JTAG Chip DFT:  

The figure shows a JTAG or boundary-scan device connected to a non-boundary-scan device. The existence of the short is not detectable by the boundary-scan (JTAG) structure if the output boundary-scan cells are drive-only cells e.g. BC_1s. The pattern can be set up and applied but there is no boundary-scan (JTAG) visibility of the response values. Consequently, there will be a loss of defect coverage unless the driver scan cell can also become a sensor. This can be achieved by replacing the drive-only JTAG cells with drive-and-sense boundary-scan cells, such as a BC_7: next slide

The BC_7 scan cell is especially designed for bidirectional signal pins and when in Drive mode, it is able to Sense the value of the driven signal i.e. the BC_7 is a self-sensing JTAG (boundary-scan) cell. Using BC_7s instead of BC_1s will allow detection of shorts between boundary-scan and non-boundary-scan devices, thereby increasing the defect coverage on the board.

An interesting way of illustrating the phenomenon of ground bounce.

Reproduced with permission of the IEEE Computer Society.

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