What are the Data Register in a Boundary Scan?
A boundary scan logic architecture must contain a minimum of two data registers – The Boundary Register and the Bypass Register.
The most important is the Boundary Register, which has a boundary-scan cell adjacent to each input and output pin. This register is used to control and observe activities on the device's input and output pins. The Boundary register is a mandatory feature of IEEE 1149.1.
Another mandatory register is the Bypass register. This register consists of only one boundary-scan cell. The Bypass Register shortens the scan chain to a single cell when testing other boundary-scan components on the board.
One other data register, the Identification Register, described by IEEE Standard 1149.1 is optional. This register contains device identification information. The register services two functions: the IDCODE and USERCODE functions.
The standard also allows device manufacturers to implement other registers. These registers are typically used to perform manufacturer-specified instructions for built-in self tests and similar functions.