1. TOPIC : Test Techniques for some complex circuits UNIT 4 : Design For Testability Module 4.2: DFT Techniques

2. Introduction There are some circuits which cause obstruction to testing when they are a part of the circuit under test. Techniques are developed to overcome the obstacles for each complex circuit. So the test set-up should ensure that it has the techniques to test the following circuits separately before starting the test procedure. Circuits that cause obstruction are:  Oscillators and clocks  Mono-stable Multi-vibrators  Counters and shift registers

3. Mono-stable Multi-vibrators Disable internal multi-vibrators during test. These produce pulses internal to a circuit and make it difficult for external test equipment to remain in synchronization with the CUT. A jumper can be used to solve this problem. Through this the multi-vibrator can be ◦ Disabled ◦ Controlled ◦ Its outputs can be observed ◦ Normally operated along with the remaining circuitry

4. Contd … A jumper can be used to gain access to the input/output ports of a multi-vibrator. Removing jumper1 and jumper2 will allow to test the multi-vibrator. The test inputs can be given from pin A2 and the outputs can be observed at pin B1. The outputs of C1 is also observable at A1 and B2 can be used as control point for C2.

5. Contd … To test the entire circuitry linked with multi-vibrator, injection circuitry needs to be added as shown in figure.

6. Contd … To test C1: In the figure, we can observe that the inputs to C1 are primary inputs. Its output can be observed at ‘X’ or can be observed at ‘y’ making A=0 & B=0. A=0, B=0  normal operation A=1  0/1 injection to the multi-vibrator depending on the other control point B. To test multi-vibrator: Making A=1 will deactivate the normal operation and B is used to apply externally generated pulses to it as input and test. Its output for different inputs from B can be observed at E(OP).

7. Contd … To test C2: Making D=0 will deactivate the normal operation and C is used to apply all the externally generated signals as inputs to C2. The outputs of C2 for different inputs can be observed at the primary output. A=0, B=0, D=1  normal operation of the circuit. A=1 deactivates the normal operation A=1, B=0/1 will inject 1/0 as inputs to multi-vibrator. D=0 deactivates the normal operation and probes for testing D=0, C=0/1 will inject 0/1 as inputs to C2. B, C are used as external inputs during testing.

8. Oscillators and Clocks Disable the internal oscillators and clocks during test. Similar to multi-vibrators oscillators and clocks also lead to synchronization problems. A method to test the circuits consisting oscillators/clocks:

9. Contd … To test Oscillator: All possible inputs are applied to the oscillator and its outputs are observed using OP by making A=0 & B=0. To test C: A=1  deactivates the normal operation A=1, B=0/1  injects 1/0 as inputs to C B is used to apply the externally generated signals as inputs to C and the outputs are observed at the primary output. Hence the oscillator and circuit C are tested.

10. Counters and shift Registers Partition large counters and shift registers into smaller units. These are difficult to test because their test sequences require many clock cycles. For better controllability and observability, these devices are to be partitioned. Figure shows how a register is partitioned.

11. Contd … The clock from circuit C can be replaced by an external clock as shown and the test equipment is also shown in the following figure: The serial inputs to R1 and R2 are easily controllable. R1 and R2 can be independently tested.

12. Contd… CP1=1  clock inhibit CP2  test clock DP1=1, DP3=1  data inhibit DP2, DP4  test data Making DP1=1, deactivates the normal operation and test data can be injected to R1 from DP2. The outputs are observed at OP. By making DP3=1, we can inject test data to R2 from DP4.