What’s going on inside that Joint Test Action Group (JTAG) debugger probe you’re using: https://t.co/vvOMKHn8wU
highlights how boundary scan works. This harkens back almost all the way to those mainframe days. The Test Access Port (TAP) controls the scan chain within a device. The scan chain is essentially a giant shift register. Information is shifted in via the Test Data In (TDI) pin and out via the Test Data Out (TDO). This is synchronized with the Test Clock (TCLK), and the Test Mode Select (TMS) helps control the operation of the system. Commands and data are shifted in a bit at a time. This information can be loaded into the boundary control and instruction registers. The bypass register is a way to short-circuit the boundary scan register, as scan chains can be large and shifting through all the scan registers in a system can slow things down. When enabled, the bypass register adds a single bit to the chain.