Method and apparatus for real time clock (RTC) brownout detection

1. A method of determining if a real time clock (“RTC”) in a global positioning system (“UPS”) receiver system is accurate enough to be used on start up of the GPS receiver, the method comprising:

transferring the time kept by the RTC clock to a GPS clock of the GPS receiver system;

estimating the position of visible satellites, wherein the GPS clock is updated with signals from the RTC clock;

based on the estimated position of the visible satellites, measuring a pseudo-noise (“PN”) code for each of the visible satellites;

calculating an expected current full PN code phase for each of the visible satellites;

computing the location of the GPS receiver using the expected current full PN code phases;

comparing the computed location of the GPS receiver to a previously-computed location of the GPS receiver in units of time;

determining whether the computed location of the GPS receiver has changed more than a predetermined threshold amount from the previously-computed location; and

responsive to the determined amount exceeding the predetermined threshold amount, establishing a new GPS clock by acquiring the visible satellites using conventional methods.

2. The method of claim 1 , wherein the step of transferring the time kept by the RTC clock to the GPS clock includes the steps of:

providing a docking signal from the RTC to an edge aligned ratio counter (“EARC”); and

aligning the clocking signal from the RTC with a clock signal from a GPS clock using the EARC.

3. The method of claim 1 , where the predetermined threshold amount is ±0.5 ms.

4. The method of claim 1 , where the RTC includes a K32 oscillator (with a nominal 32,768 Hz) that generates a K32 clock signal.

5. The method of claim 4 , further including the steps of:

measuring an operating temperature of the K32 oscillator of the RTC;

calculating a frequency error associated with the K32 oscillator based upon the measured operating temperature of the RTC; and

updating the RTC with the frequency error.

6. The method of claim 5 , further including the steps of:

measuring an operating temperature of the GPS clock;

calculating a frequency error associated with the GPS clock based upon the measured operating temperature of the GPS clock; and

updating the GPS clock with the frequency error.

7. The method of claim 1 , further including the step of responsive to the determined amount not exceeding the predetermined threshold amount, updating the RTC clock with the updated GPS clock.

8. The method of claim 7 , wherein the step of updating the RTC clock with the updated GPS clock includes the steps of:

providing a clocking signal from the updated GPS clock to the EARC; and

aligning the clocking signal from the updated GPS clock with a clock signal from the RTC clock using the EARC.

9. The method of claim 7 , wherein the step of updating the RTC clock with the updated GPS clock includes the steps of:

providing a clocking signal from the updated GPS clock to the EARC; and

aligning the clocking signal from the updated GPS clock with a clock signal from the RTC clock using the EARC.

10. A method of updating a real time clock (“RTC”) in a global positioning system (“GPS”) receiver system that uses significantly less power than a GPS clock of the GPS receiver system, the method comprising:

receiving a wake up command at a GPS receiver of the GPS receiver system while the GPS receiver is in a powered down mode;

powering up selected components of the GPS receiver in response to receiving the wake up command;

measuring an operating temperature of the RTC of the GPS receiver;

calculating a frequency error associated with the RTC based upon the measured operating temperature of the RTC;

updating the RTC with the frequency error;

determining a status of the updated RTC, wherein the status is one of GOOD and NOT GOOD;

responsive to determining that the status of the updated RTC is GOOD; configuring the updated RTC to keep time of the GPS receiver system instead of the GPS clock:, and

powering down the selected components of the GPS receiver.

11. The method of claim 10 , wherein the wake up command is a predetermined wake up command provided on a periodic basis.

12. The method of claim 10 , wherein the wake up command is initiated by a user of the GPS receiver corresponding to a positional query of the GPS receiver.

13. The method of claim 10 , further including the step of storing the operating temperature and the frequency error of the RTC in a temperature/frequency error table in a memory of the GPS receiver.

14. The method of claim 10 , further including the steps of:

measuring an operating temperature of a GPS clock of the GPS receiver;

calculating a frequency error associated with the GPS clock based upon the measured operating temperature of the GPS clock;

updating the GPS clock with the frequency error.

15. The method of claim 14 , further including the step of storing the operating temperature and the frequency error of the GPS clock in the temperature/frequency error table.

16. A method of updating a real time clock (“RTC”) in a global positioning system (“GPS”) receiver system that uses significantly less power than a GPS clock of the GPS receiver system, the method comprising:

receiving a wake up command at a GPS receiver of the GPS receiver system while the GPS receiver is in a powered down mode;

powering up selected components of the GPS receiver in response to receiving the wake up command;

measuring an operating temperature of the RTC of the GPS receiver;

calculating a frequency error associated with the RTC based upon the measured operating temperature of the RTC;

updating the RTC with the frequency error

powering down the selected components of the GPS receiver, wherein the wake up command is initiated by a user of the GPS receiver corresponding to a positional query of the GPS receiver;

providing a clocking signal from the RTC to an edge aligned ration counter (“EARC”);

aligning the clocking signal from the RTC with a clock signal from a GPS clock using the EARC; and

transferring the time kept by the RTC clock to the GPS clock.

17. The method of claim 16 , further including the step of:

determining whether the RTC clock is losing cycles, wherein the loss of cycles is above a predetermined threshold; and

responsive to the loss of cycle being below the predetermined threshold, using the GPS clock to acquire satellites.

18. The method of claim 17 , wherein the determination of whether the RTC clock is losing cycles is made by a brownout detection circuit coupled to the RTC.

19. The method of claim 18 , wherein the brownout detection circuit comprises:

a detection circuit that receives the RTC clock signal and determines whether the RTC clock is losing cycles, wherein the detection circuit is calibrated to determine whether a loss of cycles is above the predetermined threshold; and

a status circuit that stores a signal output by the detection circuit and outputs a status signal indicating the RTC clock is one of GOOD and NOT GOOD.

20. A method of updating a real time clock (“RTC”) in a global positioning system (“GPS”) receiver system that uses significantly less power than a GPS clock of the GPS receiver system, the method comprising:

receiving a wake up command at a GPS receiver of the GPS receiver system while the GPS receiver is in a powered down mode;

powering up selected components of the GPS receiver in response to receiving the wake up command;

measuring an operating temperature of the RTC of the GPS receiver; wherein the step of measuring the operating temperature of the RTC includes:

measuring a current operating temperature of the RTC;

retrieving an operating temperature of the RTC taken at the time of an earlier wake up command from the temperature/frequency error table;

averaging the current operating temperature and the operating temperature taken at the earlier time; and

calculating the frequency error associated with the RTC based upon the average operating temperature of the RTC

calculating a frequency error associated with the RTC based upon the measured operating temperature of the RTC;

updating the RTC with the frequency error; and

powering down the selected components of the GPS receiver.