Macein, You have given a good explanation for the difference between stuck at and transition faults. But when you can confirm that a node can have transition from high-to-low and low-to-high at the functional frequency (testing transition faults) doesn't it confirm that node is definitely not stuck-at any value (which otherwise should have been tested through stuck-at patterns) ?
Hi Ranger01,
Good question, you made me think...!!
It is important to run separate tests for stuck-at and transition (atspeed) testing. The two runs will give different test-coverage values, where transition coverage will be 3-4% lower than stuck-at test-coverage. The total list of faults tested under stuck-at are much greater in no. than in transition.
In stuck-at testing we shift-in the values, initialize the primary inputs of the combinatorial logic, observe the outputs,ONLY ONE CAPTURE pulse is given and shift-out the values. The circuit is already in steady state.
While when we are performing transition testing, we shift-in the values, then pulse two clock pulses at the functional clock frequency (Primary input changes are also made with the application of the first clock pulse). The first pulse will launch the transition and the second pulse will capture the response from the combinatorial portion of the circuit. Thus there is a time duration in which the circuit needs to respond correctly otherwise there is a defect.
Also a lot of nodes are left out during transition testing, which can only be covered by stuck-at testing. Suppose a device has a 2.2 Mhz functional freq, but shows slow-to-rise defect for some nodes, this defect goes away when we change the clock freq to 2.0 Mhz or when we are running stuck-at testing. If no stuck-at testing had been performed than it would seem that the device is not working correctly and the chip/device would come under fail category. But actually this is a transition defect not stuck-at and the device can still work correctly. Thus, in scenarios like this both stuck-at and transition tests become necessary and the device will operate under lowered freq.
Regards
Macein