Boundary-Scan Tutorial:

Using the Scan Path

At the device level, the boundary-scan elements contribute nothing to the functionality of the internal logic. In fact, the boundary-scan path is independent of the function of the device. The value of the scan path is at the board level as shown in Figure 11.

The figure shows a board containing four boundary-scan devices. Notice that there is an edge-connector input called TDI connected to the TDI of the first device. TDO from the first device is permanently connected to TDI of the second device, and so on, creating a global scan path terminating at the edge connector output called TDO. TCK is connected in parallel to each device TCK input. TMS is connected in parallel to each device TMS input.

What the tester sees from the edge connector is simply the concatenation of the various boundary-scan registers – that is, a single register that provides access to all device outputs now considered to be drivers (sometimes called a transmitter ) onto an interconnect and all device inputs now considered to be sensors (sometimes called a receiver ) from the interconnect – see Figure 12.

In this way, particular tests can be applied across the device interconnects via the global scan path by loading the stimulus values into the appropriate device-output scan cells via the edge connector TDI ( shift-in operation), applying the stimulus across the interconnects ( update operation), capturing the responses at device-input scan cells ( capture operation), and shifting the response values out to the edge connector TDO ( shift-out operation).

Essentially, boundary-scan cells can be thought of as virtual nails , having an ability to set up and apply tests across the interconnect structures on the board.

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