Computer-aided design and analysis of circuits and semiconductor – Nanotechnology related integrated circuit design
Reexamination Certificate
1999-05-17
2001-08-07
Phan, Trong (Department: 2818)
Computer-aided design and analysis of circuits and semiconductor
Nanotechnology related integrated circuit design
Reexamination Certificate
active
06272663
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a system and method for reducing undesired radiation generated from LSIs. More specifically, the invention relates to an undesired radiation reducing system and method for use in automated design for LSIs.
2. Description of Related Arts
As undesired radiation (which will be also hereinafter referred to as a “noise”) generated from LSIs, the following three kinds of noises shown in
FIG. 7
are typically known.
(1) Power Source Noise: Swing in potential of power source lines
41
a
,
41
b
due to power consumption in an LSI circuit
40
.
(2) I/O Noise: Swing in potential of power source lines
41
a
,
41
b
propagated to an output signal of an input-output buffer
44
.
(3) Electromagnetic Radiation Noise (Radiation Noise): Electromagnetic wave radiated from the whole surface of a chip
40
.
Main noise sources are circuit elements relating to clock signals (which will be also hereinafter referred to as “clock cells”), such as a flip-flop
42
or a clock buffer. Since the clock signals are synchronism with each other, currents flow simultaneously through the clock cells. Since an LSI has numerous clock cells, the total current is great. Therefore, a great number of charges are drawn out of power source lines in a short time. In order to compensate the charges drawn out of the power source lines, it is required to supply with charges from the outside of the circuit. The flow of the supplied charges serves as a power source noise observed outside the LSI.
As a method for reducing a power source noise, there is known a method for adding a capacitor to a power source line in the vicinity of a noise source. The reason why this method can reduce the power source noise is as follows. The added capacitor serves as a charge storage area. Thus, a part or a large part of current to be consumed by the noise source can be temporarily supplied from the added capacitor. Therefore, it is not required to supply charges from the outside of the LSI in a short time, so that the power source noise is reduced.
However, it is generally difficult to determine the dimension of the capacitor to be added. If the capacitor is inadvertently added, there is a possibility that the added capacitor is combined with an inductance in the circuit to cause resonance (a state of a very large noise). In addition, an addable capacitor is restricted for the following two reasons.
(1) Holding of Reliability of LSI
The capacitor in the LSI is formed of an oxide film. This oxide film is very thin, so that it is breakable. Therefore, as the number of capacitors increases, the area of the oxide films increases, so that there is a higher probability that the LSI will fail.
(2) Constraint on Available Capacitor
Circuit elements are spread all over the interior of an LSI in order to increase the degree of integration of the LSI. Thus, the area occupied by the capacitor in the vicinity of a certain noise source is restricted.
It is required to obey the aforementioned two constraints and to add capacitors so as not to cause resonance. It is very difficult to do so by the engineer's manual operation. Therefore, it is required to provide a system for automatically calculating a capacity to be added. However, such a system is not known in the present circumstances.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to eliminate the aforementioned problems and to provide an undesired radiation reducing system and method, which can reduce undesired radiation generated from an LSI.
In order to accomplish the aforementioned and other objects, according to one aspect of the present invention, there is provided a system for reducing undesired radiation generated from an LSI, the system comprising: first storage means for storing a circuit connection information for the LSI and a current waveform of an undesired radiation source of the LSI; transfer function calculating means for calculating a transfer function of undesired radiation, which is transmitted from an undesired radiation source in the LSI to a power source terminal connected to the outside of the LSI, on the basis of the circuit connection information and the current waveform; second storage means for storing a constraint on an impedance added in the vicinity of the undesired radiation source, and an allowable level of undesired radiation at the power source terminal; and undesired radiation optimizing means for calculating an impedance, which causes the undesired radiation at the power source terminal to be less than or equal to the allowable level, using the transfer function under the constraint.
The impedance added in the vicinity of the undesired radiation source is preferably a capacitor.
Preferably, the undesired radiation optimizing means derives a relationship between a current vector of the undesired radiation source and a current vector flowing through the power source terminal, and derives undesired radiation so that an objective function, which is an absolute value of the current vector flowing through the power source terminal, is minimum under the constraint.
Preferably, the undesired radiation optimizing means derives a differential of the objective function with respect to the added capacitor, and derives undesired radiation so that the differential is minimum under the constraint.
The differential may be weighted by a weighting factor determined by the allowable level.
According to another aspect of the present invention, there is provided a method for reducing undesired radiation generated from an LSI, the method comprising: a first step of deriving a transfer function of undesired radiation, which is transmitted from an undesired radiation source in the LSI to a power source terminal connected to the outside of the LSI, on the basis of a circuit connection information for the LSI and a current waveform of the undesired radiation source of the LSI; a second step of setting a constraint on an impedance added in the vicinity of the undesired radiation source, an allowable level of undesired radiation at the power source terminal, and a frequency range to be considered; a third step of deriving undesired radiation at the power source terminal under the constraint; a fourth step of determining whether the undesired radiation derived at the third step is not higher than the allowable level in the frequency range; and a fifth step of changing the added impedance so as to reduce the undesired radiation to repeat the third and fourth steps when the derived undesired radiation exceeds the allowable level, and outputting the added impedance when the derived undesired radiation is not higher than the allowable level.
The impedance added in the vicinity of the undesired radiation source is preferably a capacitor.
REFERENCES:
patent: 5559997 (1996-09-01), Tsuchida et al.
Foley & Lardner
Kabushiki Kaisha Toshiba
Phan Trong
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