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11.2.1. Wrapper

There are seven input signals that control the logic of the core to support the Wrapper Boundary Register (WBR) and the IEEE 1500 standard:

  • WEXTEST

  • WINTEST

  • WSE

  • CAPTUREWR

  • TESTMODE

  • SERIALTEST

  • SHIFTWR.

This logic:

  • separates the shift and capture for IEEE 1500 compliance so that the shared wrapper cell can hold state when neither shifting nor capturing

  • requires only one external wrapper scan enable and prevents unknown states in wrapper cells with multiple capture cycles, which is preferable for delay testing and for testing through the memories.

Figure 11.26 shows the RTL logic for a set of input WBR cells.

Figure 11.26. Input wrapper boundary register cell control logic

Figure 11.26. Input wrapper boundary register cell
control logic

Figure 11.27 shows the RTL logic for a set of output WBR cells.

Figure 11.27. Output wrapper boundary register cell control logic

Figure 11.27. Output wrapper boundary register
cell control logic

The hold control logic in Figure 11.26 and Figure 11.27 has capture and shift signals that enable the WBR cell to hold data during test mode while both these signals are deasserted. The only difference between the input wrapper and output wrapper cells is that the WINTEST and WEXTEST connections switch polarity. The type of IEEE 1500 compliant-wrapper cell used with this logic is shown in Figure 11.28.

This utilization provides the benefit of requiring only one external wrapper scan enable and preventing unknown states from being output from the WBR cells during patterns with multiple capture cycles. If you use a standard multiplexed-scan flip-flop in the WBR in place of the WBR cell as shown in Figure 11.28, you can use the shift_outputs and shift_inputs signals for the scan enable to the output and input WBR cells, respectively.

Figure 11.28 shows the type of WBR cell required to meet IEEE 1500 compliance.

Figure 11.28. IEEE 1500-compliant input wrapper boundary register cell

Figure 11.28. IEEE 1500-compliant input wrapper
boundary register cell

Note

The IEEE 1500-compliant output wrapper boundary register cell uses the shift_outputs and capture_outputs_n signals.