Patent ID: 7751725
Filing Date: 2010-07-06
Classification: H04B

Abstract:
1. A method for detecting a consecutive identical state in a HIGH level or in a LOW level of an optical signal transitional between the HIGH level and the LOW level, and for generating an alarm signal when the consecutive identical state continues by a predetermined period, the method comprising steps of: (a) converting the optical signal into a corresponding electrical signal; (b) setting a first comparative output when the electrical signal detected through a coupling capacitor is in a first state corresponding to the HIGH level of the optical signal, and setting a second comparative output when the electrical signal detected through the coupling capacitor is in a second state corresponding to the LOW level of the optical signal, wherein the electrical signal detected through the coupling capacitor includes a second order lag; (c) counting a first period by a first counter when the first comparative output is set and a second period by a second counter when the second comparative output is set; (d) latching an output of the first counter by a first latch when the first counter counts at least a predetermined number and latching an output of the second counter by a second latch when the second counter counts at least a predetermined number; and (e) performing a logical sum of the output of the first latch and the output of the second latch to generate the alarm signal.