Patent ID: 8854031

Claim:
A clock detecting circuit, comprising: a microprocessor; a clock circuit; and a zero-cross detecting circuit; wherein: said clock circuit is connected to said microprocessor; an input end of said zero-cross detecting circuit is connected to a utility power AC input; an output end of said zero-cross detecting circuit is connected to an input end of said microprocessor; said zero-cross detecting circuit operates to detect a plurality of zero crossing points of the utility power AC input; said microprocessor operates to count a number of oscillation periods of said clock circuit in a time interval between two adjacent zero crossing points of the utility power AC input; and said microprocessor compares the number of oscillation periods with a minimum threshold value and a maximum threshold value, and (a) if the number of oscillation periods is smaller than the maximum threshold value and is greater than the minimum threshold value, said microprocessor determines that said microprocessor meets a precision requirement; and (b) if the number of oscillation periods is smaller than the minimum threshold value or is greater than the maximum threshold value, said microprocessor determines that said microprocessor does not meet the precision requirement.