a system and method have been provided for determining a frequency tolerance between a generated clock and an input data rate . the invention analyzes beatnotes , externally generated through a comparison of clock and input data rates , and an overflow count of the clock . the occurrence of overflow counts , without intervening beatnotes , indicates that the clock and data rate are close in frequency . the occurrence of beatnotes without intervening overflow counts indicates that the clock and data rates are not close in frequency . hysteresis is built into the system , preventing the system from indicating an out - of - lock condition when the beatnotes immediate follow the an overflow count , or when the system monitors occasional beatnotes .