大牛问答 | LBIST

今早把一篇老文《葵花宝典：DFT问答第二篇》，文中有一段LBIST 的描述：

A：LBIST （Logic Built-In Self Test）是针对逻辑电路的自测试。测试激励由片上 PRPG (Pseudo-randompattern generation) 来产生。输出响应通过 MISR（Multiple Input Signature Register）来压缩，最后对得到的特征值进行比对。LBIST 多应用于对可靠性要求较高的芯片（如汽车电子，工业级应用）的系统自检测试。LBIST产生的激励是随机的，所以天生的缺陷是测试覆盖率不充分，通过在设计中增加测试点(Test Point Insertion)可以在一定程度上得到改善。

某兄看了这段描述后提问：

有个疑问没想明白原因：「LBIST 多应用于对可靠性要求较高的芯片（如汽车电子，工业级应用）的系统自检测试。LBIST产生的激励是随机的，所以天生的缺陷是测试覆盖率不充分」既然有覆盖率不充分的天生缺陷，那怎么还多用于可靠性要求高的芯片产品呢？

某兄给出了精彩回答：

可靠性要求高的芯片，如航空，医药，汽车电子类，都是详细复杂的安全标准，并且有多种安全测试校验机制，比如POST( power on/off self test ) 就是其中一种，而Bist (mbist &lbist) 就是一种实现POST 很好的手段。lbist coverge 和测试结果，会提交给 safety island, 然后再结合多个其他测试 (mbist) 或者安全校验 (如CRC ECC 等)，得出一个最终的安全结果和指令。

不同安全标准，对Functional Safety 的coverge 要求不一样, Functional Safety 的coverge， 是function 的计算方法，不同于我们DFT 里的coverge. 比如，Functional 的coverge 最主要的两个是SPFM 和LFM, 在B level的时候分别是90% 和60%, 这个很低的，但并不是直接对应DFT 的coverge, 需要主要考虑Functional 的安全可靠，DFT 的bist 提供了一种便捷的辅助统计手段。

驴谷歌到了一篇《 Functional Safety Methodologies for Automotive Applications 》, 截取两段：

From the vocabulary in ISO 26262:1-2011, a safety mechanism is a technical solution implemented by E/E functions or elements, or by other technologies, to detect faults or control failures to achieve or maintain a safe state. Examples of safety mechanisms include:

• Error correction code (ECC)
• Cyclic redundancy check (CRC) – Hardware redundancy
• Built-in-self-test (BIST)

The effectiveness of the solution to detect these random failures is measured by three metrics to detect fault and failure in time (FIT), as well as the overall likelihood of risk:

• Single-point fault metric (SPFM)
• Latent fault metric (LFM)
• Probabilistic metrics for hardware failures (PMHF)

The most notable example of safety mechanism already automated in the design ow is the BIST, used for automotive in-system/ eld testing for lifetime reliability to achieve the desired ASIL.

There are two general categories of BIST techniques for testing random logic. They have different impacts on the safety metrics and require different timing performance:

• Online BIST: This test is performed when the functional circuitry is in normal operational mode (mission mode). It contributes to the SPFM metric and has more stringent timing requirements because it must complete within the DTI.
• Of ine BIST: This test is performed when the functional circuitry is not in normal mode, e.g., during power-on reset at the engine startup. It contributes to the LFM and timing requirements are more relaxed.

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