GB2350743A

GB2350743A - Signal detecting circuit

Info

Publication number: GB2350743A
Application number: GB9912934A
Authority: GB
Inventors: Bar-Chung Hwang
Original assignee: Winbond Electronics Corp
Current assignee: Winbond Electronics Corp
Priority date: 1999-06-03
Filing date: 1999-06-03
Publication date: 2000-12-06
Also published as: GB9912934D0

Abstract

A telephone signal detecting circuit is provided, which can identify whether an input signal is a line reversal signal or a ring signal. Providing this telephone signal detecting circuit in a ring detector of a conventional telephone set can lower the burden on a microprocessor of the telephone set and lessen the loading of the power supply. Schmitt triggers 32, 31 of two different trigger levels are used to respectively detect the voltage change of the discharge of a capacitor (32) and the ripple signal produced due to the charge of the capacitor by the ring signal (31). The input signal can be easily identified to be a line reversal signal or a ring signal through a simple flip-flop circuit 40 that can reduce the influence of the bouncing effect. The telephone signal detecting circuit is operated in a static mode and is almost free of power-consumption when it is not working.

Description

2350743 TELEPHONE SIGNAL DETECTING CIRCUIT This invention relates to a

telephone signal detecting circuit, which can distinguish between a line reversal signal and a ring signal.

A telephone system (telephone set) is either on hook or off-hook. When a telephone system is on-hook, there are different signalling sequences to interrupt that state according to different standards. Referring to Fig. 1, which illustrates timing diagrams of Caller ID signals, Fig. la represents the timing of a Caller ID for the BT (British Telecom) standard. That is, a Caller ID signal is sent to the telephone system before a ring signal. However, the telephone system stays in a standby state, which is very low in power consumption when it is on-hook. Therefore a line reversal signal is used to wake up the microprocessor of the telephone system before the arrival of the Caller ID signal.

Fig. lb shows Bellcore's standard, in which a caller ID signal is sent after the first ring. However, the caller ID signal is not to be received when the line reversal signal appears. A telephone set, which can identify the calling party, normally determines whether a received signal is a line reversal signal or a ring signal according to the period of interrupting the microprocessor of the telephone set, and then determines whether to activate the caller identifying IC. Another simple method is to activate the IC regardless of whether the microprocessor is interrupted by a line reversal signal or a ring signal. After a given period of time has passed, the caller identifying IC is turned off if no caller ID signal is received.

However, power is wasted no matter which method is adopted. Moreover, the first method also wastefully uses the microprocessor in the telephone set.

Fig. 2 shows a diagram of a detection circuit for the line reversal signal or the ring signal in a normal telephone set. In Fig. 2 there are only illustrated a rectifier circuit 10 and a ring detector 20 since these components are relevant to the detection of the signal S input to the telephone set. In the rectifier circuit 10, the capacitors C1, C2 can block the DC voltage in the tip/ring line. The AC signal can be obtained by the resistor R1 and capacitor C1 or the resistor R2 and the capacitor C2 and then rectified by the rectifier consisted of D1, D2, D3 and D4. The rectified signal is input to the ring detector 20. A part of the signal is sent to the input terminal RNGDI (ring detector input) of the Schmitt trigger 201 through the resistors R3 and R4. Therefore, if the change of the signal in the tip/ring line is large enough, Q1 will discharge the capacitor C3, and the output terminal RNGON becomes low level. When the signal in the tip/ring line is stable, Q1 will not discharge the capacitor C3. The capacitor C3 is then charged by Vcc through the resistor R5, and the output terminal RNGON becomes high level. Fig. 3 illustrates a diagram of the waveform for the signal on RNGRC and RNGON while inputting the line reversal signal or the ring signal. The output terminal RNGON is low level when the voltage of RNGRC drops off. Generally speaking, it can be identified whether there is a line reversal signal or a ring signal in accordance with the voltage level of the output terminal RNGON.

However, a bouncing phenomenon often occurs when the exchange activates a line reversal signal or a ring signal, therefore the transistor Q1 is in a state of "on, off, on, off" at the beginning of discharging the capacitor C3, as shown in Fig. 4. A series of rises and f alls of the signal similar to a ripple causes an error in signal detection.

Embodiments of this invention can provide a telephone signal detecting circuit, which can identify whether an input signal is a line reversal signal or a ring signal before the end of the detection for the line reversal signal or the ring signal.

The telephone signal detection method employed in this invention is to latch the identifying signal before the end of the detection for the line reversal signal or the ring signal and interrupt the microprocessor. The microprocessor recognizes that the input signal is a line reversal signal or a ring signal according to the identifying signal. Then the interrupt signal is clear, and the microprocessor can go on the next task.

In addition, embodiments of this invention use Schmitt triggers of different trigger levels, wherein Schmitt triggers having a higher trigger level are used to detect the input line reversal signal and ring signal, whilst Schmitt triggers having a lower trigger level are used to detect the ripple signals of the capacitor while detecting the ringing, and accordingly determine the ring signal. The ripple detecting circuit can overcome the signal detecting error caused by the bouncing effect occurring when the exchange activates the line reversal signal and the ring signal.

A telephone signal detecting circuit embodying this invention includes Schmitt triggers of two different trigger levels, which are a first level and a second level. As the capacitor is charged and discharged, the trigger having the first trigger level outputs a ring detecting signal when the voltage of the capacitor is lower than the first level, and the trigger having the second trigger level outputs a pulse signal corresponding to the ripple signal when the voltage of the capacitor is lower than the second trigger level. Furthermore, the ripple detecting circuit can filter out the bouncing noise by the driving of the trigger having the first trigger level and under the control of the trigger having the second trigger level, and outputs a ring signal after receiving the pulse signal. Thus the input signal is determined to be a line reversal signal when the ring detecting signal is low level and no ripple signal outputs. otherwise the input signal is determined to be a ring signal if the ring detecting signal is low level and a ripple signal is output.

The following detailed description, given by way of example and not intended to limit the invention solely to the embodiments described herein, will best be understood in conjunction with the accompanying drawings in which:

is Fig. 1 is a timing diagram of a call ID signal in a communication standard; Fig. 2 is a diagram illustrating the detection circuit for detecting the line reversal signal and the ring signal in a conventional telephone set; Fig. 3 is a timing diagram for the detecting circuit of Fig. 2; Fig. 4 is a diagram illustrating the bouncing phenomenon occurring when the exchange activates a line reversal signal or a ring signal; Fig. 5 is a diagram illustrating an embodiment of the telephone signal detecting circuit according to this invention; and Fig. 6 is a timing diagram for the detecting circuit of Fig. 5 Fig. 5 illustrates the circuit diagram for a telephone signal detecting circuit embodying this invention, in which the circuit for inputting RNGDI signal is omitted to simplify the description. In the drawing, the flip-flops A, B, and C constitute a ripple detecting circuit 40, which can effectively absorb the bouncing pulse produced by activating the line reversal signal or ring signal, and can detect the ripple signal of the capacitor while ringing. This circuit uses Schmitt triggers 31 and 32 of two different trigger levels V1 and V2, wherein the trigger level V1 < V2, and the trigger having the trigger level V2 is used to detect the envelope of the ring signal. The trigger having the trigger level V1 is used to detect the ripple signal produced by the charge /discharge of the capacitor while ringing. A signal, which can be used to identify the line reversal signal or the ring signal, can be obtained after the flip-flops are triggered.

Fig. G illustrates the timing diagram of the above circuit, wherein RNGDI represents input signal, Q1 is a transistor, V2 SCHMITT O/P indicates the output of the Schmitt trigger having the trigger level V2, V1 SCHMITT O/P indicates the output of the Schmitt trigger having the trigger level V1, and RING represents the ringing signal. In Fig. G, the bouncing at point I is absorbed by flip-flop A, and the bouncing results in a small charge/discharge for the capacitor. If the voltage change of the charge /discharge is V1, the flip-flop is triggered at point J, then the flip-flop B absorbs the bouncing of the voltage. The ripple signal, which is produced when the ring signal causes the charge /discharge of the capacitor, is received by the flip-flop C at point K. This causes the ring signal RING to be high level. The input voltage RNGDI is changed along with the input voltage RNGDI after passing through the Schmitt trigger 30. The transistor Q1 turns ON (i.e. conducts) to discharge the capacitor C3, or turns OFF (i.e. non-conducts) so that the power supply VCC can charge the capacitor C3 through the resistor R5. The Schmitt trigger 32 having the trigger level V2 is used to detect the envelope of the ring signal, i.e., the output of the Schmitt trigger 32 becomes high level when the input voltage is lower than V2. The Schmitt trigger 31 having the trigger level V1 is used to detect the ripple signal produced by the charge /discharge of capacitor while detecting the ringing.

In the circuit of Fig. 5, the flip-flops A, B, C of the ripple detecting circuit 40 can also be negative flip-flops to obtain an identical result. The ripple detecting circuit 40, which can be a counter having the functions of Enable and Clear, can determine a ring signal only if the counter can count the pulse signal output by the Schmitt trigger 31 having the trigger level V1 corresponding to the ripple signal produced by the charge /discharge of the capacitor by the ring signal.

While the present invention has been particularly shown and described with reference to a preferred embodiment, it will be readily appreciated by those of ordinary skill in the art that various changes and modifications may be made without departing from the scope of the invention. It is intended that the claims be interpreted to cover the disclosed embodiment, those alternatives which have been discussed above and all equivalents thereto.

Claims

1. A telephone signal detecting circuit for determining an input signal to be a line reversal S signal or a ring signal, comprising:

a capacitor, controlled by a switch, which is discharged when the switch is closed (i.e. conducting), and is charged through a power supply when the switch is opened (i.e. non-conducting); a first trigger having a first trigger level, for outputting a ring detecting signal when a voltage of the capacitor is lower than the first trigger level during the charge/discharge of the capacitor; a second trigger having a second trigger level, for outputting a pulse signal corresponding to a ripple signal produced due to the charge /di s charge of the capacitor when the voltage is lower than the second trigger level during the charge/discharge of the capacitor; a ripple detecting circuit, which is driven by the first trigger and controlled by the second trigger, for filtering out the bouncing noise, and outputting a ring signal after receiving the pulse signal, so that the input signal is determined to be a line reversal signal when the ring detecting signal is low level and no ripple signal outputs, otherwise the input signal is determined to be a ring signal if the ring detecting signal is high level and a ripple signal is output.

2. A circuit as claimed in claim 1 wherein the first trigger and the second trigger are Schmitt triggers.

3. A circuit as claimed in claim 1 or 2, wherein the ripple detecting circuit includes three flip-flops which are serially connected.

4. A circuit as claimed in claim 1, 2, or 3, wherein the ripple detecting circuit is a counter having enable and clearance functions.

5. A circuit as claimed in any preceding claim, wherein the first trigger level is higher than the second trigger level.

6. A circuit as claimed in any preceding claim, wherein the ripple detecting circuit comprises flip flops each having an enable input, in which the enable input of the flip-flops is high level, and the output and the input of the flip-flops are the same on the rising edge of a clock signal.

7. A telephone signal detecting circuit for determining an input signal to be a line reversal signal or a ring signal, comprising:

a rectifier circuit for receiving and rectifying an AC signal from a telephone line to obtain a signal; and a detecting circuit, which is used to determine whether an input signal is a line reversal signal or a ring signal, for latching an identifying signal when the detection of the line reversal signal or the ring signal ends, and interrupting a microprocessor so that the microprocessor can identify whether the input signal is a line reversal signal or a ringing signal in accordance with the identifying signal.

8. A circuit as claimed in claim 7, wherein the detecting circuit includes a capacitor and two Schmitt triggers respectively having a first trigger level and a second trigger level, in which the trigger having the first trigger level outputs a ring detecting signal when the voltage of the capacitor is lower than the first level, and the trigger having the second trigger level outputs a pulse signal corresponding to the ripple signal when the voltage of the capacitor is lower than the second trigger level during the charge /di s charge of the capacitor.

9. A circuit as claimed in claim 8, wherein the Schmitt triggers of the first and second trigger levels are respectively used to detect an envelope of the ring signal and a ripple signal produced due to the charge /di s charge of the capacitor while detecting ringing.

10. A circuit as claimed in claim 9, wherein the detecting circuit includes a ripple detecting circuit including three flip-flops, which apply the output of the Schmitt trigger detecting the envelope of the ring signal to enable and clear inputs of the flip-flops, and apply the output of the Schmitt trigger detecting the ripple signal to a clock input of the flip-flops.

11. A circuit as claimed in claim 10, wherein the output of the three flip-flops equals the input thereof when an enable input of the flip-flops is high level and on the rising edge of the clock input.

12. A circuit as claimed in any of claims 8 to 11, wherein the first trigger level is higher than the second trigger level.

13. A circuit according to any of claims 1 to G, wherein the switch is a transistor.

14. A telephone signal detecting circuit, substantially as hereinbefore described with reference to Figures 5 and 6 of the accompanying drawings.