For a high reliability

on-line testing is efficient in detecting faults on the early stage.A novel scan cell is proposed based on the conventional scan cell

which can protect the operational status from being changed during test.The test process and the functional operation of sequential circuit go in parallel.With a flexible clock selection mechanism utilized

both non-concurrent test and concurrent test can be operated.Simulation results show that these circuits can be test on-line with moderate hardware redundancy

high stability

and negligible time redundancy.