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发表于 2009-7-4 15:14:16
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Spectre Warning Messages
Warning messages tell you about conditions that might cause invalid results. Unlike error
messages, warnings do not stop a simulation. When you receive a warning message, you
must decide whether the particular condition creates a problem for your simulation. This
section describes some common Spectre warning messages. It also tells you how to modify
parameters to correct conditions that might produce invalid simulation results.
The Spectre simulator often prints warnings and notices that are eventually determined to be
“uninteresting,” and there is a natural tendency after a while to ignore them. We recommend
that you carefully study them the first few times you simulate a particular circuit and whenever
the simulator gives you unexpected results.
P-N Junction Warning Messages
Almost every semiconductor device includes at least one p-n junction. Normally, these p-n
junctions are biased in a particular operating region. Three types of warning messages are
available for each p-n junction, one for exceeding a maximum current, one for exceeding a
melting current, and one for exceeding a breakdown voltage.
Explosion Region Warnings
All Spectre models that include p-n junctions identify the maximum current with the imax
parameter. Junctions are modeled accurately for current values up to imax. For current
values greater than imax, Spectre models the junction as a linear resistor and issues a
warning. For most devices, the default value of imax is 1 ampere, although the value scales
with the size of the device. For the bjt model, the default imax value is 1000 A.When the
current through a junction diode exceeds imax, the Spectre simulator displays a warning like
one of the following:
Dl: Junction current exceeds 'imax'
Ml: The bulk-drain Junction current exceeds ' lmax'
The warning message identifies the relevant component name (D1 and M1) and. when necessary, it also identifes the affected junction (bulk-dram junction).
The Spectre simulator limits the current through a junction in its explosion region to prevent numerical overflow. If you receive the previous message, the results of the simulation are not reliable. You must decide if you really want a junction current to be that large. If you do. increase imax and resimulate. Otherwise, correct whatever is causing The current to be that large.
Melting Current Warnings
A separate model parameter, imelt, is used as a limit warning for the junction current. This parameter can be set to the maximum current rating of the device. When any component of the junction current exceeds imelt. note that base and collector currents are composed of many exponential terms. Spectre issues a warning and the results become inaccurate. The junction current is linearized above the value of imelt to prevent arithmetic exception, with the exponential term replaced by a linear equation at imelt.
Breakdown Region Warnings
Messages like the following are breakdown region warnings:
D2 ¦ Breakdown voltage exceeded.
Q1: The collector-substrate voltage, exceeded breakdown voltage.
The warning message identifies the relevant component name (D2 and Q1) and, when necessary, it also identifies the affected junction.
The Spectre simulator issues breakdown region warnings only when you specify conditions for them. For information on setting parameters to identify a breakdown region, see"Customizing Error and Warning Messages" ,Missing Diode Would Be Forward-Biased
For efficiency, diodes in real semiconductor devices are sometimes not modeled. For example, it is common to model only the bipolar junction transistor (BJT) substrate junction capacitor, not the diode itself. You car set the saturation current to zero on MOSFETs to eliminate the channel-to-bulk diodes if The diodes you eliminate are never forward-biased. If these diodes inadvertently become forward-biased, all simulators give inaccurate results.
For this reason, the Spectre simulator displays this message if an eliminated diode would have been forward-biased.
M2: Missing bulk-source diode would be forward biased.
The warning message identifies the relevant component name (M2). and it also identifies The affected junction.
Note: When the resistive part of a junction is turned off, the Spectre simulator assumes The resistive part of the junction does not exist Therefore. The imax parameter cannot be used to flag junction explosion warnings. In such cases, the Spectre simulator issues a warning if the voltage across the omitted diode is over 10 times the thermal voltage.
Tolerances Might Be Set Too Tight
When you simulate high-voltage or high-current circuits, the default tolerances might be tight enough to make convergence difficult or impossible. If you get a Tolerances might be set too tight" message, try relaxing tolerances by increasing the value of reltol. iabstol, and vabstol.
Parameter Is Unusually Large or Small
The Spectre simulator checks the parameter values to see if they are within a normal range of expected values. This check can catch data entry errors or identify situations that can cause the Spectre simulator to have difficulties simulating the circuit.
The Parameter is unusually large or small' message issues a notice about a parameter value. The message looks like one of the following:
NPNbjt, 'rb' has the unusually small value of lmOhms.
PNPbjt: 'If has the unusually large value of IGs.
OA1.Q16 of ua741: 'region' has the unusual value of rev.
If you receive such a message, check the parameter, if the unusual parameter value is correct, you can ignore this message.
The limits settings that generate These warning messages are soft limits, as opposed To hard limits settings. Hard limits stop a simulation if they are violated, the Spectre simulator has automatic soft limits on a few parameter values. However, you can override these limits orspecify your own limits for parameters that do not have automatic limits. For more information,see "Customizing Error Find Warning Messages".
gmin Is Large Enough to Noticeably Affect the DC Solution
By default. the Spectre simulator adds a very small conductance of 10-12 Siemens called gmin across nonlinear devices. This conductance prevents nodes from floating if the nonlinear devices are turned off. The gmin parameter usually has a minimal effect on circuit behavior. However, some circuits, such as charge storage circuits, are very sensitive to the small currents that flow through gmin. For example. the current through gmin to the storage capacitor or a sample-and-hold might significantly affect the droop and invalidate the hold-time measurement.
You see the "gmin is Large Enough..." message if the current flowing through the gmin conductors, when treated as an error current, does not meet the convergence criteria. In other words, the current thai enters any node from all attached gmin conductors is larger than iabstol or reltol multiplied by The sum of The absolute values of individual currents that enter the node.
Total gmin Current > abstol or reltol*(|/ec1| + Iec2| + IIec3I + ...IIecn)
If your circuit is not sensitive to small leakage currents, you can ignore the message. If your circuit is sensitive to these currents, reduce the gmin value or set it to zero.
The Spectre simulator also estimates the change in the signal that occurs if you remove gmin. The amount of information displayed is controlled by The gmin_check parameter of the options statement.
Minimum Timestep Used
If this problem occurs, the analysis continues, and a warning message is displayed at each lime point that does not meet the convergence criteria. In the Spectre simulator, this is very rare, but it does occur. Occasionally, this needs To be remedied To get The correct solution.
1. Make sure devices have junction and overlap capacitance specified.
2. Increase maxiters, but do not go higher than 200.
3. Change to the gear2 or gear2only method of integration.
4. Reduce other occurrences of the local truncation error cutting the timestep. Increase lteratio and increase the absolute error tolerances vabatol and iabstol. Do not go too high with any of these.
5. Combine 2,3 and 4 and set cmin to prevent instintaneous change at every node in the circuit.
6. Relax reltol in combination with 5. |
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