1 What Is JTAG?

JTAG is named after the group that formed the industry standard for boundary scan testing of printed circuit boards (PCBs). The results of the JTAG effort were later codified in the Institute of Electrical and Electronics Engineers (IEEE) standard: IEEE Std 1149.1. Soon after introduction, the standard became very widely used, which resulted in several additional standards, including the implementation of the on-chip Test Access Port (TAP). This made JTAG the most adapted means for embedded systems development, debug, and testing. JTAG uses a 5-pin implementation in most systems:

• Test Data In (TDI)
• Test Data Out (TDO)
• Test Clock (TCK)
• Test Mode Select (TMS)
• Test Reset (TRSTn)

The necessary biasing of these pins on custom board designs is found in the device-specific data sheet or from reference designs from TI.