JTAG Chip DFT:   

Ground bounce is a dip in the DC supply voltage caused by a sudden power surge and, if it occurs it can disrupt the operation of the TAP controller. The 1149.1 Standard states that there should be no ground bounce inside a compliant device under worst-case switching conditions in the boundary-scan (JTAG) cells – for example, if all output scan cells were updated from, say, all-1s to all-0s. It’s interesting to ask whether an 1149.1 synthesis tool actually checks this requirement!!

Allowing access to other internal chip DFT features, such as internal scan (known as Scan Thru TAP) and internal memory BIST or logic BIST (using RUNBIST-like instructions) allows re-use of these features once the chip is assembled to the board. Such re-use supports improved board-level diagnostics.

Using the optional RUNBIST or INTEST public instructions places the device in test mode. The outputs of the device are under the control of the JTAG or boundary scan register cells and can either be preloaded with safe values (using the PRELOAD instruction) or placed in a disabled non-drive (high impedance) state.

Using private instructions such as “INSCAN” to access internal scan paths (sometimes called “Scan Thru TAP”), the values driven out of the device output pins should also be under the control of the boundary scan/JTAG register cells and can either be preloaded with safe values (using the PRELOAD instruction) or placed in a disabled non-drive (high impedance) state.

In general, it is desirable that JTAG/boundary-scan (and non-boundary-scan) devices power up in safe states. That is, three-state outputs power up with their outputs in the high-Z state, and bidirectional outputs power up in the input mode rather than their output drive mode. In this way, the risk of powering up into a state of contention (bus fight) is reduced. Safe-state power up can be achieved by internal power-on circuitry (preferred) or by controlling Output_Enable control cells via their JTAG/boundary-scan cells and using the PRELOAD instruction during board initialisation (not so good as the board may power up in an unsafe state to start with).

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