Methods and systems for detection of zero crossings in a signal

1. A method of determining true zero crossings in a signal, the method comprising:

receiving a time-varying signal;

identifying a zero crossing in the time-varying signal based on a transition of the time-varying signal from a first state to a second state; and

determining a true zero crossing if a flag is set and after the time-varying signal has been at the second state for a first predefined amount of time,

wherein the flag is set based on whether the time-varying signal was at the first state for a second predefined amount of time.

2. The method of claim 1 , further comprising:

establishing a value of a signal status counter;

at a repeating interval, if the time-varying signal is a logic low value, decrementing the value of the signal status counter if the signal status counter is greater than a first value, otherwise setting the flag to enable detection of a given zero crossing in the time-varying signal; and

at the repeating interval, if the time-varying signal is a logic high value:

incrementing the value of the signal status counter; and

if the signal status counter is equal to a second value and the flag is set,

determining the true zero crossing of the time-varying signal.

3. The method of claim 2 , further comprising at the repeating interval, determining if the time-varying signal is a logic low value or a logic high value.

4. The method of claim 2 , further comprising at the repeating interval, incrementing a time stamp counter to measure a period between zero crossings of the time-varying signal.

5. The method of claim 2 , further comprising, if the time-varying signal is the logic high value, incrementing the value of the signal status counter if the signal status counter is not equal to a threshold value.

6. The method of claim 2 , wherein the second value is set based according to the first predefined amount of time for the time-varying signal to be a logic high value before a zero crossing is declared.

7. The method of claim 2 , wherein the first value is set based according to the second predefined amount of time for the time-varying signal to be a logic low value before the flag is set to enable detection of a zero crossing in the time-varying signal.

8. The method of claim 2 , wherein the repeating interval is between about 2 microseconds and about 100 microseconds.

9. The method of claim 2 , wherein the repeating interval is a time interval set based on a maximum pulse width of an expected noise transient.

10. The method of claim 2 , the repeating interval and the second value are set based according to an acceptable delay between a zero crossing occurring in the time-varying signal and determining the zero crossing in the time-varying signal.

11. The method of claim 1 , wherein the time-varying signal is a power signal, and the method further comprises:

determining a zero crossing in a second power signal; and

performing an in phase load transfer from the power signal to the second power signal at a time that the zero crossing in the second power signal is substantially aligned with the zero crossing in the power signal so that the second power signal is within about 60° of the power signal.

12. The method of claim 1 , further comprising after determining the true zero crossing of the time-varying signal, clearing the flag to disable detection of a zero crossing in the time-varying signal until the flag is set again.

13. A non-transitory computer readable medium having instructions stored thereon that, in response to execution by a computing device, cause the computing device to perform operations comprising:

receiving a time-varying signal;

establishing a value of a signal status counter;

at a repeating interval, if the time-varying signal is a logic low value, decrementing the value of the signal status counter if the signal status counter is greater than a first value, otherwise setting a flag to enable detection of a zero crossing in the time-varying signal; and

at the repeating interval, if the time-varying signal is a logic high value:

incrementing the value of the signal status counter; and

if the signal status counter is equal to a second value and the flag is set,

determining a true zero crossing of the time-varying signal.

14. The computer readable medium of claim 13 , further comprising instructions that, in response to execution by the computing device, cause the computing device to perform operations comprising:

after determining the true zero crossing of the time-varying signal, clearing the flag to disable detection of a zero crossing in the time-varying signal until the flag is set again.

15. The computer readable medium of claim 13 , wherein the repeating interval is a time interval set based on a maximum pulse width of an expected noise transient.

16. A system comprising:

an up/down counter for establishing a signal status counter; and

a processor coupled to the up/down counter, the processor for receiving a power signal and identifying zero crossings in the power signal, wherein at a repeating interval the processor determines if the power signal is a logic low value or a logic high value, and wherein at the repeating interval if the signal is the logic low value the processor instructs the up/down counter to decrement a value of the signal status counter if the signal status counter is greater than a first value otherwise the processor sets a flag to enable detection of a true zero crossing in the signal, and wherein at the repeating interval if the signal is the logic high value the processor instructs the up/down converter to increment the value of the signal status counter and after incrementing if the signal status counter is equal to a second value and the flag is set the processor declares a true zero crossing of the signal.

17. The system of claim 16 , wherein after the processor declares the true zero crossing of the signal, the processor clears the flag.

18. The system of claim 16 , further comprising a time stamp counter to record a time stamp at each repeating interval, wherein the time stamps are used to measure a time from a last zero crossing to a present zero crossing.

19. The system of claim 16 , further comprising:

conditioning circuitry to remove high-frequency noise from the power signal; and

a comparator with hysteresis to receive an output of the conditioning circuitry and to provide a discrete signal output to the processor.

20. The system of claim 16 , wherein the processor identifies zero crossings in the power signal that occur when the power signal transitions from a logic low value to a logic high value.