Real time clock

1. A real time clock apparatus for use with a legitimate source, the real time clock apparatus comprising:

a counter for storing a count value representing a time signal;

a check register for storing a check value, wherein the check value is used for determining whether a write operation performed on the counter is an authorised write operation; and

means for comparing the check value stored in the check register with an authorised value, and means for determining if an unauthorised write operation has been performed on the counter if the check value does not match the authorised value.

2. A real time clock apparatus as claimed in claim 1 , further comprising updating means for automatically updating the check value each time a write operation is performed on the counter.

3. A real time clock apparatus as claimed in claim 2 , wherein the updating means is configured to increment the check value stored in the check register each time a write operation is performed on the counter.

4. A real time clock apparatus as claimed in claim 3 , wherein the updating means is configured to increment the check value by one each time a write operation is performed on the counter.

5. A real time clock apparatus as claimed in claim 3 , further comprising means for limiting the number of write operations performed on the counter in a given period of time.

6. A real time clock apparatus as claimed in claim 3 , wherein the check value stored in the check register is initialised to a random number following a power up sequence.

7. A real time clock apparatus as claimed in claim 2 , wherein the updating means is configured to update the check value with a new random number each time a write operation is performed on the counter.

8. A real time clock apparatus as claimed in claim 2 , wherein the updating means is configured to update a first portion of the check register in an incremental manner, and a second portion of the check register in a random manner, each time a write operation is performed on the counter.

9. A real time clock apparatus as claimed in claim 2 , wherein the updating means is implemented in hardware.

10. A real time clock apparatus as claimed in claim 2 , wherein the updating means is implemented in software.

11. A real time clock apparatus as claimed in claim 1 , wherein the counter comprises a plurality of registers.

12. A real time clock apparatus as claimed in claim 1 , wherein the counter is periodically updated at predetermined intervals to represent a real time signal.

13. A real time clock apparatus as claimed in claim 1 , further comprising means for selectively enabling or disabling the use of the check register.

14. A real time clock apparatus as claimed in claim 13 , wherein the means for selectively enabling or disabling the check register is controlled by a control signal received from a remote source.

15. The real time clock of claim 1 , wherein the real time clock apparatus is incorporated into a processor.

16. The real time clock of claim 1 , wherein the real time clock is incorporated into an electronic device.

17. The real time clock of claim 1 , wherein the real time clock is incorporated into a communications device.

18. The real time clock of claim 1 , wherein the real time clock is incorporated into a portable telephone.

19. The real time clock of claim 1 , wherein the real time clock is incorporated into an audio device.

20. The real time clock of claim 1 , wherein the real time clock is incorporated into a computer device.

21. A method of detecting an unauthorised write operation on a real time clock, the real time clock comprising a counter for storing a count value representing a time signal, wherein the counter can be written to set the time signal, the method comprising the steps of:

storing a check value in a check register, wherein the check value is used for determining whether a write operation performed on the counter is an authorised write operation; and comparing the check value stored in the check register with an authorised value, and determining if an unauthorised write operation has been performed on the counter if the check value does not match the authorised value.

22. A method as claimed in claim 21 , further comprising the steps of automatically updating the check value each time a write operation is performed on the counter.

23. A method as claimed in claim 22 , wherein the updating step comprises the step of incrementing the check value each time a write operation is performed on the counter.

24. A method as claimed in claim 23 , wherein the updating step comprises the step of incrementing the check register by one each time a write operation is performed on the counter.

25. A method as claimed in claim 23 , further comprising the step of initialising the check value of the check register to a random number following a power up sequence.

26. A method as claimed in claim 22 , further comprising the step of updating the check value with a new random number each time a write operation is performed on the counter.

27. A method as claimed in claim 22 , wherein the updating step comprises the step of updating the check value with a new random number each time a write operation is performed on the counter.

28. A method as claimed in claim 22 , wherein the updating step comprises the steps of updating a first portion of the check register in an incremental manner, and a second portion of the check register in a random manner, each time a write operation is performed on the counter.

29. A method as claimed in claim 21 , wherein the counter comprises a plurality of registers.

30. A method as claimed in claim 21 , wherein the counter is periodically updated at predetermined intervals to represent a real time signal.

31. A method as claimed in claim 21 , further comprising the step of selectively enabling or disabling the use of the check register.

32. A method as claimed in claim 31 , wherein the step of selectively enabling or disabling the check register is controlled by a control signal received from a remote source.