Comparison of LFSR and CA for BIST Sachin Dhingra ELEC 7250: VLSI Testing 4/26/05 Dhingra: ELEC7250

Introduction Built-In Self Test Implementation of BIST Circuit capable of testing itself Two major components Test Pattern Generator Output Response Analyzer Implementation of BIST Linear Feedback Shift Register (LFSR) Shift Register with feedback path linearly related to the nodes using XOR gates Cellular Automata (CA) A collection of nodes logically related to their neighbors using XOR gates 4/26/05 Dhingra: ELEC7250

Built-In Self Test Test Mode Normal Operation System Inputs System Input Circuit Outputs Isolation Under Circuitry Test Test Output Pattern Response Generator Analyzer Test Controller TPG generates pseudo – random test vectors Input Isolation Circuitry isolates the normal system inputs from the CUT Output Response Analyzer performs polynomial division for test data compaction (signature analysis) 4/26/05 Dhingra: ELEC7250

Linear Feedback Shift Register (LFSR) Two Types External Feedback Internal Feedback Characteristic Polynomial All zero state is invalid Max. Sequence Length = 2n – 1 Primitive and Non-primitive Reciprocal of primitive polynomial is also primitive P*(x) = xnP(1/x) Compact Design Less than one gate per node Parallel Pattern generation Signature Analysis Signature Analysis Register (SAR) Multiple Input Signature Register (MISR) P (x) = x0 + x1 + x3 + x4 4/26/05 Dhingra: ELEC7250

Cellular Automata (CA) Rule 150 Rule 90 Rule 90 Rule 90 Null boundary condition One-Dimensional Linear CA Linear Hybrid Cellular Automata (LHCA) Linear Cellular Automata Register (LCAR) “Rules” define the logical relationship of a node with its neighbors Rule 90 xi(t+1) = xi-1(t)  xi+1(t) Rule 150 xi(t+1) = xi-1(t)  xi(t)  xi+1(t) Combination of Rules ≡ Characteristic Polynomial of LFSRs Boundary Condition Null Boundary Condition – No Feedback ⇒ Faster Cyclic Boundary Condition – Feedback ⇒ Slower Highly Random Vectors 4/26/05 Dhingra: ELEC7250

Comparison Characteristic LFSR CA Area Overhead Max. Length Sequence Least Less than one Gate/node Higher than LFSR One Gate/node Max. Length Sequence Easy to implement Well defined P(x) Harder to implement Combination of rules not well defined Performance Lower – External Feedback XOR gates in Feedback Higher – Internal Feedback Max. one gate/path High No gates in feedback Parallel Pattern Randomness Low Shifting of Data Logical relation with neighbors Stuck-at-fault detection Stuck-open and Delay fault Detection Less number of transitions Higher number of transitions due to higher randomness CAD friendliness No Nodes cannot be cascaded Yes Nodes can be easily cascaded Signature Aliasing Higher Probability Lower Probability 4/26/05 Dhingra: ELEC7250

Summary and Conclusion LFSRs are more popular because of their compact and simple design CAs are more complex to design but provide patterns with higher randomness CAs perform better in detection of faults such as stuck-open or delay faults, which need two-pattern testing In applications where area overhead is a big concern, LFSRs prove to be a better choice CAs provide a good alternative for LFSRs when high fault coverage is needed 4/26/05 Dhingra: ELEC7250

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