1980-12-22
1982-06-29
Robinson, Thomas A.
H04M 322
Patent
active
043373780
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention pertains to telephone communication circuits and more particularly to an integrated circuit for detecting on-hook and off-hook conditions.
BACKGROUND OF THE INVENTION
A variety of integrated circuits have been developed for use in applications associated with conventional telephone circuits. These integrated circuits must interface with existing telephone equipment and comply with telephone communication standards. The existing equipment and standards were not necessarily designed for use with integrated circuits and as such it is often difficult to directly implement a telephone function with an integrated circuit.
Integrated circuits offer the possibility of adding many new features to enhance the use of telephones. In many of these features it is required that the circuit be cognizant of the on-hook or off-hook condition of the telephone handset. Although a power terminal is present which makes a transition as the handset is moved on-hook and off-hook, this power signal is subject to intermittent operation and noise such that it cannot serve directly as a reliable indication of the on-hook or off-hook condition of the handset. Therefore, there exists a need for a circuit which can reliably determine the on-hook and off-hook condition of a telephone handset despite the presence of noise and the intermittent operation of telephone circuits.
SUMMARY OF THE INVENTION
In a selected embodiment of an on-hook/off-hook detector circuit in accordance with the present invention there is included a series combination of a resistor and capacitor connected together at a junction node between first and second power terminals. A current path is provided from the junction node to the second power terminal when a status signal is received at a first state. The junction node is clamped at a first preset voltage when the current path is provided to the second power terminal. The current path is opened when the status signal is received at a second state. When the current path is opened the capacitor is charged through the resistor to raise the voltage at the junction node to approach the voltage at the first power terminal. A transistor connected to the junction terminal changes state when the junction terminal reaches a second preset voltage to drive an output terminal from a first state to a second state. The state change at the output terminal occurs after a time delay from the state change of the status signal due to the charging time of the capacitor.
BRIEF DESCRIPTION OF THE DRAWINGS
For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following description taken in conjunction with the accompanying drawings in which:
FIG. 1 is a schematic illustration of the on-hook/off-hook detector circuit of the present invention; and
FIG. 2 is a illustration of waveforms which are present in the circuit illustrated in FIG. 1.
DETAILED DESCRIPTION OF THE INVENTION
Referring to FIG. 1 a detector circuit 10 is included within the dotted lines and comprises an on-chip part of an overall integrated circuit. The integrated circuit has a plurality of external terminals for transmitting and receiving various signals. A number of these terminals are related to the operation of the detector circuit 10. A tone generator power terminal 12 receives a voltage which also powers a tone generator circuit V.sub.G. (A tone generator circuit is not illustrated.) The signal V.sub.G is voltage divided down from a power voltage supplied from the central telephone office. The signal V.sub.G can serve as a status signal to indicate the position of a switch hook (not shown) for a telephone handset. When the switch hook is in the on-hook position the signal V.sub.G is at essentially 0 volts, but when the switch hook is in the off-hook position the signal V.sub.G is at approximately -2 volts and is utilized to power a tone generator circuit. The signal V.sub.G is illustrated in FIG. 2. Although it is shown as directly corresponding to switch ho
REFERENCES:
patent: 3205312 (1965-09-01), Brightman et al.
patent: 4220825 (1980-09-01), Fahey
Mostek Corporation
Robinson Thomas A.
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