A circuit generally including a control function and a checksum function is disclosed. The control function may be configured to assert (i) a start signal in response to a signal having a predetermined sequence of values matching an entry value and (ii) a stop signal in response to the signal matching an exit value. The checksum function may be configured to (i) generate a checksum value for the signal between assertions of the start signal and the stop signal and (ii) generate a result signal in response to comparing the checksum value with an expected value.