请问什么是边界扫描？

_ * Nutshell中的边界扫描* _ ICT测试需要“测试访问”。
这是指设计到PCB中的测试点（通常是焊盘或过孔），探针可以连接到测试点以提供被测板和测试仪之间的电气连接。
该测试标准由一个公司联盟于1990年组建，主要是为了解决由于减小PCB尺寸和增加包装到产品中的功能而越来越缺乏PCB组件的测试访问。
IEEE 1149.1（也称为边界扫描）是一种测试标准，涉及设计有移位寄存器的器件，如图1所示。每个移位寄存器称为边界扫描单元。
这些边界扫描单元允许您控制和观察每个输入和输出引脚上发生的情况。
当这些单元连接在一起时，它们形成一个称为边界寄存器的数据寄存器链。
边界扫描设备中还有其他寄存器。
•指令寄存器解码指令位，允许器件执行各种功能。
•旁路寄存器提供一位路径，最小化扫描输入和扫描输出之间的距离。
•标识寄存器，称为IDCODE寄存器，用于标识设备和制造商。
•其他设计人员指定的数据寄存器通常执行内部测试功能。
边界扫描设备有一个专用端口，称为测试访问端口（TAP），它将输入信号路由到称为TAP控制器的控制器和寄存器单元。
TAP控制器是一个控制边界寄存器的16状态机器。
图中所示的TAP信号用于控制边界扫描设备。
它们包括以下内容：•测试数据输入（TDI）：测试数据和指令位的串行输入。
•测试数据输出（TDO）：测试数据的串行输出。
•测试时钟（TCK）：用于驱动器件的独立时钟。
•测试模式选择（TMS）：这提供了将TAP控制器从状态更改为状态所需的逻辑电平。
•测试复位（TRST）可选：此可选输入提供TAP控制器的异步初始化，从而导致设计中包含的其他测试逻辑的异步初始化。
边界扫描设备可以执行许多测试功能。
其中三个，EXTEST，SAMPLE / PRELOAD和BYPASS，对于每个边界扫描设备都是必需的。
对于其他测试功能，INTEST，RUNBIST，IDCODE，CLAMP，HIGHZ和USERCODE由IEEE 1149.1标准描述，但是是可选的。
制造商还可以添加其实施遵循IEEE标准的测试功能。



     以下为原文

  _*Boundary Scan in a Nutshell*_
Testing at ICT requires “test access”. This refers to the testpoints (usually pads or vias) that are designed into the PCB, that probes can connect to in order to provide the electrical connectivity between the board-under-test and the tester. The test standard was formed by a consortium of companies in 1990 mainly to address the increasing lack of test access on PCB assemblies due to reducing PCB sizes and increasing functionalities being packed into the product.  

IEEE 1149.1 (also known as Boundary scan) is a test standard that involves devices designed with shift registers placed between each device pin and the internal logic as shown in Figure 1. Each shift register is called a boundary scan cell. These boundary scan cells allow you to control and observe what happens at each input and output pin. When these cells are connected together, they form a data register chain, called the Boundary Register.
  
There are other registers within a boundary-scan device.
• An Instruction Register decodes instruction bits that allow the device to perform various functions.
• A Bypass Register provides a one-bit path that minimizes the distance between the scan input and the scan output.
• An Identification Register, called the IDCODE Register, identifies the device and manufacturer.
• Other designer-specified data registers typically perform internal test functions.

Boundary scan devices have a dedicated port, called the Test Access Port (TAP), that routes input signals to a controller, called the TAP Controller, and the register cells. The TAP Controller is a 16-state machine that controls the Boundary Register.
  
The TAP signals shown in the illustration are used to control the boundary scan device. They include the following:
• Test Data In (TDI): The serial input for test data and instruction bits.
• Test Data Out (TDO): The serial output for test data.
• Test Clock (TCK): An independent clock used to drive the device.
• Test Mode Select (TMS): This provides the logic levels needed to change the TAP Controller from state to state.
• Test Reset (TRST) Optional: This optional input provides asynchronous initialization of the TAP Controller, which in turn causes asynchronous initialization of other test logic included in the design.

Boundary scan devices can perform many test functions. Three of these, EXTEST, SAMPLE/PRELOAD, and BYPASS, are mandatory for every boundary-scan device. For other test functions, INTEST, RUNBIST, IDCODE, CLAMP, HIGHZ, and USERCODE, are described by the IEEE 1149.1 standard, but are optional. Manufacturers can also add test functions whose implementation is guided by the IEEE standard.   
附件

• Boundary_Scan_Device.PNG1.2 MB