Editing JTAG (section)

{{Short description|Serial interface for testing integrated circuits}}
{{For|the Xbox 360 hardware modification|Homebrew (video games)#Xbox 360}}
{{Use dmy dates|date=January 2021}}

'''JTAG''' (named after the '''Joint Test Action Group''' which codified it) is an [[Technical standard|industry standard]] for verifying designs of and testing [[printed circuit board]]s after manufacture.

JTAG implements standards for on-chip instrumentation in [[electronic design automation]] (EDA) as a complementary tool to [[logic simulation|digital simulation]].<ref name=stollon>{{cite book|author=Neal Stollon|title=On-Chip Instrumentation|publisher=Springer|year=2011}}</ref> It specifies the use of a dedicated [[debug port]] implementing a [[serial communication]]s interface for low-overhead access without requiring direct external access to the system address and data buses. The interface connects to an on-chip '''Test Access Port''' (TAP) that implements a [[stateful protocol]] to access a set of test registers that present chip logic levels and device capabilities of various parts.

The Joint Test Action Group formed in 1985 to develop a method of verifying designs and testing [[printed circuit board]]s after manufacture. In 1990 the [[Institute of Electrical and Electronics Engineers]] codified the results of the effort in IEEE Standard 1149.1-1990, entitled ''Standard Test Access Port and Boundary-Scan Architecture''.

The JTAG standards have been extended by multiple semiconductor chip manufacturers with specialized variants to provide vendor-specific features.<ref>[https://web.archive.org/web/20170830070123/http://www.intel.com/content/dam/www/public/us/en/documents/white-papers/jtag-101-ieee-1149x-paper.pdf Randy Johnson, Steward Christie (Intel Corporation, 2009), ''JTAG 101—IEEE 1149.x and Software Debug'']</ref>