Abstract: A boundary scan cell including a three-state output buffer, a test data scan flip-flop for providing an input to the three-state buffer, a control data scan flip-flop for receiving a serial control data input, independent clock signals for independently clocking the test data scan flip-flop and the control data scan flip-flop, and control circuitry for controllably providing the output of the control data scan flip-flop to the three-state output driver such that the enabled state of the three-state output buffer is controlled by the output of the control data scan flip-flop, whereby the enabled state of the three-state output driver is controlled independently of the test data in the test data scan flip-flop.

Abstract: Boundary scan cells including mask circuitry having a mask latch for storing a mask flag that is serially scanned into the cell via a scan flip-flop. In a boundary scan cell having an output function, control circuitry responsive to the mask flag forces or holds the output of the cell at a state determined by one or more values scanned into the cell via the scan flip-flop if the mask flag is of a predetermined state that indicates the cell is masked. In a boundary scan cell having an input function, control circuitry responsive to the mask flag connects the output of the scan flip-flop to the input of the scan flip-flop if the mask flag is of a predetermined state that indicates the cell is masked.