of the scan register retains its state and hence retaining the state of the circuit in a functional mode after the diagnosis data has been scanned out se 620 can be deasserted to allow the circuit to continue to function from the last state test cycle 6 se 620 is deasserted when the system clock signal 630 is low to ensure that the q output 660 of the scan register stays substantially unchanged the next rising edge on the clock signal 630 may capture the next state in the functional mode not in the scan mode in one embodiment se 620 is pulsed when the clock signal 630 is high to capture the data into the scan latch of the scan register however to enter or exit a shift operation se 620 is toggled when the clock signal 630 is low this ensures that the scan and functional latches of the scan register do not change their data when se 620 is toggled to enter or exit the shift operation as shown in fig6 the data 640 output by the system stays substantially steady during the shift mode and the scan out data 670 stays substantially steady during the functional mode one should appreciate that the applications of the improved scan registers described above are examples for illustrating the concept the above examples are not intended to limit the application of the improved scan registers one of ordinary skill in the art would be able to recognize from the above description that other applications of the improved scan registers in circuit testing are possible the foregoing discussion merely describes some exemplary embodiments of the present invention one skilled in the art will readily recognize from such discussion the accompanying drawings and the claims that various modifications can be made.