Online proceedings - EDA Publishing Association

24-26 September 2008, Rome, ItalyConsideration of Thermal Effects in Logic SimulationGergely Nagy, György Horváth, András Poppe{nagyg | gyuri | poppe}@eet.bme.huAbstract-This paper presents a method that considersthermal effects in logic simulations. The aim is to develop a toolthat is capable of modeling the thermal behavior of a digitalcircuit and, at the same time, yields results almost at the speedof ordinary logic simulators. The importance of such asimulation is that thermal effects can be the cause of signalintegrity problems.The structure and the basic operation of the simulator arediscussed as well as the issues that need to be addressedthroughout the development. A detailed description of theimplementation of the tightly integrated thermal and logicsimulation is also given.I. INTRODUCTIONA logithermal simulator performs a digital simulation withsimple logic gates. Apart from determining the Booleanvalues of the gates in a circuit, it also estimates thetemperature of the gates and takes the gates’ delay intoaccount [1] [6] [11].Digital simulators can usually deal with delays but mostlyas constant values. This is a rather simple and inaccurateway of approaching this phenomenon. The characteristics oftransistors is temperature dependent and thus, so is the speedof logic gates. This means that the delay of digital circuits atdifferent temperature values is not the same.The greatest problem is not that at an elevated ambienttemperature our devices might work a little slower. This mayalso cause flaws in the operation, but there is another effect,that can be responsible for errors much harder to investigate.The temperature distribution in a chip or a PCB is notnecessarily homogeneous: the temperature of the devices isnot only determined by the ambient temperature. Asignificant factor to be considered is the dissipation of thegates. When a gate is working (its input has changed and it isin the move from one state to another), it consumes anddissipates and thus its temperature changes. In addition, agate’s dissipation affects the temperature of the adjacentgates.This effect implies that if some parts of a digital circuit aremuch more active than other parts, then the temperaturedissipation and thus the delays of the gates might differsubstantially throughout the chip. This means that a circuitthat is dependant on timing may operate impeccably at roomtemperature when it is switched on, and it might produceerrors after a certain time when some parts have warmed upwhile others not. Thermal effects can corrupt signal integrityand this can be traced by a logithermal simulator.A great challenge throughout the design and developmentof the simulator engine is to stay as close to simple digitalsimulation as possible in order to maintain a relatively highspeed but, at the same time, generate much more accurateresults than those of ordinary logic simulators. Thetemperature values of the gates are needed to be determinedby an analog thermal simulator, the execution time of whichis a crucial factor in the overall simulation time. Thus thealgorithm performing this task needs to be constructed withperformance as a priority. The thermal engine also needs tobe simplified; its input is a rough layout comprising the gatesand the interconnections.In order to achieve considerably precise results, othereffects are also needed to be taken into account. In moderndigital circuits glitches are of great concern. There existways to address this phenomenon in a logithermalsimulation.The novelty of this method is that is uses two differentsimulator engines to examine the same circuit in order togain a deeper insight to the logic and thermal behavior of thesystem. The engines are tightly integrated so as to achieve ahigher speed and a better resolution.II. THE BUILDUP OF THE SIMULATOR ENGINEThe simulator is built up of two cooperating engines: alogic engine and an electro thermal solver as seen on Fig. 1.The input of the logic engine is a gate level description ofthe circuit and a set of test signals that serve as a basis forthe simulation. The thermal solver is provided with thephysical layout of the circuit and the initial temperaturevalue.Fig. 1. The buildup of the simulator engineThe simulation is directed by the logic engine. Itdetermines the logic values of the gates in each simulationcycle. It also registers the gate activities and the glitches.The logic engine sends the acquired data (signaltransitions and glitches) to the thermal engine which©EDA Publishing/THERMINIC 2008 229ISBN: 978-2-35500-008-9