Dft (design for testability)

design for testability vlsi

1. Dft ( design for testability)

2. objectives What is DFT? Why we need DFT? DFT methods. What is scanned flip flop? What is scan chain? Fault models. Stuck at fault models.

3. What is DFT? DFT is a technique, which facilitates a design to become testable after production. Its the extra logic which we put in the normal design, during the design process, which helps its post-production testing.

4. Why we need DFT? Post-production testing is necessary because, the process of manufacturing is not 100% error free. There are defects in silicon which contribute towards the errors introduced in the physical device. Of course a chip will not work as per the specifications if there are any errors To increase Productivity To improve Quality how to detect that?

5. Basic principles:- Controllability Observability

6. DFT methods There are two types of dft methods are DFT methods for digital circuits: Ad-hoc methods Structured methods

7. DFT techniques Ad-hoc technique :- it Is temporary technique Good design practice learnt through experience are used as guide lines for ad-hoc DFT Structural technique:- it provides more systematic & automatic approach to enhance the design testability. It targets manufacturing defects.

8. Sequential circuit( before adding scan chain)

9. What is scanned flip flop? D flip flop followed by mux…

10. Scan chain A scan chain is formed by connecting the scanned flip -flops in serial manner , it will function like a shift register. And it is used to perform controllability and observability.

11. After adding scan chain

12. Fault models FAULT MODELS Stuck-at fault model Transistor faults Bridging faults Delay faults • Delay faults can be classified as: Gate delay fault 2) Transition fault 3) Hold Time fault 4) Slow/Small delay fault

13. Fault modeling Def:- Due to defect during manufacturing of integrated circuit, There is need to model the possible faults that might occur during fabrication process, this is called fault modelling. Stuck-at-fault. Transition fault

14. Stuck at fault:- It is most popular fault model used in practice . These faults occurred due to process variations , manufacturing defects.

15. Stuck at fault models

16. Thank you