Joint Test Action Group
IEEE Standard 1149.1
(JTAG, or "IEEE Standard 1149.1") A standard specifying how to control and monitor the pins of compliant devices on a printed circuit board.
Each device has four JTAG control lines. There is a common reset (TRST) and clock (TCLK). The data line daisy chains one device's test data out (TDO) pin to the test data in (TDI) pin on the next device.
The protocol contains commands to read and set the values of the pins (and, optionally internal registers) of devices. This is called "boundary scanning". The protocol makes board testing easier as signals that are not visible at the board connector may be read and set.
The protocol also allows the testing of equipment, connected to the JTAG port, to identify components on the board (by reading the device identification register) and to control and monitor the device's outputs.
JTAG is not used during normal operation of a board.
JTAG Technologies B.V..
Boundary Scan/JTAG Technical Information - Xilinx, Inc..
Java API for Boundary Scan FAQs - Xilinx Inc..
JTAG Boundary-Scan Test Products - Corelis, Inc..
"Logic analyzers stamping out bugs at the cutting edge", EDN Access, 1997-04-10.
IEEE 1149.1 Device Architecture - Boundary-Scan Tutorial from ASSET InterTech, Inc..
"Application-Specific Integrated Circuits", Michael John Sebatian Smith, published Addison-Wesley - Design Automation Cafe.
Software Debug options on ASIC cores - Embedded Systems Programming Archive.
Designing for On-Board Programming Using the IEEE 1149.1 (JTAG) Access Port - Intel.
Built-In Self-Test Using Boundary Scan by Texas Instruments - EDTN Network.