A timer unit includes a timer for timing the period of time the logic circuit has been in the self-test mode. A comparator is connected to the timer, for comparing the period of time with a maximum for the period of time the logic circuit is allowed to be in the self-test mode and outputting an error signal when the period of time exceeds the maximum. The test timer unit further includes a mode detector for detecting a switching of the logic circuit to the self-test mode. The mode detector is connected to the timer, for starting the timer upon the switching to the self-test mode and stopping the timer upon a switching of the logic circuit out of the self-test mode. The timer unit can be used in a system for testing a logic circuit which includes a test routine module containing a set of instructions which forms a test routine for performing a test on a tested part of the logic circuit. The system has a mode control unit containing a set of instructions which is executable by the logic circuit, for switching the logic circuit from and to a test mode in which a part of the logic circuit can be subjected to a selected test by executing a selected test routine.