JTAG - Standard Test Access Port and Boundary - Scan Architecture - By: Allan Michael Taylor

When you have an integrated circuit (IC) or many such devices on a board, where each IC has 100 or more pins, it is mandatory to have some tool to test connections and verify operation. Having this in mind, an expert group prepared a specification for testing that was standardized in 1990 as the IEEE Std. 1149.1. It is also known as the JTAG (Joint Test Action Group) standard.

JTAG is a method for testing connections on printed circuit boards (PCBs) that are implemented at the integrated circuit (IC) level. It is very difficult to test complex circuits with traditional in-circuit testers. Because of physical space constraints and inability to access very small components and BGA devices, the cost for board testing has increased significantly. JTAG is an elegant solution to overcome problems with physical in-circuit testers.

With JTAG you can test interconnects between integrated circuits on a board without using physical test probes. This is a big advantage because you don't need any additional customized tool for testing. Of course, the device has to be JTAG enabled. This means an additional cell (a boundary-scan cell) for each pin. Boundary-scan cell can set or read data on each pin. Boundary-scan cells are connected together and the data is serially shifted into the boundary-scan cells. The process is controlled from a serial data path called the scan path or scan chain. This is the basic principle of the JTAG interface.

JTAG eliminates the need for a large number of test vectors, which would be needed to initialize all the devices. Using JTAG means shorter test times, increased diagnostic capability higher test coverage, and lower equipment cost. Although there are many variations of the JTAG header on the board it is possible to use standard JTAG signals with almost any JTAG interface and boundary-scan software.

An additional benefit with JTAG interface is that it can be also used for programming and debugging. Many microcontrollers, FLASH memories, FPGAs and similar devices can be programmed via JTAG interface. And the same interface can be used for debugging. JTAG is a big step toward standard interfaces in electronics industry.

There are many JTAG cables that can be used on more than one device. In fact, JTAG cable is more than a cable. Usually it has some small electronics to boost signals and to provide standard computer interface. The price of the simplest JTAG cable can be as low as $5.

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If you are interested in details you can visit http://jtagcables.com/ website and read more about the JTAG standard and various JTAG cables used by the industry.

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