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BSDL Tutorial

Boundary-scan is a well established test technology. Boundary-scan has been in use since the early 1990s when the Joint Test Action Group (JTAG) devised a solution to testing the many new printed circuit boards that were being developed and manufactured where there was little or no physical access for test probes. Once boundary-scan was established, the next step was to develop a standard modeling language for silicon vendors to model their boundary-scan devices, for tool vendors to develop automation tools, and for end-users to create boundary-scan tests. Thus the Boundary-Scan Description Language (BSDL) was created.

BSDL is the standard modeling language for boundary-scan devices. Its syntax is a subset of VHDL and it complies with IEEE 1149.1-2001. It is used by boundary-scan test developers, device simulators, semiconductor testers, board level testers, and anyone using boundary-scan. The use of BSDL promotes consistency throughout the electronics industry. Additionally, it enables the specification of any boundary-scan functions on a device in a useful, understandable, and consistent manner.

BSDL came out of the development of the boundary-scan test philosophy. The initial IEEE 1149.1-1990 (see [IEEE 1149.1 (JTAG)]) standard describing boundary-scan was approved and released in 1990, and as a result the use of boundary-scan techniques started to grow. The next revision of the standard occurred in 1993. In 1994 a further revision incorporated BSDL into the IEEE 1149.1-1994 standard.


What is BSDL?

The Boundary-Scan Description Language enables users to provide a description of the way in which boundary-scan is implemented in any particular device. As each chip designer tends to apply the boundary-scan standard in a slightly different way, it is necessary to express tests in a comprehensible, specific and usable fashion.

BSDL is written within a subset of VHDL. VHDL is commonly used as a design-entry language for FPGAs and ASICs in electronic design automation of digital circuits, and as such it is suitable for work with boundary-scan since design of many chips is performed using this language. However BSDL is a "subset and standard practice" of VHDL, i.e., the scope of VHDL is thereby limited for boundary-scan application.

During the design of BSDL there were two main criteria for the language:

  • It should be easy to use

  • It should be parsable by a computer in a simple and unambiguous fashion

BDSL enables accurate and useful descriptions of the features of a device that uses boundary-scan. The BSDL file is used by the boundary-scan tools to make use of the device features to enable test program generation, failure diagnosis, as well as use in any testability analysis. BSDL is not a language that can be used for hardware description; rather, it is used to define the data transport characteristics of the device, i.e. how it captures, shifts, and updates scanned data. This is then used in defining the test capability. The BSDL file includes the following data:

  • Entity Declaration: The Entity Declaration is a VHDL construct that is used to identify the name of the device that is described by the BSDL file.

  • Generic Parameter: The Generic Parameter is the section that specifies which package is described.

  • Logical Port Description: This description lists all the connections on the device. It defines its basic attributes, i.e., whether the connection is an input (in bit;), output (out bit;), bi-directional (inout bit;) or if it is unavailable for boundary-scan (linkage bit;).

  • Package Pin Mapping: The Package Pin Mapping is used for determining the internal connections within an integrated circuit. It details how the pads on the device die are wired to the external pins.

  • Use Statements: This statement is used to call the VHDL packages that contain the data that are referenced in the BSDL File.

  • Scan Port Identification: The Scan Port Identification identifies the particular pins that are used for the boundary-scan / JTAG implementation. These include: TDI, TDO, TMS, TCK and TRST (if used).

  • Test Access Port (TAP) Description: This entity provides additional information on the boundary-scan or JTAG logic for the device. The data includes the instruction register length, instruction opcodes, device IDCODE, etc.

  • Boundary Register Description: This description provides the structure of the boundary-scan cells on the device. Each pin on a device may have up to three boundary-scan cells, each cell consisting of a register and a latch.


How is BSDL used?

When a board is designed, boundary-scan-compliant devices are organized into "chains". Scan chains form the basis for board-level and system-level tests that can detect and diagnose pin-level structural faults such as opens and shorts. Automated tools are used to generate test programs or procedures for the boards. The most important inputs to this process are the BSDL files for boundary-scan-enabled devices, and the netlist that describes the interconnects between the devices of the board. The generated test program, when applied to a target board, reports the structural test failures and can be used to help with board repair.

Some tools are able to use boundary-scan to create test patterns for clusters of components that include non-boundary-scan-compliant devices, and other tools can generate test patterns that an on-board processor can run to enable at-speed functional testing. These test procedures are applied stand-alone or in conjunction with other test techniques, such as In-Circuit Testing (ICT), with an overall goal of producing optimal test coverage at the lowest cost and the shortest test development time.


Summary

The Boundary-Scan Description Language, BSDL, is widely used within the IEEE 1149.1 / JTAG community to enable consistent, accurate and useful information to be defined for a boundary-scan-enabled device. In this way, the chip can be incorporated into a design, and its capabilities used to their full in the most efficient manner.

References

The Boundary-Scan Description Language (BSDL) specification is contained in the IEEE 1149.1 standard which can be obtained from http://www.ieee.com/.

 

     

 

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