Patent ID: 7747917

Claim:
A scan cell comprising: a scan cell data input and a scan cell shift input; input selection circuitry for selecting between the scan cell data and scan cell shift inputs during normal, capture, and shift modes in response to only a first control signal and a second control signal, the first and second control signals each switching between first and second logic levels, comprising: a first storage element for storing a bit representing a current one of the first and second logic levels of the first control signal in response to an edge of the second control signal; and multiplexing circuitry including a multiplexer having data inputs coupled to the scan cell data and scan cell shift inputs and a control input coupled to a gate, the gate having inputs receiving the bit stored in the first storage element and the second control signal, wherein the multiplexing circuitry is operable to: in the normal mode, select the scan cell data input in response to the first logic level of the second control signal; in the capture mode, select the scan cell data input when the bit stored in the first storage element represents the second logic level of the first control signal at a first edge of the second control signal, the first edge of the second control signal delayed from a transition of the first control signal to the second logic level; and in the shift mode, select the scan cell shift input when the bit stored in the first storage element represents the first logic level of the first control signal at a second edge of the second control signal, the second edge of the second control signal delayed from a transition of the first control signal to the first logic level; a second storage element for storing, in response to the second logic level of the first control signal, data presented at the selected one of the scan cell data and scan cell shift inputs; and a third storage element for storing, in response to the second logic level of the first control signal and the second logic level of the second control signal, data presented at an output of the second storage element.