mains failure detection in Multi cycle mode (topswitch-jx)

Hi

Do you have any good mains failure detection circuits, so that mains failure can be quickly detected by a microcontroller on the secondary of an isolated, offline flyback…..for use with topswitch-jx in multi-cycle mode?

In MCM , the usual mains failure detection circuits that get used with flybacks tend not to work, because switching is too sparse and irregular