What is JTAG?

JTAG (also known as boundary-scan) has enjoyed growing popularity for board
level manufacturing test applications since its introduction as an industry
standard in 1990. JTAG has rapidly become the technology of choice for building
reliable high technology electronic products with a high degree of testability.
Due to the low-cost and IC level access capabilities of JTAG, its use has
expanded beyond traditional board test applications into product design and
service.

This overview provides a brief overview of the JTAG architecture and the new
technology trends that make using JTAG essential for dramatically reducing
development and production costs. The article also describes the various uses of
JTAG and its application.

JTAG, as defined by the IEEE Std. 1149.1 standard, is an integrated method for
testing interconnects on printed circuit boards that is implemented at the IC
level. The inability to test highly complex and dense printed circuit boards
using traditional in-circuit testers and bed of nail fixtures was already
evident in the mid eighties. Due to physical space constraints and loss of
physical access to fine pitch components and BGA devices, fixturing cost
increased dramatically while fixture reliability decreased at the same time.

In the 1980s, the Joint Test Action Group (JTAG) developed a specification for
JTAG testing that was standardized in 1990 as the IEEE Std. 1149.1-1990. In 1993
a new revision to the IEEE Std. 1149.1 standard was introduced (titled 1149.1a)
and it contained many clarifications, corrections, and enhancements. In 1994, a
supplement that contains a description of the boundary-scan Description Language
(BSDL) was added to the standard. Since that time, this standard has been
adopted by major electronics companies all over the world. Applications are
found in high volume, high-end consumer products, telecommunication products,
defense systems, computers, peripherals, and avionics. Now, due to its economic
advantages, smaller companies that cannot afford expensive in-circuit testers
are using JTAG.

The boundary-scan test architecture provides a means to test interconnects
between integrated circuits on a board without using physical test probes. It
adds a boundary-scan cell that includes a multiplexer and latches, to each pin
on the device. Boundary-scan cells in a device can capture data from pin or core
logic signals, or force data onto pins. Captured data is serially shifted out
and externally compared to the expected results. Forced test data is serially
shifted into the boundary-scan cells. All of this is controlled from a serial
data path called the scan path or scan chain. Figure 1 depicts the main elements
of a JTAG device. By allowing direct access to nets, JTAG eliminates the need
for large number of test vectors, which are normally needed to properly
initialize sequential logic. Tens or hundreds of vectors may do the job that had
previously required thousands of vectors. Potential benefits realized from the
use of JTAG are shorter test times, higher test coverage, increased diagnostic
capability and lower capital equipment cost.

Figure 1: Main elements of a JTAG device


The principles of interconnect test using JTAG are illustrated in Figure 2.
Figure 2 depicts two JTAG compliant devices, U1 and U2 that are connected with
four nets. U1 includes four outputs that are driving the four inputs of U2 with
various values. In this case we will assume that that the circuit includes two
faults: A short between Nets 2 and 3, and an open on Net 4. We will also assume
that a short between two nets behaves as a wired-AND and an open is sensed as
logic 1. To detect and isolate the above defects, the tester is shifting into
the U1 boundary-scan register the patterns shown in Figure 2 and applying these
patterns to the inputs of U2. The inputs values of U2 boundary-scan register are
shifted out and compared to the expected results. In this case the results
(marked in red) on Nets 2, 3, and 4, do not match the expected values and
therefore the tester detects the faults on Nets 2, 3, and 4.

JTAG tool vendors provide various types of stimulus and sophisticated algorithms
to not only detect the failing nets but also isolate the faults to a specific
nets, devices, and pin numbers.

Figure 2: Principles of interconnect test using JTAG