It's not just for test anymore: Recent extensions to JTAG not only enhance its test coverage, but also make it useful for design as well, says this National Semiconductor technologist. Suppliers of CPLD's and FPGA's were quick to support IEEE 1532 standard for Boundary-Scan-based In-System-Configuration of programmable devices. Now designers have a way to develop flexible systems where functionality can be adapted to a specific customer or application, he says.
Now that the IEEE 1149.1 (JTAG) test standard has become standard tackle for most advanced electronic systems, a significant number of new uses for JTAG are evolved. Design teams now rely on the JTAG for In-System-Programming (ISP), In-system Configuration (ISC), and emulation. JTAG is no longer just for test, it's considered to be part of the basic architecture of the system design. With all these new uses beyond test, designers are now taking back the test bus for system configuration and programming after manufacturing test is complete.
Test engineers have long known the benefits of using JTAG for structural and interconnect test at the board level. With simple PC-based test software and hardware tools, otherwise undetectable faults can be easily identified and isolated with a very high degree of fault coverage.
Recent extensions to JTAG enhance the test coverage. Embedded at-speed BIST in drivers and receivers "initiated and verified through the JTAG bus"enables testing of high-speed interconnects previously untestable using lower speed JTAG techniques. IEEE 1149.6 describes how AC-coupled (capacitive) interconnects can be tested using compliant drivers and receivers. IEEE 1149.4 defines extensions for mixed-signal test capability.
With this JTAG infrastructure installed in a system, additional uses have emerged of interest to designers. Suppliers of CPLD's and FPGA's were quick to support IEEE 1532 standard for Boundary-Scan-based In-System-Configuration of programmable devices. Now designers have a way to develop flexible systems where functionality can be adapted to a specific customer or application. Firmware changes can be loaded through the system JTAG port during production, or in the field when an upgrade is available or necessary. JTAG emulation tools also use the JTAG infrastructure to control the local system bus. These tools emulate the local bus cycles necessary for FLASH programming.
So how do design and test teams share the former test-only bus? The answer lies in partitioning the JTAG bus to provide isolation. IC's like multidrop JTAG multiplexers enable several JTAG chains on an individual board, and interface multiple boards into a single system-level backplane JTAG test bus to provide targeted access to specific devices on JTAG chains.
The promotion for Brian Stearns at National's "Analog University" says he's been employed by National for 19 years, in various Marketing and Product Engineering roles"especially on Designfor- Test (DfT) issues. We remember him attracting audiences with his expertise at the engineeringintensive DesignCon seminars.