state machine also exists: it will return to the reset state when TMS is held high for at least five rising edges of TCK.Instruction RegisterThe instruction register includes at least two shift-register-based cells that hold instruction data shifted in from the TDI input. The instruction is used to select the test to be performed or the data register to be "targeted" between TDI and TDO.Note:In IEEE Std 1149.1, signals that are asserted or active in the low state have an asterisk suffix.
17. . . . .Understanding DFT Concepts1.3JTAG Boundary-ScanTest Data RegistersEach instruction selects a unique test data register path to be enabled to shift data between TDI and TDO. There are the following types of test data registers.Boundary-Scan RegisterThe boundary-scan register (BSR) is a shift-register-based structure mandated by IEEE Std 1149.1. It is inserted between the on-chip system logic and package pins as shown in Figure 1–6 and provides a means of both controlling and observing the I/O pads of an IC. the boundary-scan register allows you to check for shorts and opens in the interconnect between ICs.Bypass RegisterThe bypass register is a mandatory 1-bit shift register. This register provides a minimum-length serial path through the IC (between TDI and TDO) when the IC is not required for the current test. The ability to "bypass" segments of the board-level serial test interconnect (boundary-scan path) allows considerable shortening of the overall path for the movement of test data.Device Identification RegisterThe device identification register is an optional, 32-bit-wide shift-register-based structure. This register allows the device identification code to be serially read from the IC that shows the manufacturer’s identify, the part number and the version number for the IC.User-Defined Test Data RegistersIEEE Std 1149.1 can be extended by adding optional user-defined test data registers. The user has a freedom to define new instructions to gain access to the test features within the IC such as a self-test function and internal scan chains.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .TAP ControllerThe TMS signal controls the sequence of transitions of the TAP controller in conjunction with the rising edge of TCK. The state diagram of the TAP controller is shown in Figure 1–8. Each arrow between states is labeled with a 1 or 0, indicating the logic value of TMS that must be set up before the rising edge of TCK to cause the transition.
18Understanding DFT Concepts1.3JTAG Boundary-Scan1Figure 1–8TAP Controller State DiagramThe following paragraphs briefly describe each of the controller states.■Test-Logic-ResetThis is the controller reset state. The controller must be placed in this state after power-up.■Run-Test/IdleIn this state, the IC is put in a test mode only when certain instructions are present. Otherwise, the instruction register and all test data registers retain their previous state.■Select-DR-Scan — Capture-DR — Shift-DR — Exit1-DR — Pause-DR — Exit2-DR — Update-DR