What is JTAG?

Answer1:

JTAG is acronym for "Joint Test Action Group".This is also called as IEEE 1149.1 standard for Standard Test Access Port and Boundary-Scan Architecture. This is used as one of the DFT techniques.

Answer2:

JTAG (Joint Test Action Group) boundary scan is a method of testing ICs and their interconnections. This used a shift register built into the chip so that inputs could be shifted in and the resulting outputs could be shifted out. JTAG requires four I/O pins called clock, input data, output data, and state machine mode control.

The uses of JTAG expanded to debugging software for embedded microcontrollers. This elimjinates the need for in-circuit emulators which is more costly. Also JTAG is used in downloading configuration bitstreams to FPGAs.

JTAG cells are also known as boundary scan cells, are small circuits placed just inside the I/O cells. The purpose is to enable data to/from the I/O through the boundary scan chain. The interface to these scan chains are called the TAP (Test Access Port), and the operation of the chains and the TAP are controlled by a JTAG controller inside the chip that implements JTAG.

For more info:

http://www.xess.com/faq/M0000297.HTM
http://www.cadreng.com/open_source/jtag/jtag_tutorial.php
http://www.ee.ic.ac.uk/pcheung/teaching/ee3_DSD/ti_jtag_seminar.pdf