Microcomputer reset device for positively resetting microcomputer before starting operation
First Claim
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1. A microcomputer reset device comprising:
- an initializing circuit for placing a microcomputer in a reset state as long as a first divided voltage proportional to a supply voltage is lower than a reference voltage, and for releasing the reset state of the microcomputer when the first divided voltage exceeds the reference voltage;
an adjusting circuit for adjusting, in response to an operation frequency of the microcomputer, a second divided voltage which is proportional to the supply voltage; and
a switching circuit for comparing the second divided voltage adjusted by said adjusting circuit with the reference voltage, and for switching, when the second divided voltage exceeds the reference voltage, a clock source of the CPU from an internal clock signal to an external clock signal.
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Abstract
A microcomputer reset device includes a switching circuit for comparing, after a reset of a CPU is released during power-up, a reference voltage Vref with a second divided voltage Vd2 which is proportional to a supply voltage and is adjusted by the CPU, and for switching, when the second divided voltage Vd2 exceeds the reference voltage Vref, a clock source of the CPU from an internal clock signal to an external clock signal. This makes it possible to solve a problem of a conventional microcomputer reset device in that it is not unlikely that the microcomputer starts its operation before its reset has been completed, which hinders the normal operation of the microcomputer.
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Citations
4 Claims
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1. A microcomputer reset device comprising:
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an initializing circuit for placing a microcomputer in a reset state as long as a first divided voltage proportional to a supply voltage is lower than a reference voltage, and for releasing the reset state of the microcomputer when the first divided voltage exceeds the reference voltage;
an adjusting circuit for adjusting, in response to an operation frequency of the microcomputer, a second divided voltage which is proportional to the supply voltage; and
a switching circuit for comparing the second divided voltage adjusted by said adjusting circuit with the reference voltage, and for switching, when the second divided voltage exceeds the reference voltage, a clock source of the CPU from an internal clock signal to an external clock signal. - View Dependent Claims (2, 3)
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4. A microcomputer reset device comprising:
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an initializing circuit for placing a microcomputer in a reset state as long as a first divided voltage proportional to a supply voltage is lower than a reference voltage, and for releasing the reset state of the microcomputer when the first divided voltage exceeds the reference voltage;
an adjusting circuit for adjusting, in response to an operation frequency of the microcomputer, a second divided voltage which is proportional to the supply voltage;
a counter for counting an interval from a time when said initializing circuit releases the reset of said microcomputer to a time when the second divided voltage adjusted by said adjusting circuit exceeds the reference voltage; and
a switching circuit for calculating a clock switching time from the interval counted by said counter, and for switching, when the clock switching time comes, a clock source of the CPU from an internal clock signal to an external clock signal.
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Specification