Design for Testability (DFT) is required to guarantee the product quality, reliability, performances, etc. Design for Testability refers to those design techniques that
- Enhances testability of device
- Ease ability to generate vectors
- Reduce test time
- Reduce the cost involved during test
There are different methods to implement the DFT Logic for Digital circuits which are listed below
- Ad-hoc methods: Good design practices learnt through experience and those methods are used as guidelines
- Avoid combinational feedback
- All flip flops must be initializable
- Avoid redundant and large fanin gates
- Provide test control for the signals which are not controllable
- While designing test logic we have to consider the ATE requirements
Ad-hoc methods had few disadvantages, and these gives more advantage to Structured methods.
- Disdvantages od ad-hoc DFT methods:
- Experts and tools not always available
- Test generation is often manual with no guarantee of high fault coverage
- Design iterations may be necessary
- Structured Methods: Structured DFT provides a more systematic and automatic approach to enhancing design testability. Structured DFT’s goal is to increase the controllability and observability of a circuit. Various methods exist for accomplishing this. The most common is the scan design technique, which modifies the internal sequential circuitry of the design.
- Scan: In the design all the flip flops are converted to scan flip flop.
- Boundary Scan
- Built-in self-test
we have came across the scan flip flop, and you may be wondering, what would be the difference between a norml flip flop and a scan flip flop. Below pictorial representation give clear picture about a flop and scan flop.
TM represents Test Mode signal and this signal should be 1 during DFT testing and 0 for functional model.