1. TOPIC : BILBO BIST Architectures UNIT 5 : BIST and BIST Architectures Module 5.2 BIST

2. BILBO BIST Architecture A circuit is partitioned into a set of registers and combinational circuits. These registers are replaced by BILBO registers. The inputs to combinational circuits are driven by BILBO registers and the outputs drive another BILBO register.

3. Contd … An architecture is shown in figure. The registers are all BILBOs. To test C1: R1 is put in PRPG mode and R2 is put in MISR mode. After the test session, R2 can be scanned out and the signature can be checked. To test C2: Similarly R2 is put in PRPG mode and R1 in MISR mode. Thus the circuit is tested in 2 sessions.

4. Contd … Another BILBO BIST architecture is shown in figure. To test C1: R1 must be in PRPG mode and R2 should be in both MISR and PRPG mode since BILBO R2 have a self loop. With the BILBO register design given earlier, it cannot be in both MISR and PRPG modes. To solve this problem concurrent BILBO is developed.

5. Concurrent BILBO (CBILBO) It can be operated simultaneously as PRPG and MISR modes. It is shown in figure below:

6. Contd … The top row of D flip-flop and associated logic form MISR. The bottom row of dual-port flip-flops and associated logic form PRPG. When B1=0, B2=1 it works in both PRPG and MISR modes. When B1=1, B2=1 it works in test mode.

7. Contd … Note that the inputs of a BILBO should be some constant value when it is in PRPG mode. Since CBILBO can operate in both PRPG and MISR modes simultaneously, problem arises because the inputs keep changes. Hence when BILBO is in PRPG mode, its inputs should be deactivated. For this a modified BILBO register with 3 control inputs is designed.

8. BILBO with 3 control inputs (modified BILBO) There will be 8 possible states, so one can be used to activate MISR mode and one can be used for PRPG.

9. Modes of Operation Since there are 3 control inputs, there will be 8 possible states. Some states are used to specify different modes of BILBO as shown in table. B1B2B3Mode of Operation 000Scan mode 100Signature analysis (MISR) 101Pattern generation (PRPG) 110Normal mode 010Reset