Method of determining the condition of a back-up battery for a real time clock

• US 5,596,512 A
• Filed: 04/17/1996
• Issued: 01/21/1997
• Est. Priority Date: 08/15/1994
• Status: Expired due to Fees

• Current Assignee: Thermo King Corporation (Trane Technologies plc)
• Original Assignee: Thermo King Corporation (Trane Technologies plc)
• Inventors: McCormack, Michael D., Jurewicz, Romuald M., Wong, Albert C. K.
• Primary Examiner(s): Trammell, James P.
• Assistant Examiner(s): Pipala, Edward J.
• Application Number:
  US08/633,328
• Time in Patent Office: 279 Days
• Field of Search: 364/481, 364/550, 364/550.01, 364/424.03, 364/424.04, 364/424.05, 364/569, 364/483, 364/273.4, 364/948.5, 324/427, 429/90, 307/64-66, 395/750, 368/203, 368/204
• US Class Current: 702/63
• CPC Class Codes: G01R 31/3648 comprising digital calculation means, e.g. for performing an algorithm

Abstract

A method of determining the condition of a battery which backs up a real-time clock (RTC) when a main power supply fails or is disconnected, in a system which includes a controller having a volatile random access memory (VRAM) and a non-volatile random access memory (NVRAM). The method includes the steps of periodically storing the time value of the RTC in NVRAM, and periodically writing a predetermined data pattern in VRAM, while the main power supply adequately powers the real time clock. The method then includes the steps of detecting a power-down condition of the main power supply, and terminating the recording and writing steps when the detecting step detects power-down. Return of main power, or power-up, is then detected, with the data pattern stored in VRAM then being compared with the predetermined data pattern. When the patterns are not the same, indicating more than a transitory loss of main power, the difference between the time value indicated by the RTC upon power-up and the last time value of the RTC stored in NVRAM before power down is compared with a predetermined constant time value. When the RTC has not incremented by at least the predetermined constant time value the method includes the step of indicating that the back-up battery is bad.

Claims

1. A method of determining the condition of a battery which backs up a real-time clock (RTC) when a main power supply for the RTC fails or is disconnected, comprising the steps of:

storing a time value of the RTC in non-volatile memory means while powered by the main power supply, just prior to a loss of the main power supply,

detecting return of the main power supply,

comparing the time value of the RTC upon return of the main power supply with said stored time value of the RTC,

and indicating that the back-up battery is bad when the comparison step finds a predetermined relationship between the compared time values.

2. The method of claim 1 wherein the step of storing the time value of the RTC includes the step of periodically storing the value of the real time clock, detecting a power-down condition of the main power supply, and terminating the step of periodically storing the time value of the RTC upon detection of a power-down condition.

3. The method of claim 1 including the step of ignoring interruptions in the main power supply which persist for less than a predetermined period of time.

4. The method of claim 1 including the step of ignoring interruptions in the main power supply which persist for less than the memory retention time of a predetermined volatile memory means.

5. The method of claim 1 including the steps of:

writing a predetermined data pattern in volatile memory means while the volatile memory means is powered by the main power supply, just prior to the loss of the main power supply,

providing a stored data pattern in a memory,

detecting return of the main power supply,

comparing said stored data pattern with the predetermined data pattern,

and ignoring an interruption of the main power supply when the comparison step finds that the stored data pattern matches the predetermined data pattern.

6. The method of claim 5 wherein the step of writing a predetermined data pattern in volatile memory means includes the steps of periodically writing the predetermined data pattern in the volatile memory means, detecting an interruption of the main power supply, and terminating the writing step when the detecting step detects an interruption of the main power supply.

7. A method of determining the condition of a battery which backs up a real-time clock (RTC) when a main power supply fails or is disconnected, in a system which includes a controller having a read-only memory including a data pattern stored therein, a volatile random access memory (VRAM) and a non-volatile random access memory (NVRAM), comprising the steps of:

periodically storing the time of the RTC in NVRAM, while the main power supply powers the RTC,

periodically writing a predetermined data pattern in VRAM, while the main power supply powers the RTC,

detecting an interruption of the main power supply,

terminating the recording and writing steps when the detecting step detects an interruption of the main power supply,

detecting return of the main power supply,

comparing the data pattern stored in VRAM with the predetermined data pattern, when the detecting step detects the return of the main power supply,

providing a difference time value between the time value indicated by the RTC after return of the main power supply and the last time value of the RTC stored in NVRAM before detection of an interruption of the main power supply, when the comparison step finds that the stored data pattern does not match the predetermined data pattern,

comparing the difference time value with a predetermined constant time value,

and indicating that the back-up battery is bad when the comparison of time values by the comparison step finds that the RTC has not incremented by at least the predetermined constant time value.

8. The method of claim 7 including the step of ignoring the detection of an interruption of the main power supply when the data comparison step finds that the stored data pattern is the same as the predetermined data pattern.

9. The method of claim 7 including the step of selecting the predetermined constant time value such that the predetermined constant time value exceeds the time for data stored in the VRAM to lose its integrity after loss of the main power supply.

10. The method of claim 7 including the step of selecting the predetermined constant time value such that the predetermined constant time value is the minimum time by which the RTC should have incremented during a loss of the main power supply for a time sufficient to destroy the integrity of data in the VRAM.

11. The method of claim 7 wherein the step of writing a predetermined data pattern in VRAM includes the step of writing a plurality of predetermined data patterns in VRAM, each at a different memory address.

12. The method of claim 7 wherein the step of indicating the back-up battery is bad includes the step of storing an alarm code which identifies the back-up battery.

13. The method of claim 7 wherein the step of indicating the back-up battery is bad includes the step of flashing a visible indicator on and off.

14. The method of claim 7 wherein the step of indicating the back-up battery is bad includes the step of displaying predetermined indicia which identifies the back-up battery.