Mission elapsed time unit

A method for synchronization of an output of a mission elapsed time (MET) core to an external pulse per second (XPPS) input signal, wherein the MET core includes a sub-seconds counter keeping track of time over scales of less than one second and a seconds counter keeping track of time over scales of larger than one second, the sub-seconds counter operably connected to the seconds counter, the XPPS input signal generated by a XPPS source operably connected to the MET core, the method comprising the steps of:

• powering up or resetting the XPPS input signal;
• determining how far apart are two one-second boundaries of the MET core;
• determining a period of the XPPS input signal;
• enabling first synchronization to the XPPS input signal using a status register;
• adjusting a freewheel limit higher or lower to achieve an acceptable drift rate of the one-second boundaries of the MET core toward the XPPS input signal;
• and
• setting a blackout limit lower than the freewheel limit with enough margin to ensure that the XPPS pulse is captured.