JTAG
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JTAG, an initialism for Joint Test Action Group, is the usual name used for the IEEE 1149.1 standard for Test Access Port and Boundary Scan, primarily used for testing integrated circuits, but also useful as a mechanism for debugging embedded systems.
A JTAG interface is a special four/five-pin (TDI (Test Data In), TDO (Test Data Out), TCK (Test Clock), TMS (Test Mode Select), optional TRST (Test Reset)) interface added to a chip, designed so that multiple chips on a board can have their JTAG lines daisy-chained together, and a test probe need only connect to a single "JTAG port" to have access to all chips on a circuit board.
Since only the one data line is available, the protocol is necessarily serial. The clock input is at the TCK pin. Configuration is performed by manipulating a state machine one bit at a time through a TMS pin. One bit of data is transferred in and out per TCK clock pulse at the TDI and TDO pins, respectively. Different instruction modes can be loaded to read the chip ID, sample input pins, drive (or float) output pins, manipulate chip functions, or bypass (pipe TDI to TDO to logically shorten chains of multiple chips). The operating frequency of TCK varies depending on the chip, but it is typically 10-100MHz (10-100ns per bit).de:Joint Test Action Group