Increase the number of controllable and observable points
As we mentioned during the presentation of the stuck-at and transition fault coverage methodology, one of the key element is the number of controllable and observable points of the system. Since every single input and every single output of any of the gate in the design must be tested, it means that we have to be able to put the required test pattern deep inside the design.
Inputs and outputs of a component cannot be sufficient to insure a high level of coverage. The question is then to establish a methodology that provides a new way to control and observe the design.
Serial transmissions through a shift register.
This is not possible to add as many ports as we would require on a given component to have a sufficient number of controllable and observable point. We need another way to increase this number of points with a very limited impact on the total number of ports, and also if possible, with a limited impact on the area.
The idea is to base the controllability and the observability on a serial transmission approach. A serial transmission allows transferring an important set of data through a limited number of channels. But what kind of serial transmission could help us? Let’s have a look to the simplest serial data transmission structure : the shift register.
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