[IBIS] BIRD 113

                       Buffer Issue Resolution Document (BIRD)

BIRD ID#: 113
ISSUE TITLE: Weak pull-up and weak pull-down resistance and voltage
REQUESTOR: Greg Edlund, IBM

DATE SUBMITTED: 08/19/10
DATE ACCEPTED BY IBIS OPEN FORUM:

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STATEMENT OF THE ISSUE:

If a chip input has a weak pull-up resistor (say 10K) and another chip
input on the same net has a weak pull-down resistor (say 10K), it is
possible for those inputs to be biased in the receiver threshold region.
This BIRD enables automated checking of this condition. To store this
information, we propose four new subparameters under the [Model Spec]
keyword.

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STATEMENT OF THE RESOLVED SPECIFICATIONS:

| Keyword: [Model Spec]
| Required: No
| Sub-Params: Vinh, Vinl, Vinh+, Vinh-, Vinl+, Vinl-, S_overshoot_high,
| S_overshoot_low, D_overshoot_high, D_overshoot_low,
| D_overshoot_time, D_overshoot_area_h, D_overshoot_area_l,
| D_overshoot_ampl_h, D_overshoot_ampl_l, Pulse_high,
Pulse_low,
| Pulse_time, Vmeas, Vref, Cref, Rref, Cref_rising,
| C_ref_falling, Rref_rising, Rref_falling, Vref_rising,
| Vref_falling, Vmeas_rising, Vmeas_falling, Rref_diff,
| Cref_diff, Weak_pull_up_r, Weak_pull_up_v,
Weak_pull_down_r,
| Weak_pull_down_v
|
| Description: The [Model Spec] keyword defines four columns under which
| specification subparameters are defined.
|
| The following subparameters are defined:
| Vinh Input voltage threshold high
| Vinl Input voltage threshold low
| Vinh+ Hysteresis threshold high max Vt+
| Vinh- Hysteresis threshold high min Vt+
| Vinl+ Hysteresis threshold low max Vt-
| Vinl- Hysteresis threshold low min Vt-
| S_overshoot_high Static overshoot high voltage
| S_overshoot_low Static overshoot low voltage
| D_overshoot_high Dynamic overshoot high voltage
| D_overshoot_low Dynamic overshoot low voltage
| D_overshoot_time Dynamic overshoot time
| D_overshoot_area_h Dynamic overshoot high area (in V-s)
| D_overshoot_area_l Dynamic overshoot low area (in V-s)
| D_overshoot_ampl_h Dynamic overshoot high max amplitude
| D_overshoot_ampl_l Dynamic overshoot low max amplitude
| Pulse_high Pulse immunity high voltage
| Pulse_low Pulse immunity low voltage
| Pulse_time Pulse immunity time
| Vmeas Measurement voltage for timing
measurements
| Vref Timing specification test load voltage
| Cref Timing specification capacitive load
| Rref Timing specification resistance load
| Cref_rising Timing specification capacitive load for
| rising edges
| Cref_falling Timing specification capacitive load for
| falling edges
| Rref_rising Timing specification resistance load for
| rising edges
| Rref_falling Timing specification resistance load for
| falling edges
| Vref_rising Timing specification test load voltage
for
| rising edges
| Vref_falling Timing specification test load voltage
for
| falling edges
| Vmeas_rising Measurement voltage for rising edge
timing
| measurements
| Vmeas_falling Measurement voltage for falling edge
timing
| measurements
| Rref_diff Timing specification differential
| resistance load
| Cref_diff Timing specification differential
| capacitive load
| Weak_pull_up_r weak pull-up resistance
| Weak_pull_up_v weak pull-up voltage
| Weak_pull_down_r weak pull-down resistance
| Weak_pull_down_v weak pull-down voltage
|
| Usage Rules: [Model Spec] must follow all other subparameters under the
| [Model] keyword.
|
| For each subparameter contained in the first column, the
| remaining three hold its typical, minimum and maximum
values.
| The entries of typical, minimum and maximum must be placed
on
| a single line and must be separated by at least one white
| space. All four columns are required under the [Model
Spec]
| keyword. However, data is required only in the typical
| column. If minimum and/or maximum values are not
available,
| the reserved word "NA" must be used indicating the typical
| value by default.
|
| The minimum and maximum values are used for specifications
| subparameter values that may track the min and max
operation
| conditions of the [Model]. Usually it is related to the
| Voltage Range settings.
|
| Unless noted below, no subparameter requires having present
| any other subparameter.
|
| Weak_pull_up and Weak_pull_down rules:
|
| If an IO circuit uses a simple weak pull-up resistor
| or transistor between the chip IO pad and a power supply,
| Weak_pull_up_r stores the resistance of this resistor or
| transistor and Weak_pull_up_v stores the voltage of the
| power supply to which the resistor or transistor is
| connected.
|
| If an IO circuit uses a simple weak pull-down resistor
| or transistor between the chip IO pad and ground,
| Weak_pull_down_r stores the resistance of this resistor or
| transistor and Weak_pull_down_v stores the voltage of the
| power supply to which the resistor or transistor is
| connected (usually ground).
|
|
| Weak pull-up or pull-down
|
Weak_pull_up_r 10K NA NA
Weak_pull_up_v 1.5V NA NA
Weak_pull_down_r 10K NA NA
Weak_pull_down_v 0 NA NA

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ANALYSIS PATH/DATA THAT LED TO SPECIFICATION:

IBM consistently sees problems whose root cause is a conflict between
pull-up and pull-down resistors. We have documentation that shows other
companies see the same problems. It can be extremely costly to redesign
hardware to fix these problems even though the fix is usually simple.

Checking programs exist, but the input data to feed these programs does
not exist in electronic format. Engineers currently have to dig through
many, many pages of component datasheets to find this information, which
may not even be documented at all. To run the checking programs, engineers
must hand code the pull-up and pull-down values into the input files.
This is a labor-intensive and error-prone process.

The industry needs a means for storing this information to enable automated
checking and insure product quality. Since the information pertains
directly to IO circuits and is not readily available in a machine-readable
format, IBIS is the natural source.

It is not possible to extract weak pull-up or pull-down resistances from
[GND Clamp] and [POWER Clamp] curves since they may contain current from
three possible sources:

1. ESD protection devices
2. termination
3. weak pull-up or pull-down

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ANY OTHER BACKGROUND INFORMATION:

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Greg Edlund
Senior Engineer
Signal Integrity and System Timing
IBM Systems & Technology Group
3605 Hwy. 52 N Bldg 050-3
Rochester, MN 55901

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