Testing
JTAG (Joint Test Action Group)
An industry standard for testing and debugging integrated circuits through a serial interface.
Detailed Explanation
JTAG provides boundary scan capability—controlling and observing chip I/Os through a serial chain. This enables board-level testing (checking solder connections) and in-system programming. Debug extensions allow processor halt, single-step, and memory examination.
The JTAG interface uses four signals: TCK (clock), TMS (mode select), TDI (data in), TDO (data out). A state machine controls operations. Multiple devices can chain together (daisy-chain).