A flip flop includes a data input, a clock input, a test chain input, a test chain output, a monitoring circuit, and an alert transmission circuit. The monitoring circuit is adapted to generate an alert if the time between arrival of a data bit and a clock edge is less than a threshold. The alert transmission circuit is adapted to apply during a monitoring phase an alert level to the test chain output in the event of an alert generated by the monitoring circuit, and to apply the alert level to the test chain output when an alert level is received at the test chain input.