Event detection circuit

A network interface circuit that screens out transient signals and provides an indication to the line side that an actual event has occurred, so that appropriate discrimination circuitry is powered up to determine the exact nature of the actual event only when actually needed. The present invention develops an AC signal that represents the events that are desired to be detected. This AC signal is timed so that it has a sustained rate (e.g., a 1 millisecond burst) that is unlike any transient that would occur on the line. Thus, unless the incoming signal meets the timing requirements of the circuit, it is disregarded as a being a transient and no action is taken to determine the exact nature of the signal. This avoids the need to invoke the discrimination circuits that have large power requirements until they are actually needed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is related to an application entitled “Combination Clock and Charge Pump for Line Powered DAA” and an application entitled “Line Interface Circuit with Event Detection Signaling”, both of which are assigned to the assignee of the present invention and both of which were filed concurrently with this application.

FIELD OF THE INVENTION

This invention relates generally to the field of telecommunications networks, and more particularly, to a network interface circuit that determines whether a signal on a TIP and RING line represents an actual event (e.g., a ring signal) or a transient noise signal (e.g., noise generated on the line as a result of a lightning strike).

BACKGROUND OF THE INVENTION

Telephone networks comprise a series of interconnected subsystems that are linked together at points called interfaces. These interfaces provide a link between old and new equipment and allow for simplified design and maintenance. A local loop is an example of an interface that connects a subscriber's telephone set and the central office.

The U.S. Federal Communications Commission (FCC) and counterpart regulatory agencies in other countries require, among other things, electrical isolation between the line side and the user devices on the user side. Electrical isolation protects the line side from damage transmitted from the device side and vice versa.

The isolation between the line side and the device side is often accomplished within the interface circuit. A modem is an example of an interface circuit that may include circuitry that provides electrical isolation from the line in addition to the signal modulation and demodulation function of the modem. Isolation transformers, optical coupling, and capacitive coupling are all examples of known methods of isolating the line side from the device side.

It is known to include in the interface circuit a discrimination circuit to identify the occurrence of signals is indicative of specific “events” that occur on the telephone line and discriminate between the different signals to determine what the particular event signifies.

FIG. 1is a block diagram of a typical telecommunication system5showing the connection between a subscriber and a central office that controls the telecommunication system. Central-office equipment10on the line side15of the telecommunication system5is connected to user device20(e.g., telephones or computer terminals) on the device side25of the telephone system5via an interface circuit30.

Many components (e.g., data access arrangements (DAAs) or CODEC's) of telephone interface circuits are PSTN line-powered circuits that also require isolation from low-voltage power supplies. Because of this required isolation, line powered interface circuits will not function until they are connected to a power source, i.e., the interface circuit must be activated when it is needed to connect the line side to the device side. When a circuit on the line side is placed in the “on-hook” state (e.g., a telephone receiver is placed in its cradle) the local loop is opened and almost all of the power to the interface circuit is cut off. Activating the DAA or CODEC requires that some power must be drawn from the loop or from another source. While in the on-hook state a small amount of current (idle-state loop current) can be drawn for a short period of time from the TIP/RING line to register that an event has occurred on the PSTN TIP and RING line. However, it is extremely inefficient to draw the extra power unless it is really needed, i.e., it is extremely wasteful of time and resources to power up the DAA or CODEC when a transient noise signal occurs on the TIP and RING line.

The primary events that the interface must detect while in the on-hook state are the application of a TIP and RING signal and a polarity reversal of TIP and RING DC voltage, either of which may be used to signal the need for more power. A problem can arise because ordinary transients on the line can have electrical characteristics very similar to those of actual events. To discriminate between transients and actual events, prior art interface circuits employ the previously mentioned discrimination circuitry that determines the exact nature of every signal introduced thereto, but these additional circuits draw higher amounts of power. While drawing additional power is acceptable on a temporary basis, the amount of power available is limited. Discrimination circuits must operate at power levels no higher than the amounts determined by regulatory agencies as “satisfactory” on-hook leakage currents. Different amounts of leakage current are allowed for short periods of time. As an example, ringer AC current may be rectified and used as a source of power. Regulatory agencies allow higher leakage currents during Calling Number Delivery (Caller-ID).

In addition to determining what an event is, the event signals themselves must be transmitted from the device side to the line side. A problem arises because high voltage isolation prevents the transmission of the event signals from the device side to the line side. Prior art circuits avoid this problem by using optical couplers to transmit the signal caused by the actual event to the low voltage interface. These circuits employ a general purpose optical coupler with an LED input and a photo-transistor output. These optical couplers require “light pipes” which are cavities on the chip between the emitter and the detector of the optical coupler to allow the light to pass between the two. These light pipes increase the size and cost of the interface circuit. Capacitive coupling, another known isolation method, allows the circuit size to remain small and low cost, but the rate at which the events occur are too slow to accurately transmit them from the device side to the line side using capacitive coupling.

To avoid powering up the discrimination circuitry except when it is needed to process an actual event, it would be desirable to employ a preliminary circuit that, before invoking the discrimination circuitry, distinguishes between actual events (e.g., a ring signal, a polarity reversal, an audio signal) and noise (transient spikes on the line caused by a variety of sources, e.g., lightning, battery noise, etc.). Accordingly, there remains a need for a simplified, smaller, and less costly detection circuit that can operate on the minimal amount of power available when the circuit is in the on-hook state to preliminarily distinguish between actual and noise events, and transmit event signals between the device side and line side while maintaining electrical isolation between the device side and the line side.

SUMMARY OF THE INVENTION

These and other aspects of the invention may be obtained generally in a network interface circuit that screens out transient signals and provides an indication to the line side that an actual event has occurred, so that appropriate discrimination circuitry can be powered up to determine the exact nature of the actual event.

According to one exemplary embodiment of the present invention, an event detection apparatus is disclosed for distinguishing between the occurrence of actual events and transient events on the tip and ring lines of a phone line, comprising, a signal generator generating an output signal upon receipt of an input signal from the tip and ring line, and a signal receiver receiving the output signal from the signal generator, wherein the signal generator outputs an event-detect signal only upon receipt by the signal generator of an input signal caused by the occurrence of an actual event.

In a preferred embodiment, the signal receiver comprises a counting device for counting the number of output signals generated by the signal generator, a clear circuit for clearing the counting device after a predetermined time period, and a gate outputting the event-detect signal if the counting device counts a predetermined number of output signals within the predetermined time period.

In a more preferred embodiment, the signal generator comprises a differential comparator having inputs coupled to the tip and ring line and an output, a timer having an input coupled to the output of the differential comparator and an output, and an oscillator having a first input coupled to the output of the differential comparator and a second input coupled to the output of the timer, and having an output coupled to the signal receiver.

In still another preferred embodiment, the signal generator is capacitively coupled to the signal receiver. Further the counting device can comprise at least two binary counters operating in parallel and in alternating sequence, each binary counter having a threshold level which, when reached, causes the counter reaching the threshold level to output to the gate a signal indicating the reaching of the threshold level.

DETAILED DESCRIPTION OF THE INVENTION

As described below in more detail and in accordance with an embodiment of the invention shown inFIG. 2, a network interface circuit30comprises a high voltage event detect generator (EDG)100and a low voltage event detection receiver (EDR)200. The EDG100is capacitively coupled to the EDR200via capacitors140and150, thereby isolating the EDG100from the EDR200and, therefore, isolating the line side from the device side. Although not part of the present invention, in practical application the interface circuit30includes additional parallel-connected processing circuitry35e.g., modem circuitry, a digital signal processor, and/or DAA/CODEC circuits that is also isolated via capacitors140and150as shown.

FIG. 3is a detailed block diagram of an event detection generator and an event detection receiver in accordance with the present invention. As shown inFIG. 3, EDG100comprises a differential comparator110coupled to the TIP and RING line, a timer120coupled to the output of differential comparator110, and an oscillator130coupled to the output of differential comparator110and the output of timer120. The oscillator130is turned on when a signal is output from differential comparator110, and the oscillator130is timed to remain on for a predetermined period of time controlled by timer120. Oscillator130operates at a frequency high enough that the capacitive coupling capacitors140and150receive sufficient energy to properly transmit a signal indicating the occurrence of an event to the low voltage circuitry of EDR200.

Whenever an initial threshold condition is met as sensed by differential comparator110in EDR100, timer120and oscillator130are triggered, causing oscillator130to operate for the time period specified by timer120. The output of oscillator130is directed across the differential interface comprising capacitors140and150to EDR200. As discussed in more detail below, the output of oscillator130sends an AC signal across capacitors140and150that is unlike any transient that would occur on the line, in the form of a high-frequency sustained burst.

In the preferred embodiment, comparator110is a hysteresis comparator. If a particular signal is below a minimum differential current level, e.g., 100 μA AC peak-to-peak, the chance of the signal being a useable valid event is very low. Thus, it would be preferable to ignore such minimum signals. Therefore, the hysteresis is designed into the differential comparator110to produce a “dead band” below which the network interface circuit30will not respond.

EDR200comprises a differential comparator210, counters220and230coupled to the output of comparator210, a clear circuit240switchably connectable between counter220and counter230via switch245, and OR gate250coupled to the outputs of counters220and230. Output counters220and230have an overflow output such that they output a digital signal when they reach capacity. Since events may occur at any time with respect to the sampling by the active counter, two parallel circuits are employed in EDR200so that one counter may be cleared by clear circuit240while the other counter continues to register the event. Thus, via switch245, clear circuit240alternately clears counter220and counter230so that an event in the process of being counted is not lost.

By way of example and without limitation, in the preferred embodiment timer120is a 1 millisecond timer and oscillator130is a low power, 1 MHz oscillator. To reduce costs, the oscillator130can have process dependant variation such as a ring oscillator and should drive timer120. By driving the timer120with the oscillator130, if there is a variation in the 1 MHz oscillator signal, the period of timer120will be reduced when the oscillator frequency increases and will increase when the oscillator frequency decreases. For example, if the oscillator130is variable between 1 and 3 MHz, timer120will operate between 1 mS and 330 microseconds, depending on (and controlled by) the frequency of the oscillator130. The low voltage circuitry of EDR200can be built to match these variations in terms of monitoring the event; This arrangement allows the use of a less-costly oscillator that does not require trimming.

In this example, if the oscillator130is controlled by timer120to remain active for 1 mS and the frequency of the oscillator130is 1 Mhz, and the capacitors140and150are 5 picoFarad capacitors, then 1000 pulses will be output to capacitors140and150when a signal is output from the differential comparator.

Since, in this example, 1,000 transients must occur within the 1 millisecond cycle of the timer120before EDR200will output an indication that an actual event has occurred, it is virtually impossible for EDR200to respond to anything other than an actual event. If the EDR200receives less than 1,000 transients within the period of clear circuit240, the active counter in EDR200is cleared and a new count begins. An actual event will appear on the counter as a 1 millisecond burst of oscillator130, or in other words, greater than 1000 transients within the period of clear circuit240will occur, and this would be registered as an actual event.

In a preferred embodiment, Counters220and230are conventional binary counters with an overflow output so that they will output a digital signal when they reach capacity. As mentioned above, dual counters220and230are employed to assure that there is constant monitoring of the line. For example, in the example set forth above, a 1 mS timer is utilized. If half-way through the occurrence of an event (e.g., 0.5 mS into the event) the clear circuit240operates to clear counter220, secondary counter230, operating in parallel with counter220, will properly register the event.

In the preferred embodiment, EDR200is designed for low power operation, e.g., it uses static CMOS circuitry operating at low frequencies, thereby drawing very little current. For example, EDR200may be operating off the battery power of a lap top computer and must be able to operate on the power provided by the lap top computer when it is operating in the “battery saver” or “sleep” mode.

The present invention develops an AC signal that represents the events that are desired to be detected. This AC signal is timed so that it has a sustained rate (e.g., a 1 millisecond burst) that is unlike any transient that would occur on the line. Thus, unless the incoming signal meets the timing requirements of the circuit, it is disregarded as being a transient and no action is taken to determine the exact nature of the signal. This avoids the need to invoke the discrimination circuits that have large power requirements until they are actually needed.

Thus, the present invention acts as a fast-acting screening device, e.g., a “first line of defense” for determining what has occurred on a TIP and RING signal. It operates on idle-state loop current and allows the determination to be made that there is an event that requires further investigation before powering up additional circuitry to identify precisely what kind of event it is. When it is known that an actual event has occurred, it is acceptable to draw more current to determine exactly what it is.

The events that would be considered valid on TIP and RING lines are those that are considered differential in nature. A differential event involves a current that flows from TIP to RING as opposed to a current that flows from TIP to system ground or RING to system ground. A current that flows to system ground is called a longitudinal or common-mode current. The present invention is designed to be sensitive to differential currents and insensitive to longitudinal or common-mode currents.

FIG. 4illustrates an example of a clear circuit240in accordance with the present invention. Obviously, clear circuit240can be configured in many different forms and the example shown inFIG. 4should not be considered as limiting. Referring toFIG. 4, a D flip-flop300has a Q output connected to counter220and a Q_ output coupled to counter230. A clock signal is input to D flip-flop300from divider320. Divider320is driven by oscillator310. Oscillator310with divider320operates to produce a clear function240that insures that events will be properly registered even for the wide variation in oscillator130. The D input of the D flip-flop330is coupled to the Q_ output of D flip-flop300. In this configuration, with every positive clock pulse from oscillator310, the D flip-flop circuit300will clear counter220, and with every negative clock pulse from oscillator310, D flip-flop300will clear counter230. Thus, if a 3 millisecond period was desired for each counter220and230, oscillator310should be a 660 Hz oscillator. This would result in the 3 millisecond period for the counters as discussed.