Computer program run-away supervisory circuit and method
TECHNICAL ART 
     The present invention relates to computer program run-away supervisory circuits and methods, and more particularly to a circuit for and method of detecting computer run-away in response to the computer deriving a run signal at an excessively high frequency. 
     BACKGROUND OF THE INVENTION 
     Microcomputers are frequently used as controllers for equipment or apparatus, et cetera. For example, in an internal combustion engine of the fuel injection type, a microcomputer is used to calculate the amount of fuel to be injected into the engine in accordance with various engine parameters. In this case, to calculate the amount of fuel to be injected it is necessary for a microcomputer to repeat prescribed operations many times during a short time period. Such repetitive operations are performed in response to program stored in a memory of the microcomputer; however, since the capacity of the memory to store the programs is desirably as small as possible, the programs are carried out in compliance with the steps of a prescribed circulating range. 
     However, in such a microcomputer, no counter-measures for noises, temperature or the like are normally taken in contrast to the measures taken with large computers. Consequently, malfunctions occur such that operations are performed without the program operating through the normal sequence thereof; in particular, program run-away may occur depending upon surrounding circumstances. Because of the possibility of program run-away, such microcomputers are frequently constructed to derive a run signal each time a prescribed step of a re-circulating section (re-circulating program) of the program is executed. The run signal confirms whether or not the microcomputer is operating according to the program. There is a known program run-away supervisory circuit for preventing program run-away by resetting the operation of the microcomputer; program run-away is detected in response to this run signal. 
     In a conventional program run-away supervisory circuit, program run-away is detected by the presence/absence of a run signal being generated in a prescribed time. It is possible to detect such a program run-away if no run signal is generated because the execution step of the microcomputer has jumped to a step out of a re-circulating program. However, if there is program run-away such that only a part of the steps including the above-mentioned prescribed step in the re-circulating program are performed, program run-away is not detected since the run signal is generated. 
     It is an object of the present invention to provide a program run-away supervisory circuit and method which can always and accurately detect a program run-away. 
     DISCLOSURE OF THE INVENTION 
     The program run-away supervisory circuit according to the invention is characterized by a run signal discriminating means for generating a program run-away signal in response to the run signal being in a frequency bank outside of the normal frequency band for derivation of the run signal. 
     The program run-away supervisory circuit of the invention is characterized by a run signal discriminating means comprising frequency-voltage converter means for generating a voltage representing the frequency at which a run signal is generated, and a window comparator for generating a program run-away signal when the output voltage of the frequency-voltage converter is out of a prescribed voltage range. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of an exemplary conventional program run-away supervisory circuit; and 
     FIG. 2 is a block diagram of an embodiment of a program run-away supervisory circuit of the present invention. 
    
    
     BEST MODE FOR PRACTICING INVENTION 
     The features and advantages of the present invention will be better understood from the following description taken in conjunction with the accompanying drawings. 
     FIG. 1 is a block diagram of an exemplary conventional program run-away supervisory circuit. In the drawing, a program run-away supervisory circuit comprises a run signal discriminator 1 and a reset signal generator 2. Generator 2 responds to the output of the run signal discriminator 1. The run signal discriminator 1 is connected to respond to a run signal at an output terminal of microcomputer 3. Reset signal generator 2 is connected to supply a reset signal to a reset terminal of the microcomputer 3. 
     When a run signal is derived by the microcomputer 3 in a prescribed time, the run signal discriminator 1 determines that the microcomputer 3 is operating in compliance with the program. However, if no run signal is derived by microcomputer 3 in a prescribed time, discriminator 1 determines that program run-away has occurred, and generates a program run-away signal. This program run-away signal activates reset signal generator 2 to reset the operation of the microcomputer 3. 
     In such a program run-away supervisory circuit, it is possible to detect program run-away if no run signal is generated because the execution steps of the microcomputer 3 has jumped to steps out of the re-circulating program. However, the prior art circuit does not accurately detect program run-away if only a part of the steps including the above-mentioned prescribed steps in the re-circulating program are continuously performed. This is because the run signal is generated and detected by discriminator 1. 
     FIG. 2 is a block diagram of a program run-away supervisory circuit according to the present invention. In FIG. 2, the run signal output terminal of the microcomputer 3 is connected to a F-V (frequency-voltage) converter 4, having an output connected to an input of window comparator 5. Two predetermined voltages Vref 1  and Vref 2  (where, Vref 1  &lt;Vref 2 ) are supplied to window comparator 5. Window comparator 5 derives an output that is supplied to an input of reset signal generator 2. A reset signal generated by the reset signal generator 2 is supplied to the reset terminal of the microcomputer 3. 
     In such a program run-away supervisory circuit with the above-stated construction, when a run signal is derived from the microcomputer 3, the F-V converter 4 derives a voltage representing the run signal generating frequency. Window comparator 5 derives a program run-away signal when the voltage supplied to it by converter 4 is smaller than the prescribed voltage Vref 1  and larger than the prescribed voltage Vref 2 . Therefore, if the microcomputer 3 correctly operates in compliance with the program, the run signals are generated in a predetermined time period, causing the output voltage of the F-V converter 4 to be between the prescribed voltages Vref 1  and Vref 2 . However, if no run signal is generated when the microcomputer 3 executes the steps other than the re-circulating program due to program run-away, the output voltage of the F-V converter 4 is smaller than the prescribed voltage Vref 1 , causing the window comparator 5 to generate a program run-away signal. When the microcomputer 3 continuously performs only a part of the steps in the circular program the run signal is generated at a higher frequency than that in normal operation, causing the output voltage of the F-V converter 4 to be larger than Vref 2 . Window comparator 5 responds to the output of converter 4 being greater than Vref 2  to generate a program run-away signal. In response to the generated program run-away signal at the output of comparator 5 reset signal generator 2 supplies a reset signal to the microcomputer 3, causing the operation of the microcomputer 3 to return to the start status. 
     As described above, in accordance with the program run-away supervisory circuit of the present invention, a program run-away signal is generated when the run signal generating frequency is out of prescribed frequency bands. For this purpose, the microcomputer continuously executes only a part of the steps in the re-circulating program; it is possible to detect such a program run-away because the run signal generating frequency is higher than a value normally associated therewith.