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Tell me about JTAG/boundary-scan!

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Boundary-scan, also known as JTAG or IEEE Std. 1149.1, is a serial interface that gives access to the special embedded logic built-in into many of today’s integrated circuits (ICs). JTAG/boundary-scan provides a quick and easy method for testing electronic Printed Circuit Board Assemblies or PCBAs for manufacturing faults. It is also widely used for programming ICs such as cPLDs, FPGAs and flash memories on the circuit boards in production as well as after product manufacture if software/ firmware updates are needed.

Basic idea

The basic idea behind boundary-scan is that with internal cells you don’t need external probing access to the pin connections you wish to test. If you can drive the output pin of Device A using a built-in drive cell, and read the result on the input pin of Device B with a built-in sense cell, you can perform the desired test.

BSDL File

The boundary-scan logic of a device is described in a single model file known as BSDL file (Boundary-Scan Description Language), which is also part of the IEEE 1149.1 standard. For every device type, the manufacturer of that device must provide a BSDL file. Using the description in this file software tools can analyze the boundary-scan logic in a device and automatically generate tests or other boundary-scan applications, such as device programming, for a PCBA

Applications

The boundary-scan logic on a PCBA can be used for testing connections. Not only direct connections between boundary-scan devices, but also connections through transparent devices such as series resistors and buffers for example, as well as connections through logic gates. The presence of pull-up and pull-down resistors and connections to memory devices and microprocessor peripherals can also be tested

Hardware and Software

Hardware and software is needed to control the boundary-scan logic and drive data through the TAP (Test access port) and the registers in the chains. Special, advanced software can automatically generate JTAG/boundary-scan applications from CAD data and device models. A broad range of hardware and software tools is available with different capabilities and performance. This gives you the possibility to configure the boundary-scan solution that optimally fits the needs of your application area. For example tools for hardware debugging during prototype bring-up, testing and device programming in production, or repair and device re-programming for product upgrades in service.

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