 Hello everyone, I am Prashant S. Malge, Assistant Professor, Department of Electronics Engineering Walsh and Institute of Technology, Swarapur. Today we will discuss the methods for testing of sequential circuits, learning outcome. At the end of the session, student will be able to explain the scan path technique for testing the sequential circuits. In previous lectures, we have seen that the combinational circuits can be tested by using either the deterministic test methods or the random test methods. But it is somewhat difficult to test the sequential circuits. The basic reason is, here in case of sequential circuits, presence of the memory elements allows a sequential circuit to be in various states and the response of the circuit to externally applied inputs depends on the state of the circuit. So, therefore, there is a need for checking that the circuit performs correctly all a transition between states and also it produces a correct output ok. So, now the problem here is, it is difficult to determine that the circuit is in a specific test, because the state variables are not observable on the external pins of the circuit. Therefore, for testing the sequential circuit, there is a need to design the sequential circuit so that they are easily testable. Now, this particular diagram shows the general model for a sequential circuits. Here the input external inputs w 1 to w 2 we have and the circuit outputs are z 1 to z m. The state next state variables is capital Y 1 to Y n in general and the present state variables Y 1 to Y n. So, now these are next state variables as well as the present state variables are not externally available. So, here the combinational circuit can be tested very easily by applying these external inputs. It is also possible to observe the outputs that is z 1 to z m, because these are externally available, but the problem is to give these present state variables. So, one possible solution is to include a two-way multiplexer in the path of each of these present state variable, when in which one of the input will be a present state variable and the other input will be a external input. But the problem here is, the second input of each multiplexer must be directly accessible through external pins, which requires many pins if there are many state variables. So, that is not a good solution. So, we can think of some alternative attractive approach, where in which provide a connection that allows shifting the test vector into the circuit one bit at a time. So, this alternative method requires less pins. So, now we will discuss one such popular method known as the scan path technique. So, here for example, a sequential circuit is shown with three flip-lops. So, this is a combination circuits. We have the input w 1 to w n, we have the output z 1 to z m. So, this combination circuit and this part consist of three flip-lops. External inputs are w 1 to w n, whereas the outputs are z 1 to z k. These next state variables are given through the multiplexers. So, these three multiplexers forms a shift register. So, the control signal normal bar oblique scan selects the active input to the multiplexer. So, in the normal operation to input to these flip-lops are these state variables as it is and in case of the scan operation, the these multiplexers makes it as a shift register and the external input is shifted into these flip-lops. So, during normal operation the flip-lops inputs are driven by the next state variables y 1, y 2, y 3 and in case for shift testing the shift register connections is used to scan in the portion of each test vector that involves the present state variables y 1, y 2, y 3. So, now the testing method consists of the following steps. So, the operation of flip-lops is tested by scanning into them a pattern of 0s and 1s say for example, 0 1 0 1 1 0 0 1 in a consecutive clock cycles and observing whether the same pattern is scanned out. For this, this normal bar oblique scan is kept to 1 and the input pattern is applied on this scanning. Now, the combinational circuit is tested by applying the test vectors w 1, w 2 dot dot dot w n as well as these present state variables y 1, y 2, y 3 and by observing the values generated on this z 1 to z k and these next state variables y 1 to y 3. Now, the external inputs w 1 to w n can be applied directly, but for applying these present state variables the y 1, y 2, y 3 portion of this test vector is scanned into the flip-lops using this normal bar oblique scan equal to 1 and it is run for three clock cycles so that these this input will be applied to y 1, y 2, y 3. Next the w 1 to w n portion of the test vector is applied and the normal operation of the sequential circuit is performed for one clock cycle. So, after performing this the output generated on z 1 to z m are observed and generated values of the next state variables y 1, y 2, y 3 are loaded into the flip-lops. So, the select input is now changed this is actually normal bar oblique scan equal to 1 and the contents of the flip-lops are scanned out during next three clock cycles which makes y 1, y 2, y 3 portion of the output observable externally. So, this is how although these present state variables and the next state variables are not externally available it is possible to apply by making use of this shift register and it is also possible to observe the next state variable part of output generated by using this. And so, this is how the sequential circuit can be tested. Thank you.