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What is a Boundary Register?

The Boundary Register is the most important part of the boundary scan, it is a shift register based structure which has one or more boundary scan cell connected to a device pin (See figure 1). There are various boundary scan register cell designs available for used according to the type of system pin concerned (input, output, 3-state, bidirectional) and according to the set of boundary scan instructions supported. Figure 2 is an example of a typical boundary scan register cell found on a boundary register. The boundary scan cell design will be able to use as either input cell or output cell. The “Parallel In” and Parallel Out” on Figure 2 can be connected to a device input or output pin while the “Shift In” and “Shift Out” are the serial inputs and outputs of the boundary register which enable the shifting of data within the device.


Figure 1 - Boundary Scan device with a Boundary Register cell.



Figure 2 –A typical Boundary Scan Register Cell