| 摘要 |
PROBLEM TO BE SOLVED: To allow a telephone set having provision for a DTMF sugnal to reduce its current consumption so as to be in operation on a telephone line and to outputs a correct DTMF signal at all times. SOLUTION: When a 1st interrupt request X is fed to an interrupt input circuit 28 in a usual operating state of a microcomputer 2, a logical '1' is set to an interrupt control register 29, which outputs an interrupt signal IRPT of a logical '0' and a reference voltage output circuit 30 receiving the signal starts its operation. When a 2nd interrupt request signal Y is fed to an interrupt input circuit 28, a logical '0' is set to the interruption control register 29, which outputs an interrupt signal IRPT of a logical '1' and the reference voltage output circuit 30 receiving the signal stops its operation. Thus, current consumption of the reference voltage output circuit 30 is limited for a period from a speech start to its end and then the current consumption is reduced. Moreover, a fault of a DTMF signal is detected and the microcomputer 2 is initialized.
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