1 引言

随着半导体工艺的发展，片上系统SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC" title="SOC">SOC已成为当今一种主流技术。基于IP复用的SOC设计是通过用户自定义逻辑(UDL)和连线将IP核整合为一个系统，提高了设计效率，加快了设计过程，缩短了产品上市时间。但是随着设计规模的增大，集成密度的提高，IP引脚的增多，IP的植入深度加大必然使得测试验证工作繁重。据统计，在SOC设计中，各种内核的测试验证工作所用的时间占整个设计过程的60％～80％，SOC及IP核的测试验证已成为SOC技术发展的瓶颈。如何在最短的时间内高效迅速地通过IP核验证与测试．并把其集成在SOC中成为业界关注的焦点和研究领域急待突破与实现的方向。

基于IP核复用的SOC，其IP核类型和来源都不相同，即使已验证好的IP核在集成时也不能确保不出差错。IP核被集成到SOC后，其输入输出端口也嵌入到SOC，原本可测的端口失去了原有的可控性和可观测性，变得不可测。

因此人们一直寻求有效的测试验证技术。本文给出了基于内建自测试方法(BIST)，在设计编译码器IP核的同时，考虑其测试外壳设计，以期提高IP核可测性。

2 测试结构

所谓测试，就是在被测电路的输入引脚施加相应的激励信号，然后检测输出引脚的响应，并将检测的输出引脚的响应与期望引脚的响应进行比较以判断电路是否存在故障的过程。

IP核测试的目的在于检测IP核是否存在功能和时序错误，从而对IP核进行修改，提高产品的可靠性。一般采用访问、隔离、控制的手段对IP核的输入端施加激励来得到响应与期望的响应进行比较。嵌入式IP核的测试结构如图1所示。

测试激励源为嵌入式IP核生成测试所需的激励。响应分析器对所得到的响应进行分析，如果相同则表明没有故障，不同则表明存在故障。测试访问机制是 SOC传送数据的一种手段，它将测试激励传送到IP核的输入端口并将测试响应从IP核的输出端口传送到响应分析器。测试外壳是IP核同访问机制及器件其他逻辑之间的接口；测试外壳以实现片上核与核之间的测试隔离，也可以为IP核提供了测试数据传送通道。

3 内建自测试原理

内建自测试是可测性设计的一种重要方法。其基本思想是让电路自己生成测试向量，而非通过外部施加测试向量，并且依靠自身判断所得到的结果是否正确。内建自测试原理图如图2所示。

其中测试外壳(wrapper)在IP核的设计之中就予以考虑。通过测试外壳外部可以控制BIST和正常模式的切换。测试外壳内部多采用边界扫描模块、移位寄存器或多路访问器等，它起到访问、隔离、控制的作用，可提高IP核的可测性。然而加入测试外壳会使IP核的面积开销增大，因此必须在IP核的可测性和面积之间进行权衡。

4 内建自测试的实现

可控性指驱动一个节点为逻辑状态0或1的难易程度；可观测性指从外部端口观察内部节点故障的难易程度；可测性就是指在整个电路全部节点的可控性和可观测性。很显然，高可测性比较容易产生测试向量并且测试效果良好。

为了提高IP核的可测性，在IP核的设计中就考虑了设计特定电路方便测试。以BIST实现IP核的测试，一般具有如下优势：简化测试接口；改善测试质量；降低测试成本；提高测试可靠性。

基于BIST的编译码器IP核测试实现框图如图3所示。

　　通过测试外壳控制输入/输出寄存器将编译码器隔离使它们互不影响，正常状态和测试状态能够切换，提高了IP核的可测性。具体的实现过程如下：

(1)正常状态下原始码输入编码器，由其输出的码再进入译码器又转换为原始码；

(2)测试状态下测试外壳的测试向量输入编码器，由其输出的码直接进入译码器，由译码器输出的码为测试响应输出，使其与期望值比较；图3中的模式选择模块的逻辑结构如图4所示。


图4中mod为模式选择控制端，置0为正常状态。当rood为1时，out_sel输入测试向量，再由in_tem输出，进入in_sel；当mod 为0时，cod_out输出片外，从而实现了正常状态和测试状态的切换。可以用硬件描述语言编写此逻辑电路。用VHDL描述如下：

此模式选择模块实现状态之间的切换，电路简单，易于实现。

5 结束语

BIST为嵌入式内核的测试提供了一个可解决的方案，其测试效果明显，故障覆盖率较高，实现简单。通过加入测试外壳可以实现对IP核的访问、隔离、控制，有效地提高了IP核的可测性。但是采用BIST会使电路面积增加额外开销，必须在IP核的可测性和面积之间进行权衡。