This invention relates to an information communicating apparatus connected to a telephone subscriber line, such as a modem device, a telephone set, a facsimile machine, etc., and, more particularly, to a line interface circuit.
In the field of communication, in order to provide for both protection of network facilities having high public use and protection of terminals, a high insulation quality is required at the boundary between a network and a terminal, and, for that reason, compact transformers for communication which make provision for a high insulation quality have been used previously. However, a problem has occurred in that, in proportion to the popularization and evolution of personal terminals, it is required for transformers which are further miniaturized and reduced in weight to be used for a portable terminal; however, this requirement can not be satisfied sufficiently only with improvement in the material and arrangement of parts in the transformers. In light of this, the possible use of an isolator has been examined.
Also, in the application of this technology to the field of measurement, medical treatment or the like, it may be necessary to isolate a signal detecting part and a signal handling part, e.g. a sensor and a signal processing circuit. It is known that an isolator can be used as an isolation and separation means in this case.
For the purpose of making the isolator miniature, reliable and low-cost, a capacitive isolator has been developed. A ceramic capacitor for power application and surge protection, which is made on the basis of a high voltage capacitor technique providing an individual part constituting an-isolation barrier, is known; and, a circuit block for signal transmission using this capacitor is referred to as a capacitive isolation amplifier or a capacitive isolation coupler. This device has been used Since the 1970s.
Also, what is disclosed in Japanese Patent Application Laid-open N6-.62-260408 (1987) achieves isolation and separation of an analogue signal transmission in which a pulse signal obtained by the modulation of the analogue signal is transmitted to the secondary side, which is insulated and separated by a capacitor; and, after decoding, the analogue signal is regenerated.
Further, Japanese Patent Application Laid-open No. 11;136293 (1999) entitled xe2x80x9cIsolator Circuit and Monolithic Isolatorxe2x80x9d and Japanese Patent Application Laid-Open No. 11-317445/1999 entitled xe2x80x9cIsolator and Modem Device using itxe2x80x9d disclose a technique by which an isolator is formed on an LSI so that it has an isolation and separation function. This technique also provides means for achieving a system LSI including an insulation and separation function for the modem, etc.
A system LSI for a modem including the insulation and separation function according to the above-mentioned technology dispenses with an insulation transformer and a photo coupler, which were needed for insulation previously, whereby it is possible to reduce the number of parts, the cost and the size of the mounting area. It also enables more suitable system design by improving the degree of freedom for chip division without encountering a restriction of the insulating location.
Also, by using electric power supplied from the system switch to operate the line side of two regions (line side: subscriber line side, host side: microprocessor side) on the LSI insulated by an isolator, it is possible to reduce the power consumption of the host side by a large amount. Therefore, it is possible to extend the life of a battery driving the host side in a portable terminal.
However, in the system LSI for a modem including the insulation and separation function disclosed in the above-mentioned Japanese Patent Application Laid-open No. 11-136293 (1999) and Japanese Patent Application Laid-Open No. 11-317445 (1999), further consideration needs to be given to supplying power to a circuit used at the time of the reception of the caller identification information (Caller ID).
In the above-mentioned prior art, the supply of power from the system switch uses a loop current which determines whether a telephone set is in an on-hook condition or an off-hook condition. In general, it is required that a loop current of 20 to 120 mA is caused to flow to inform the system switch of the fact that the telephone set is in an off-hook condition, depending upon the standards defined by the government. Also, in order to inform the system switch of the fact that the telephone set is in the on-hook condition, the loop current must be maintained below a predetermined constant value (for example, 3 mA). Further, at the time of the reception of the Caller ID indication, the on-hook condition must be established in many countries (for example, in Japan), and a circuit for the reception of the Caller ID indication must operate with the consumption of electric power below a predetermined constant value (for example, 3 mA).
In order to solve the above-mentioned problems, the present invention utilizes the following means:
(1) An isolator interface has a parallel/serial conversion circuit for converting the output of a digital analogue converter having plural bits to a serial signal, or a multiplexer for transmitting in a time divisional manner the output of the analogue digital converter and the status information of a line interface circuit. At the time of a normal operation, the output of the analogue digital converter is input to the isolator via the isolator interface, and at the time of the reception of a call signal, or at the time of the reception of a caller identification signal, the output of the analogue digital converter is input directly to the isolator.
(2) The isolator interface has a parallel/serial conversion circuit for converting the output of the analogue digital converter having plural bits to a serial signal, or a multiplexer for transmitting in a time divisional manner the output of the analogue digital converter and the status information of the line interface circuit, and a clock generating circuit for producing a clock signal of a first frequency and a clock signal of a second frequency lower than the first frequency. At the time of normal operation, the analogue digital converter and the isolator interface are driven by the clock signal of the first frequency output from the clock generating circuit, and, at the time of the reception of the call signal, or at the time of the reception of the caller identification information, the analogue digital converter and the isolator interface are driven by the clock signal of the second frequency output from the clock generating circuit.