The present invention relates to a technology on microcomputers incorporating a reset function.
Conventionally, in many systems, a one-chip microcomputer is provided with an external reset IC for prevention of a runaway due to supply voltage drop. In recent years, however, in response to requests for cost reduction, one-chip implementation of semiconductors and the like, incorporation of a reset IC in a microcomputer is in progress.
FIG. 6 illustrates a configuration of a conventional reset function incorporated microcomputer. Referring to FIG. 6, a reset signal generation circuit 61 incorporated in a reset block 60 of a microcomputer constantly detects a variation in the supply voltage applied to the microcomputer, and generates a reset signal SR when the supply voltage drops to a reset voltage. The generated reset signal SR propagates along an internal reset signal line, and the reset function of the microcomputer is activated in response to the reset signal SR. Alternatively, a reset signal may be given to the microcomputer via an external reset terminal 62.
The conventional technology described above has the following problems.
The value of the reset voltage used for decision of a drop of the supply voltage is determined depending on the semiconductor fabrication process. Therefore, the value may greatly vary among products, and this may cause a problem that it is not possible to decide whether or not reset operation should be performed using a common reset voltage.
To eliminate a variation in reset voltage, microcomputers may be adjusted after fabrication so that the reset voltage is newly set at a specific value. In this case, however, since adjustment of the reset voltage is required for respective microcomputers, mere implementation of the microcomputers is not enough for operation of products, and thus the fabrication process is disadvantageously complicated.
An object of the present invention is providing a reset function incorporated microcomputer capable of suppressing a variation in reset voltage with a simple configuration.
Specifically, the reset function incorporated microcomputer of the present invention includes a reset decision section for generating a reset signal instructing execution or non-execution of reset operation. The reset decision section includes: a stabilized power supply circuit for generating a predetermined reference voltage; a reset voltage generation circuit for generating a reset voltage based on the reference voltage generated by the stabilized power supply circuit; and a comparator for comparing the supply voltage applied to the microcomputer with the reset voltage, the reset signal being generated based on the comparison result from the comparator.
According to the invention described above, the reset voltage is generated based on a predetermined reference voltage generated by the stabilized power supply circuit. This suppresses a variation in reset voltage. In other words, the reset function can be operated with a fixed reset voltage common among microcomputers, and thus adjustment after fabrication is no more necessary. Moreover, since a normal microcomputer is originally provided with a stabilized power supply circuit for operating a D/A converter and the like, it is not necessary to newly provide a stabilized power supply circuit for the purpose of the present invention.
Preferably, the microcomputer of the present invention includes a reset decision control section for controlling operation/non-operation of the reset decision section. Therefore, with the control of operation/non-operation of the reset decision section by the reset decision control section, power consumed for reset decision operation can be reduced. In particular, since the stabilized power supply circuit consumes a large amount of current, great reduction in power consumption is possible according to the present invention.
Preferably, the reset decision control section includes: a timing control circuit for operating the reset decision section intermittently; and timing control means for operating the reset decision section by software at a predetermine timing, and operation/non-operation of the reset decision section is controlled with the timing control means when the microcomputer is in a normal operation mode, and is controlled with the timing control circuit when the microcomputer is in a low current consumption mode in which the microcomputer does not execute software. With this configuration, the timing control circuit controls operation/non-operation of the reset decision section in the low current consumption mode. Therefore, reset recognition is possible in the low current consumption mode without the necessity of software operation, and thus current consumption can be reduced.
Preferably, in the microcomputer of the invention, the reset decision section includes output control means for outputting a signal instructing non-execution of reset operation as the reset signal irrespective of the comparison result from the comparator as long as the operation of the microcomputer satisfies predetermined conditions. With this configuration, it is possible to avoid occurrence of reset for a time period during which a problem may occur if reset occurs. This enables stable operation of a system using the microcomputer of the invention.