LBIST Architecture
Introduction
Logic Built‑In Self‑Test (LBIST) enables a digital system to test its own logic without relying on external testers. It uses pseudo‑random pattern generation and signature compression to detect structural faults in safety‑critical and high‑reliability applications. LBIST is a key enabler for autonomous diagnostics, periodic self‑test, and power‑on safety checks.
Why LBIST Matters
LBIST provides capabilities that traditional scan‑based testing cannot offer:
- No external tester required — patterns are generated on‑chip, reducing tester time and dependency.
- Startup self‑test for safety — automotive and aerospace systems must verify logic integrity before enabling safety‑critical functions.
- Periodic online diagnostics — LBIST can run during field operation in short time slices.
- High structural coverage — pseudo‑random patterns exercise large portions of the logic with minimal overhead.
These properties make LBIST essential in modern safety‑critical SoCs.
Core Components
A complete LBIST architecture is composed of five main blocks:
- PRPG (Pseudo‑Random Pattern Generator) — typically an LFSR that produces pseudo‑random test patterns.
- Phase shifters — decorrelate PRPG outputs and distribute them across multiple scan chains.
- Scan chains — act as the delivery mechanism for test patterns and capture responses.
- Response compactors / MISR — compress the captured responses into a final signature.
- LBIST controller — orchestrates initialization, shift, capture, and signature evaluation.
Together, these blocks implement the classical STUMPS architecture used in most LBIST systems.
Safety Connection
LBIST is tightly linked to functional safety standards such as ISO 26262:
- Mandatory for ASIL C/D startup tests — logic must be verified before enabling safety‑critical functions.
- Detects latent faults — periodic LBIST reduces the exposure time of undetected failures.
- Supports in‑field diagnostics — segmented LBIST can run during operation without disrupting system behavior.
- Contributes to PMHF reduction — improves diagnostic coverage and reduces the probability of dangerous failures.
This makes LBIST a foundational element in safety‑critical DFT strategies.
Execution Modes
LBIST can operate in different modes depending on system constraints:
- Power‑up LBIST — runs at startup to ensure the logic is fault‑free before system activation.
- Online LBIST — executed during normal operation in short, time‑sliced segments to maintain diagnostic coverage.
- Periodic maintenance LBIST — triggered at scheduled intervals or during service operations.
Each mode balances diagnostic coverage, execution time, and system availability.
Figures
Figure 1 – High‑level LBIST architecture showing PRPG, phase shifter, scan chains, MISR, and the LBIST controller coordinating initialization, shift, and capture phases.
Figure 2 — LBIST timing showing the three phases of a self‑test cycle. During SHIFT‑IN, the PRPG shifts pseudo‑random patterns into the scan chains. During CAPTURE, SCAN_EN is de‑asserted and the functional logic evaluates, producing the response. During SHIFT‑OUT, the captured response is shifted out and compacted into the MISR.
Figure 3 — Example of a STUMPS architecture showing a 4‑bit PRPG driving a 4→16 phase shifter, a set of 16 scan chains, and a 4‑bit MISR compacting the responses. The LBIST controller orchestrates initialization, shift, and capture phases across all blocks.
