Abstract:
A cordless telephone system includes a base station having a base clock indicative of the time of day, and a handset having a handset clock indicative of the time of day. The handset is synchronized to the base station. When power is lost in the handset or the base station, the unit with power detects loss of synchronization therebetween. Upon restoration of the power, the continually powered unit provides the power restored unit with clock recovery information for updating the clock of the power restored unit. The clock recovery information is generated in the continually powered unit by counting elapsed time when the loss of synchronization is detected.

Description:
FIELD OF THE INVENTION 
     The invention relates to a cordless telephone comprising at least a base station with a clock, a handset and clock correction means. 
     The invention also relates to a base station and a handset intended to be used in such a telephone. 
     The invention also relates to a method of managing clocks for a cordless telephone. 
     The invention finds important applications, notably for cordless telephones of which the base station has a function of answering machine with date-time stamping of the received messages. For such applications it is necessary to have a calendar in the base station(s). 
     BACKGROUND OF THE INVENTION 
     Patent abstract of Japan no. A-06 120 881 describes a cordless telephone having a base station and a handset which include each a clock circuit. The clock of the base station is controlled by a clock generation circuit in the telephone switch that connects the base station to the telephone line. And clock correction means are provided for correcting the clock of the handset based on the clock of the base station. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to propose a cordless telephone having an autonomous, simple and economic time system. 
     For this purpose, a telephone according to the invention and as described in the opening paragraph is characterized in that said clock correction means comprise: 
     means for the base station to detect a loss of said clock, 
     means for at least one handset to generate clock recovery information, 
     means for the handset to transmit said clock recovery information to the base station. 
     The invention thus makes use of the fact that handsets have a power supply battery which gives them large autonomy in case of a power cut. When the base station has lost its time, for example, after a power cut, recovery information can be transmitted thereto by the handsets. The invention thus enables to do without the use of a specific autonomous back-up and counting element in the base station to continue calculating the clock notably in case of a power cut. 
     In a first embodiment of the invention, said base station comprises a non-volatile back-up memory of said clock and said recovery information generating means comprise calculation means for calculating the time elapsed between the detection of the loss of the clock and the transmission of the recovery information. 
     In a second embodiment of the invention, the handset has a clock slaved to that of the base station for delivering said recovery information. 
     In this embodiment, the clock of the handsets is normally updated by the base station. This embodiment thus enables to do without the use of a non-volatile memory for regularly backing up the system clock. The use of a non-volatile memory actually has the following drawback: the counting of the duration of a power cut by the handset contains an error that is equal to the delay between the last backing up of the clock in the base station and the start of the power cut. To remedy this, the rate of the clock back-ups (of the order of several seconds) is to be increased. The number of times a non-volatile memory is written is limited, which provides that after a certain period of time (several years) the clock system will no longer function correctly. 
     Finally, a clock management method according to the invention and as described in the opening paragraph is characterized in that it comprises: 
     detecting a loss of said clock by the base station, 
     generating recovery information of said clock in at least one handset, 
     transmitting said recovery information to the base station by the handset. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter. 
     In the drawings: 
     FIG. 1 diagrammatically shows an example of a telephone according to the invention, 
     FIG. 2 diagrammatically shows the operation of a handset according to the invention as regards the clock management in the first embodiment of the invention, 
     FIG. 3 diagrammatically shows the operation of a base station according to the invention as regards the clock management in the first embodiment of the invention, 
     FIG. 4 diagrammatically shows the operation of a handset according to the invention as regards the clock management in the second embodiment of the invention, and 
     FIG. 5 diagrammatically shows the operation of a base station according to the invention as regards the clock management in the second embodiment of the invention. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     In FIG. 1, the cordless telephone shown is a telephone meeting the DECT standards. Reference  1  shows a base station to which one or various handsets  2 ,  3  . . . are connected by radio. This base station  1  notably comprises a line circuit  10  which enables the base station to be connected to the switched circuit via a telephone line  12  and a radio circuit  14 , which authorizes the dialogue with the various handsets  2 ,  3 , . . . , by transmitting and receiving waves via an antenna  16 . The circuits  10  and  14  are connected via a signal processing circuit  15  formed around a signal processor DSP. All these elements of the base station  1  are managed by a microprocessor assembly  20  which is usually notably formed by a random-access memory  24 , a read-only memory  26  which contains operating instructions of the telephone and by a management processor  28  which contains a time base  29 . 
     In the first embodiment of the invention, the microprocessor assembly  20  moreover includes a non-volatile memory  27 , for example, a FLASH memory or an EEPROM memory. 
     Only the handset  2  is shown in detail in FIG. 1, as the handsets  3 , . . . can have an identical structure. It comprises a communication assembly  40  having an antenna  41  which enables it to communicate with the base station  1 . This assembly processes the information coming from the microphone  42  and also produces signals for an earphone  44 . The handset  2  also includes a display  60  on which some information can be displayed, a keyboard  61  and a power supply battery  63 . A management element  70  is also included. Just like assembly  20 , it is formed by a random-access memory  74 , a read-only memory  76  which contains operating instructions and a management processor  78  which contains a time base  79 . 
     In the first embodiment of the invention, clock recovery information is transmitted to the base station  1  by the handsets  2 ,  3 , . . . when they detect that the base station has lost its time. In the example that has just been described, such a loss is detected when there is a desynchronization of base station and handset. In fact, the power cuts (which prevent the base station from keeping its time updated and which thus cause the loss of this time) are one of the causes of a loss of synchronization of base station and handset. The choice of this criterion enables to maximize the limitation of unwanted transmissions of recovery information from the handset to the base station. 
     FIG. 2 diagrammatically shows the operation of the handset as regards the clock management. 
     When a handset loses the synchronization with the base station (box  101 ), it knows that this is no longer provided and thus that there is a risk of losing its time. The handset thus keeps this information updated for the base station: in box  102  it initializes a variable X 2  (called recovery information) and in box  103  it starts counting the time T that elapses. The variable X 2  is regularly updated, for example, every second, whereas the synchronization of base station and handset is not re-established (this is symbolized for box  104  by the arrow t). When the synchronization of base station/handset is re-established (box  105 ), the handset sets up a link (called proprietary link) to the base station in box  106 . By this proprietary link it transmits the recovery information X 2  to the base station (box  107 ). And in box  108  it stops counting. The operation is terminated in box  109 . 
     FIG. 3 diagrammatically shows the operation of the base station as regards the clock management. 
     In box  200  the base station is switched off. In box  201  it is switched on. In box  202  the base station carries out various initialization operations, notably the initialization of a variable X 1  which indicates the need to set the clock of the base station. Then, in box  203 , the base station goes to the standby mode. 
     When a proprietary link is set up (box  204 ), the base station tests the nature of this link (box  205 ). If it is a link dedicated to the clock management (branch Y 1  in the Figure), the base station tests the variable X 1  (box  206 ). And if the contents of the variable X 1  are equal to 1 (branch Y 2  in the Figure), it updates its clock CLK stored in the non-volatile memory  27  on the basis of the recovery information X 2  which the handset has transmitted to the base station by the proprietary link (box  207 ). Then, in box  208 , the base station sets the variable X 1  to zero. 
     When the user has set the time of the base station (box  210 ) by hand, the base station updates its clock CLK stored in the non-volatile memory  27  (box  211 ). 
     At the end of the boxes  208  and  211 , and when the tests made in boxes  205  and  206  are negative (branches N 1  and N 2 ), the operation is resumed in box  203 , that is to say, when the base station has gone to the standby mode again. 
     The clock of the base station is then kept updated thanks to the time base  29  of the processor  28 . This clock is periodically backed up in the non-volatile memory  27 . The contents of the non-volatile memory are not erased when there is a power cut, so that it will thus be sufficient to add thereto the contents of the variable X 2  transmitted by one of the handsets for restoring a calendar. 
     The second embodiment of the present invention enables to do without the use of a non-volatile memory. Therefore, both the handset and the base station have a simple memory for backing up its clock, and counting means for keeping the information backed up every day. When the handset has lost its clock, the base station will transmit its clock thereto and vice versa. In this embodiment, the recovery information is formed by the clock itself. 
     FIG. 4 diagrammatically shows the operation of the handset as regards the clock management in this second embodiment. 
     In box  300 , the handset detects a resynchronization of base station and handset. In box  301  it thus sets up a link (called proprietary link) to the base station. By this proprietary link it transmits a request for a clock (MES 1 ) to the base station in box  302 . When it receives the response from the base station, the handset tests the type of this response. In box  303  it tests whether the response from the base station is a clock request (RES 1 ). In that case, the handset transmits its own clock (MES 2 ) to the base station in box  304  by the proprietary link set up earlier. If not, it tests in box  305  whether the response from the base station is a message (RES 2 ) containing recovery information. If that is the case, it updates its own clock CLKc based on the recovery information received CLKb (box  306 ). After the boxes  304 ,  305  and  306 , the operation is terminated in box  307 . 
     FIG. 5 diagrammatically shows the operation of the base station as regards the clock management in this second embodiment. 
     In box  400 , the base station is switched off. In box  401  it is switched on. In box  402  the base station carries out various initialization operations, notably the initialization of a variable X 1  that indicates the need to set the time of the base station. Then, in box  403 , the base station goes to the standby mode. 
     When a proprietary link is set up (box  404 ), the base station tests the type of this link (box  405 ). If this is a link dedicated to the clock management (branch Y 1  in the Figure), the base station tests the variable X 1  in box  406  (if not, the operation is resumed in box  403 ): 
     If the contents of the variable X 1  are equal to 1 (branch Y 2  in the Figure), when it receives a message coming from the handset via this proprietary link, the base station tests the type of this message (box  407 ). If this is a message asking for the time (MES 1 ), it sends a message (RES 1 ) asking for the time back to the handset (box  408 ), and goes to the standby mode to receive a message containing recovery information (MES 2 ). When the base station receives this message (box  409 ), it updates its clock CLKb based on the recovery information received CLKc, after which it resets the variable X 1  to zero (box  410 ). If not, the operation is resumed with box  403 . 
     If the contents of the variable X 1  are equal to 0 (branch N 2  in the Figure), when it receives a message coming from a handset and asking for the time (MES 1 ), the base station returns a message (RES 2 ) containing recovery information formed by its own clock (box  411 ) to the handset. 
     When the user has set the clock of the base station by hand (box  420 ), the base station updates its clock CLKb, sets the variable X 1  to zero, and broadcasts the new time to all the handsets, for example, by setting up a clock broadcasting proprietary link (box  421 ). 
     After the boxes  410 ,  411  and  421 , and when the tests carried out in boxes  405  and  407  are negative, the operation is resumed in box  403 , that is to say, the base station goes to the standby mode again. 
     In conformity with the DECT standard and by taking the same terminology, a proprietary link is set up, for example, in the following manner: 
     a handset sends to the base station a message {CC-SETUP} whose information element &lt;&lt;BASIC-SERVICE&gt;&gt; indicates a “service call set-up” (for more information the reader be referred to paragraphs 9.3.3.1 case D, 6.3.2.1 and 7.6.4 of part 5 of the standard); 
     the base station responds by sending back a message {CC-SETUP-ACK} (paragraph 6.3.2.3 of part 5 of the standard); 
     a proprietary link thus being established between the handset and the base station, messages {CC-INFO} can be exchanged between them. These messages contain, for example, a first reference which indicates the type of link (a clock management link or clock broadcasting link, for example), a second reference which indicates the type of message (messages MES 1  and RES 1  asking for the time or messages MES 2  and RES 2  for transmitting recovery information), and a data field containing the message itself, for example, the recovery information in the first embodiment and in the case of the messages MES 2  and RES 2  (paragraph 6.3.2.2 of part 5 of the standard). 
     Within the framework of the DECT standard, the loss of synchronization and the resynchronization are detected for example, in the following manner: part 3 of the DECT standard describes in paragraph 11.3.2 an “Idle-Locked” state which is the most neutral state in which the handset can be when it is located within the coverage area of the base station. The object of this state is to maintain the synchronization of base station and handset. Here it is considered that a leaving of the “Idle-Locked” state corresponds to a loss of synchronization and that, when the “Idle-Locked” state is entered, this corresponds to a resynchronization. 
     Although the invention has been described within the framework of the DECT standard (standard ETS 300 175 defined by ETSI), it is obvious that it is applicable to other cordless telephony systems.