Joint Test Action Group (JTAG) is the common name used for the IEEE 1149.1 standard entitled Standard Test Access Port and Boundary-Scan Architecture for test access ports used for testing printed circuit boards using boundary scan. JTAG is often used as an IC debug or probing port.

History

JTAG was an industry group formed in 1985 to develop a method to test populated circuit boards after manufacture. At the time, multi-layer boards and non-lead-frame ICs were becoming standard and making connections between ICs not available to probes. The majority of manufacturing and field faults in circuit boards were due to solder joints on the boards, imperfections in board connections, or the bonds and bond wires from IC pads to pin lead frames. JTAG was meant to provide a pins-out view from one IC pad to another so all these faults could be discovered. The industry standard finally became an IEEE standard in 1990 as IEEE Std. 1149.1-1990 after many years of initial use. That same year Intel released the first processor with JTAG: the 80486 which led to quicker industry adoption by all manufacturers. In 1994, a supplement that contains a description of the boundary scan description language (BSDL) was added. Since then, this standard has been adopted by electronics companies all over the world. Boundary-scan is nowadays mostly synonymous with JTAG.

While designed for printed circuit boards, JTAG is nowadays primarily used for accessing sub-blocks of integrated circuits, and is also useful as a mechanism for debugging embedded systems, providing a convenient "back door" into the system. When used as a debugging tool, an in-circuit emulator - which in turn uses JTAG as the transport mechanism - enables a programmer to access an on-chip debug module which is integrated into the CPU, via the JTAG interface. The debug module enables the programmer to debug the software of an embedded system.

Besides debugging, the second purpose of the JTAG interface is allowing device programmer hardware to transfer data into internal non-volatile device memory. Some device programmers serve a double purpose for programming as well as debugging the device.

In most ICs today, all internal registers are on one of many scan chains. This allows all combinational logic to be tested completely even while an IC is in the circuit card and possibly while in a functioning system. When combined with built-in self-test (BIST), the JTAG scan chain enables a low overhead, completely embedded solution to testing an IC for certain static faults (shorts, opens, and logic errors). The scan chain mechanism does not generally help diagnose or test for timing, temperature or other dynamic operational errors that may occur.

Electrical characteristics

A JTAG interface is a special four/five-pin interface added to a chip, designed so that multiple chips on a board can have their JTAG lines daisy-chained together if specific conditions are met, and a test probe need only connect to a single "JTAG port" to have access to all chips on a circuit board. The connector pins are