What is BIST?

BIST = Built-In Self-Test

Test logic embedded inside the chip. No expensive external tester needed.

Two Types of BIST

MBIST - Memory BIST

Architecture

How It Works

1. Address Generator: Cycles through all memory locations
2. Data Generator: Creates test patterns (0s, 1s, checkerboard)
3. Write to SRAM: Store test data
4. Read from SRAM: Retrieve stored data
5. Compare: Match expected vs actual
6. Report: PASS if all match, FAIL otherwise

March Algorithm

Industry-standard memory test pattern.

March C- Algorithm

Step 1: ⇑(w0)      Write 0 to all locations, ascending
Step 2: ⇑(r0,w1)   Read 0, write 1, ascending  
Step 3: ⇑(r1,w0)   Read 1, write 0, ascending
Step 4: ⇓(r0,w1)   Read 0, write 1, descending
Step 5: ⇓(r1,w0)   Read 1, write 0, descending
Step 6: ⇓(r0)      Read 0, descending

Notation:
  ⇑ = Ascending address order
  ⇓ = Descending address order
  w0 = Write 0
  r1 = Read and expect 1

What March Catches

LBIST - Logic BIST

Tests random logic using pseudo-random patterns.

Components

1. LFSR: Generates pseudo-random test patterns
2. Circuit Under Test: Your logic (via scan chains)
3. MISR: Compresses outputs into signature
4. Comparison: Match expected signature

LFSR - Pattern Generator

LFSR = Linear Feedback Shift Register

Generates pseudo-random sequence using XOR feedback.

module lfsr_4bit (
    input            clk, rst_n, en,
    output reg [3:0] q
);
    // Polynomial: x^4 + x^3 + 1
    // Taps at positions 4 and 3
    
    always @(posedge clk or negedge rst_n) begin
        if (!rst_n)
            q <= 4'b1111;  // Non-zero seed required!
        else if (en)
            q <= {q[2:0], q[3] ^ q[2]};  // Shift + feedback
    end
endmodule

LFSR Sequence

4-bit LFSR cycles through 15 values (2^4 - 1):

1111 → 0111 → 0011 → 0001 → 1000 → 0100 → 
0010 → 1001 → 1100 → 0110 → 1011 → 0101 → 
1010 → 1101 → 1110 → (back to 1111)

Never produces 0000 (would get stuck!)

MISR - Signature Analyzer

MISR = Multiple Input Signature Register

Compresses many outputs over many cycles into single signature.

module misr_4bit (
    input            clk, rst_n, en,
    input      [3:0] data_in,
    output reg [3:0] sig
);
    always @(posedge clk or negedge rst_n) begin
        if (!rst_n)
            sig <= 4'b0;
        else if (en)
            sig <= {sig[2:0], sig[3] ^ sig[2]} ^ data_in;
    end
endmodule

How Signature Works

Cycle 1: Output = 1010 → MISR = 0x1A
Cycle 2: Output = 1100 → MISR = 0x3F
Cycle 3: Output = 0110 → MISR = 0x7B
...
Cycle N: Final Signature = 0xA5B7C3D1

If ANY cycle has wrong output → Signature changes!
Compare with expected: Match = PASS, Mismatch = FAIL

BIST vs External Test Comparison

When to Use BIST

MBIST - Always!

Every embedded memory should have MBIST. It's:

• Standard practice
• Required by most customers
• Catches memory-specific defects

LBIST - Sometimes

Use for:

• Automotive (safety-critical)
• Medical devices
• High-reliability applications
• In-field testing