| 摘要 |
PURPOSE:To simplify the circuit, by using an oscillator and a counter for a key input circuit and a tone ringer circuit in common. CONSTITUTION:A tone ringer control signal is a signal which rises at the same time as the leading of a detection signal VTH of a tone ringer circuit power supply voltage and falls down at the same time as the trailing of an incoming signal detection signal IR. When an incoming signal is inputted, a counter start signal RTS attains to high level, a counter 21 is operated, a tone ringer control signal attains to high level, each clock signal from the counter 21 is made effective and a call tone is generated. In this constitution, even if the incoming signal is inputted just after a handset is hooked on, the counter 21 is operated and the tone ringer circuit is operated, then malfunction of the tone ringer circuit at the transient state is prevented. Even if a key is depressed when the handset is hooked off with the input of the incoming signal, a key input circuit 10 is not malfunctioned. |
