terminal.
3. An the spec only says "It can also serve as a reference for C_comp".
a. Can is only one option.
b. IBIS also says
All tables assume that the die capacitance is included. Potential
numerical problems associated with processing the data using the effective
C_comp (or C_comp_* values as appropriate) for effective die capacitance
may be handled differently among simulators.
What problems are introduced by not knowing how C_comp is distributed to
Pullup, Power Clamp, Pulldown and GND Clamp.
1. If the [Pullup Reference], [Power Clamp Reference], [Pulldown
Reference] and [GND Clamp Reference] are supplied as constant voltages in
the simulation, then the current flowing from the buffer A_signal terminal
will be the same independent on how the C_comp is distributed among the
Pullup, Power Clamp, Pulldown, GND Clamp, and Node 0 terminals.
2. If the Pullup, Power Clamp, Pulldown and GND Clamp terminals are
supplied by the simulator and are time varying, then all currents from
A_signal to the C_comps will be in error based on the spectral content of
the rail voltages at the buffer, the spectral content of A_Signal and how
the simulator allocates C_comp to the various rail voltages.
If the model maker cares about the currents flowing through C_comp to the
various rail voltages he should not use C_Comp, but use C_Comp_*. If the
model makers uses C_Comp, the EDA tool can distribute it among any rail
voltage it chooses.
The Terminator Model is fundamentally flawed. The "GND Clamp" IV curve is
shown hooked up to the rail voltage GND or the constant voltage [GND Clamp
Reference], but the "Power Clamp" IV curve is shown hooked up the constant
voltage [Voltage Range] or [Power Clamp Reference]. The "Power Clamp" IV
curve should also be able to be hooked up to the rail voltage Vcc (a
signal_name in the component pin list).
Walter
Walter Katz
<mailto:wkatz@xxxxxxxxxx> wkatz@xxxxxxxxxx
Phone 303.449-2308
Mobile 303.335-6156
