Abstract:
A method and apparatus for sequencing an integrated circuit which receives an external clock signal consists of the use of an internally generated random clock signal and of the use of either of these clock signals depending on the instruction to be performed.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method and apparatus for sequencing an integrated circuit, and can be applied to integrated circuits designed for chip cards or circuit boards. 
     2. Discussion of the Related Art 
     Integrated circuits designed for chip card or circuit board type applications usually have an input designed to receive an external clock signal. This clock signal is used to generate an internal clock signal capable of ensuring the operation of the integrated circuit. Therefore, the internal operation of the internal circuit is sequenced with respect to the external clock signal. 
     For chip card applications, circuit board applications, or for any other application requiring high operating security, it is imperative to have the ability to prevent a situation where the presence of parasites on the external clock signal could disturb the internal clock signal and modify the operation of the integrated circuit. A disturbing action by parasites would be one way of carrying out fraudulent activity. 
     It has become common practice to use a filtering circuit in the internal clock signal generator. This filtering circuit is capable of overcoming disturbances of this kind in the external clock signal. An example of such a generator is described in the French patent No. 2 708 809. Structures also exist that can be used to obtain an internal clock signal with characteristics that depend little on variations in temperature or manufacturing method. A structure of this type is described for example in the patent No. FR 2 707 058. 
     However, it has been discovered that such structures, while they make it possible to prevent the disturbing of the internal operation of an integrated circuit, do not however make it possible to prevent another type of fraud, namely the observation of the behavior of the integrated circuit. Since the internal clock signal is based on the external clock signal, this external clock signal may be used as a synchronization signal. This would, for instance, enable an ill-intentioned individual in particular to obtain confidential data or even information by which the individual could reconstitute the program performed by the integrated circuit. 
     SUMMARY OF THE INVENTION 
     The present invention provides a method and an apparatus which prevents an external clock signal from possibly serving also as an internal synchronization signal so as to protect the integrated circuit from corresponding types of fraudulent activity. 
     An embodiment of the present invention is directed to an integrated circuit which internally generates a random clock signal, which allows the integrated circuit to use either the external clock signal or the random clock signal according to the instruction to be performed by the integrated circuit. Preferably, the random clock signal will be used for operations that process confidential data elements. 
     Also, in another embodiment of the invention, the integrated circuit can use the random clock signal by default so that the external clock signal is switched over only for operations requiring external synchronization. Therefore, the internal operation of the integrated circuit is no longer synchronous with the external clock signal except for operations which have to be synchronized. 
     The internal random clock signal being completely independent from the external clock signal, the integrated circuit can go on operating even if no external clock signal is applied, using the internal clock signal. In particular, all the secured checking operations can go on. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a block diagram of an integrated circuit according to the invention, and 
     FIG. 2 shows a timing diagram of the corresponding clock signals. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 shows a block diagram of an integrated circuit 1 enabling the implementation of the invention. The integrated circuit 1 has an input to receive an external clock signal CK-ext. According to the invention, it furthermore has a random generator 2 which provides a random clock signal CK-al. 
     The two clock signals CK-ext and CK-al are applied to two inputs of a switch-over circuit 3. The circuit 3 is capable of switching either of the clock signals over to an input E of a circuit for the generation of an internal clock signal CK-in, this switch-over being done according to the level of a binary command K which may be 0 or 1. This internal clock signal CK-in is applied to the circuitry of the integrated circuit. 
     This circuitry 5 is not shown in detail in the figure, but it typically has memory circuits in which there are stored, in particular, the application program and confidential data elements (an identification code for example) and means to manage the external exchanges and these memory circuits (processor, microcontroller or other circuits). 
     The switch-over circuit 3 receives the binary switch-over command K from circuitry 5. Depending on whether its logic state is low or high, this command enables either the random clock signal or of the external clock signal to be switched over to the input E of the clock signal generator circuit 4. 
     This generator circuit 4 provides an output signal whose pulses are stabilized 15 in time. Typically, and as shown in FIG. 2, the detection of a leading edge on the input E makes the output signal go from a low state to a high state during a calibrated period of time d1, and then the output signal goes back to the low state for at least a determined period of time d2. It is only at the end of this period d2 that the generation circuit can respond to a new pulse at its input E. A generation circuits, such as the circuits described in the patent application No. FR 2 708 809 may be an appropriate generation circuit which could be used in the present invention. 
     An integrated circuit according to the invention, comprises a random generator capable of providing a random clock signal and a switch-over circuit capable of either the external clock signal or the random clock signal as an input pulse signal of the generator 4 of the internal clock signal. 
     The switch-over circuit thus makes it possible to impose the random clock signal as an input pulse signal for operations processing confidential data elements (contained in the memory circuits of the circuitry 5). Also, the switch-over circuit enables the use of the external clock signal for operations requiring external synchronization. Therefore, the confidential data elements contained in the integrated circuit are protected from fraudulent activity since it is no longer possible to find any external synchronization in order to observe the operations of the integrated circuit. 
     In one embodiment of the invention, the integrated circuit according to the invention advantageously makes it possible to impose the random clock signal by default as the input pulse signal and to use the external clock signal as an input pulse signal only for operations of the integrated circuit requiring external synchronization. 
     With an internal sequencing method of this kind, it then becomes impossible to find an external synchronization. The only operations of the integrated circuit that will be synchronized with the external clock signal will be those relating to the transmission of data elements with an external system, for example transmission by a serial link. 
     In practice, when the integrated circuit carries out the program and when it reaches instructions corresponding to a transmission (sending or receiving) with an external system, it activates the command K(K=1) to temporarily switch the external clock signal to the generator circuit input. 
     As soon as the transmission is ended, the integrated circuit deactivates the command K(K=0) to again switch the random clock signal to the generator circuit input. 
     A sequencing method of this kind is illustrated in FIG. 2: so long as the command K is at zero, the internal clock signal CK-in is based on the random clock signal. As soon as the command K goes to one, the internal clock signal CK-in is based on the external clock signal. 
     Although the internal sequencing method according to this embodiment is particularly simple to implement, the internal sequencing method enables the integrated circuit to work always at its highest speed. When the sequencing is based on the random clock signal, it is possible to work at the optimum speed of the integrated circuit (for example 40 MHz). When it is based on the external clock signal, the speed of the integrated circuit is then matched with the external conditions (lower speed, for example in the range of 10 MHz). 
     Having thus described at least one illustrative embodiment of the invention, various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description is by way of example only and is not intended as limiting. The invention is limited only as defined in the following claims and the equivalents thereto.