1. TOPIC : Test Points UNIT 4 : Design For Testability Module 4.1: Basics of DFT

2. What are test points? Points that are used to give test inputs to a block in the circuit and observe their outputs. Employ test points to enhance controllability and observability. Two types of test points ◦ Control points ◦ Observation points Control points are primary inputs used to enhance controllability. Observation points are primary outputs used to enhance observability.

3. Contd … Different ways to introduce test points are: Use jumpers Add new control inputs ◦ 0-injection ◦ 1-injection ◦ 0/1-injection Use multiplexer

4. Jumpers Signal ‘G’ can be routed to 2pins A & A’ and connected by a removable wire. This is called a jumper. For testing, the external wire is removed and the wire stays connected during normal operation. Original CircuitCircuit using jumper

5. Contd … After removing the wire, the external test equipment can Test C2 by applying arbitrary signals to A’ and observing the primary output. Test C1 by applying arbitrary signals at the primary inputs and observing the outputs at A. Thus A acts as an observation point and A’ acts as a control point.

6. 0 – injection circuit A new control input CP has been added to gate G to make it G*. If CP=0, original and modified gates will be the same and the circuit operates in normal mode. If CP=1, OP is forced to ‘0’ and ‘0’ is injected to C2. Since ‘0’ is injected we call it as 0-injection circuit.

7. 1-injection circuit Similar to 0-injection, a 1 can also be injected. A control point is added to a OR gate to inject 1. If CP=0, the functioning of the OR gate doesn’t change and the circuit works in normal mode. If CP=1, it deactivates the normal operation and OP is forced to 1 which will be injected to C2. Since a 1 is injected, we call it as 1-injection circuit.

8. 0/1 – injection circuit Additional input CP1 has been added to gate G and another gate G’ has been added. If CP1=CP2=0, then the circuit works in normal mode. CP1=1 sets G* to 0, then G’=CP2, CP2 can be any value. Thus G’ can be controlled to either 0 or 1 and this value can be injected to C2. Hence we call it as 0/1- injection circuit.

9. Test points using MUX A multiplexer is added to the circuit. CP2 is the select line. If CP2=0, then the circuit works in normal mode. If CP2=1, G’=CP1 which can be any value. Hence a 0/1 can be injected into C2 to test.

10. General way to inject 0/1 For a given signal line G, Insert an AND/NOR gate for 0-injection Insert a NAND/OR gate for 1-injection Insert series of gates, example NOR-NOR for 0/1 injection. The problem in the above cases is the number of output pins and input pins are increased.