Method and apparatus for synchronizing clocks coupled to network

• US 5,848,028 A
• Filed: 11/24/1997
• Issued: 12/08/1998
• Est. Priority Date: 10/21/1993
• Status: Expired due to Term

• Current Assignee: Thomson Consumer Electronics Incorporated (Vantiva SA)
• Original Assignee: Thomson Consumer Electronics Incorporated (Vantiva SA)
• Inventors: Burklin, Helmut
• Primary Examiner(s): Roskoski, Bernard
• Application Number:
  US08/976,676
• Time in Patent Office: 379 Days
• Field of Search: 368/41-47
• US Class Current: 368/46
• CPC Class Codes: G04G 7/00 Synchronisation radio-controlled time-pieces G04R
• CPC Class Codes: H04J 3/0641 Change of the master or reference, e.g. take-over or failure of the master

Abstract

Many devices (in particular white goods or brown goods) in a household may contain a clock. Some systems allow connection of such devices and their related clocks to a common bus or network. The commands for controlling the clocks are in general limited to reading or writing a given time or to broadcasting the time of a given clock to one dedicated device or to the entire system or to a part of the system. In such known systems it is not specified in which manner a clock should react if it receives the time broadcast by another clock. It is possible to initialize all clocks to a given time, but due to tolerances in the different clocks, so achieved synchronization will not be maintained. Regular broadcasting by a special device master clock has the disadvantage of introducing one device with different capabilities. In case there are several master clocks, conflicts may occur and the advantages of a high precision clock will fade away if a lower precision master clock overrides it. In the present invention, only the clock with the highest precision broadcasts its time to the system, but whenever it fails the next lower precision clock takes over the broadcasting automatically. The broadcast periods are defined as factorial periods of a preselected time period. In an alternate embodiment, all devices broadcast the time with an equal period but with a different phase, and only if no broadcast was received in the preceding time period.

Claims

1. Apparatus for synchronizing clocks within a network system, said apparatus being connectable to the network system, comprising:

a plurality of clocks connected to a network;

at least one clock having one out of a plurality of different precision classes, ranging from a first high precision class to a last low precision class, wherein each said precision class is associated with a particular broadcast time period, and wherein a particular said broadcast time period of shorter duration than another said broadcast time period is associated with a correspondingly higher said precision class than said precision class associated with said another broadcast time period;

time period counting means;

bus interface means coupled to said clock for receiving a time transfer of said clock time and broadcasting the time of said clock to all other clocks within said network after expiration of the broadcast time period associate with said clock, and also for receiving the time of another clock within said network system;

interrupt means for interrupting the time transfer from said clock to said bus interface means for a certain waiting time period after each reception of time information by said bus interface means;

wherein the time of said clock is broadcast only when the bus interface means have not received time information from said another clock within said network system;

updating means for updating the time of said clock when receiving said time of another clock within said network system; and

resetting means for resetting said time period counting means when receiving said time of another clock within said network system.

2. Apparatus according to claim 1, further comprising digital filter means for comparing the internal time of said clock and the time broadcast within said network;

whereby updating of the clock time occurs only when a different exceeding a pre-selected threshold over a pre-selected observation period is detected.

3. Apparatus for synchronizing clocks within a network system, said apparatus being connectable to the network system, comprising:

a plurality of clocks connected to a network;

at least one clock having one out of a plurality of different precision classes ranging from a first high precision class to a last low precision class, said clock having a broadcast time period independent of said precision class and a broadcast time associated with said clock precision class, wherein a particular said broadcast time temporally precedes another said broadcast time when said clock precision class is correspondingly higher than said another clock precision class;

time period counting means;

bus interface means coupled to said clock for receiving a time transfer from said clock and broadcasting the time of said clock to all other clocks within said network, and for receiving the time of another clock within said network system;

interrupt means for interrupting the time transfer from said clock to said bus interface means for a certain listening time period after each reception of time information by said bus interface means;

wherein the time of said clock is broadcast only when the bus interface means have not received time information from said another clock within said network system; and

updating means for updating the time of said clock when receiving said time of said another clock within the network system.

4. Apparatus according to claim 3, further comprising digital filter means for comparing the internal time of said respective clock and the time broadcast within said network;

whereby updating of the respective clock time occurs only when a difference exceeding a pre-selected threshold over a pre-selected observation period is detected.

5. A clock synchronizing method for synchronizing a plurality of clocks coupled to a network, comprising the steps of:

assigning a predetermined broadcast time period corresponding to a precision class indicative of a precision to each respective clock, wherein the broadcast time period is inversely related to the precision of the corresponding precision class, the magnitude of said broadcast time period being representable by a count within the range of a broadcast time period counter;

for each of said clocks during the associated broadcast time period, whenever a time value is broadcast over said network, setting the time kept by each said respective clock to that broadcast time value;

resetting and restarting the broadcast time period each time the associated clock receives a broadcast time value and after expiration of the broadcast time period; and

for each of said clocks, broadcasting a time value corresponding to the current time of said clock over said network at the expiration of the associated broadcast time period.

6. Method according to claim 5, wherein every clock receiving a time broadcast of any precision class does not broadcast its time within a further time period which is higher than said pre-selected time period.

7. Method according to claim 5, wherein said further time period is one of a random and pseudo-random value, depending on some internal state of a device to which the clock belongs and depending not only on the present time, whereby these values are changed periodically or after each time broadcast.

8. Method according to claim 5, wherein said time is broadcast only to a part of the clocks belonging to said network.

9. Method according to claim 5, wherein said pre-selected time period is a factorial period of a minute.

10. Method according to claim 5, wherein tuners related to said clocks receive the time of different time zones, one of said zone times is selected for broadcasting within said network.

11. Method according to claim 5, wherein when evaluating the internal time of a clock and the time broadcast within said network, the delay of time distribution within said network is compensated.

12. Method according to claim 5, wherein digital filter means compare the internal time of a clock and the time broadcast within said network and allow updating only when a different exceeding a pre-selected threshold over a pre-selected observation period is detected.

13. A clock synchronization method for synchronizing a plurality of clocks coupled to a network, comprising the steps of:

assigning one of a plurality of precision classes to each respective clock, said precision classes being based upon the degree of time-keeping precision of the assigned clocks;

for each of said clocks, broadcasting a signal representing a time value corresponding to the current time of the respective clock when triggered;

for each of said clocks, listening for time values broadcast over said network by other clocks;

varying at least one characteristic of the signal representing the broadcast time values to identify the precision class of said broadcast time value; and

each of said clocks being triggered to broadcast a time value whenever a broadcast time value belonging to a precision class representing greater precision than that of said respective clock does not occur during a pre-selected listening period.

14. Method according to claim 13 wherein every clock receiving a time broadcast of the same or higher precision class does not broadcast its time within a further time period which is higher than said pre-selected time period.

15. Method according to claim 13, wherein said further time period is one of a random and pseudo-random value, dependent upon the internal state of a device to which the clock belongs and depending not only on the present time, whereby these values are changed periodically or after each time broadcast.

16. Method according to claim 13, wherein said time is broadcast only to a part of the clocks belong to said network.

17. Method according to claim 13, wherein said pre-selected time period is a factorial period of a minute.

18. Method according to claim 13, wherein tuners related to said clocks receive the time of different time zones, one of said zone times being selected for broadcasting within said network.

19. Method according to claim 13, wherein when evaluating the internal time of a clock and the time broadcast within said network, delay of time distribution within said network is compensated for.

20. Method according to claim 13, wherein digital filter means compare the internal time of a clock and the time broadcast within said network and allow updating only when a difference exceeding a pre-selected threshold over a pre-selected observation period is detected.