Boundary-Scan Tutorial:

Target Register Modes

Whenever a register is selected to become active between TDI and TDO, it is always possible to perform three operations on the register: parallel Capture followed by serial Shift followed by parallel Update. The order of these operations is fixed by the state-sequencing design of the TAP controller. For some target Data registers, some of these operations will be effectively null operations, no ops.

Open Circuit TDI, TMS and TRST*?

The 1149.1 Standard mandates that an open circuit TDI, TMS or TRST* input must go to logic 1. This can be achieved with internal weak resistive pull ups, or with active transistor pull ups. The reasons are as follows:

For TDI. If the Instruction register is selected as the target register between TDI and TDO ready to be loaded with a new instruction, then a safe instruction (Bypass, all-1s code) is loaded and executed into the device with the open-circuit TDI and to all devices downstream of this device.

For TMS. In a maximum of 5 x TCK cycles, the TAP controller of this device will be placed into its Test_Logic Reset state. In this state, the boundary-scan logic is inactive but the device can continue to operate functionally.

For TRST*, logic 1 is the inactive state and so the device is not prevented from being used either in functional mode or in 1149.1 modes. The device must be reset with the synchronous reset cycle (TMS = 1, 5 x TCK) rather than through its asynchronous TRST* signal.

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