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Interface JTAG

Interface JTAG JTAG is short for "Joint Test Action Group", an organisation that started handling the problem of test access to boards assembled by means of surface-mounting technology in 1980s. This working group suggested a method for direct access to active components' outputs by using a serial interface known as TAP (Test Access Port). A testing port was essentially a combination of four or five dedicated outputs of a microchip: ТСК, TMS, TDI, TDO, TRST. TRST output is not compulsory, though is present in many microcontrollers. Embedding of the BST architecture into modern chips ensures access to chip's outputs (to be precise, to special input/output units) via a 4-conductor wired serial JTAG interface. This architecture enables not only supervision of their statuses, but also control over them. Thus, bulky physical contact testers can be avoided. In 1990 this method was stipulated in IEEE Std 1149.1 International Standard (Standard Test Access Port and Boundary-Scan Architecture). Today, thousands of components feature the mentioned port as an extra option to test the boards on which they are installed. Therefore, boards containing modern microcontrollers are usually equipped with a JTAG interface. JTAG interfaces are normally connected to an LPT computer port, but versions are available that can work via a COM port as well, provided appropriate software is available. This is related to the fact that these input/output ports enable easier programming and status control. Due to widespread development of USB interface, USB-JTAG adapters received huge popularity as well. JTAG's versatility explains its universal use for debugging and in-circuit programming, as well as in boundary scanning.