Variable length scan string and cell for same

Claim 2

A variable length, serial scan string having a plurality of serial scan cells coupled in series to define said serial scan string, at least one of the serial scan cells comprising: a scan-in node for receiving first serial scan data, the first serial scan data including a bypass control signal; a scan-out node for outputting second serial scan data; a flip flop having an input operatively coupled to receive the first serial scan data from the scan-in node and an output for outputting a time delayed version of the first serial scan data received at the flip flop input; a controllable scan-out selector having a first input coupled to the scan-in node, a second input coupled to the flip flop output, a control terminal, and an output coupled to the scan-out node for selectively outputting to the scan-out node a selected one of first and second signals respectively applied to the first and second scan-out selector inputs in response to a scan-out control signal supplied to the scan-out selector control terminal; and scan-out selector control means, operatively coupled to the scan-in node and to the controllable scan-out selector, for receiving the bypass control signal from the scan-in node and for outputting the received bypass control signal to the control terminal of the controllable scan-out selector as said scan-out control signal.

Claim 3

The variable length, serial scan string of claim 1 wherein the scan-out selector control means consists essentially of an unclocked, bypass control latch having an input connected to the flip flop output for receiving the bypass control signal from the flip flop output for storage in the bypass control latch, an output for outputting a stored bypass control signal as the scan-out control signal to the controllable scan-out selector, and said control terminal for receiving a load-and-hold instruction signal, said load-and-hold instruction signal causing the bypass control latch to load-and-store the bypass control signal output by the flip flop output.

Claim 4

A variable length, serial scan string according to claim 2 wherein the scan string has plural serial scan cells each with the structure recited in claim 2.

Claim 5

The variable length, serial scan string of claim 3 wherein the control terminals of the plural serial scan cells receive a common load-and-hold instruction signal.

Claim 6

The variable length, serial scan string of claim 3 wherein the flip flops of the plural serial scan cells are clocked flip flops all receiving a common clock signal.

Claim 7

The variable length, serial scan string of claim 6 wherein the scan-out selector control means includes a bypass control latch having an input connected to the flip flop output for receiving the bypass control signal from the flip flop output for storage in the bypass control latch, an output for outputting a stored bypass control signal as the scan-out control signal to the controllable scan-out selector, and a control terminal for receiving a load-and-hold instruction signal, said load-and-hold instruction signal causing the control latch to load-and-store the bypass control signal output by the flip flop output.

Claim 8

The variable length, serial scan string of claim 1 wherein the at least one serial scan cell further comprises: a data-in node for receiving data from a first circuit part outside the serial scan string; a data-out node for outputting data to a second circuit part outside the serial scan string, said data-out node being operatively coupled to the flip flop output; a controllable scan-in selector having a first input coupled to the scan-in node, a second input coupled to the data-in node, a control input, and an output coupled to the flip flop input, for selectively outputting to the flip flop input a selected one of first and second signals respectively applied to the first and second scan-in selector inputs in response to a selector shifting signal supplied to said control input.

Claim 9

The variable length, serial scan string of claim 7 wherein the scan string has plural serial scan cells each with the structure recited in claim 7 and wherein the control inputs of the plural serial scan cells receive a common selector shifting signal.

Claim 10

The variable length, serial scan string of claim 7 wherein the scan string has plural serial scan cells each with the structure recited in claim 7 and wherein: the data-in nodes of at least two such cells are respectively coupled to corresponding first circuit parts; the bypass control signal designates one of more of the corresponding first circuit parts as circuit part that are not to be tested; and the bypass control signal further directs the scan-out selector control means in each cell whose first circuit part is designated as one not to be tested to output a scan-out control signal selecting the first signal applied to the first scan-out selector input as the signal to be output by the output of the controllable scan-out selector.

Claim 11

A variable length, serial scan string having a plurality of serial scan cells coupled in series to define said serial scan string, each of the serial scan cells comprising: a scan-in node for receiving first serial scan data, the first serial scan data including a bypass control signal; a scan-out node for outputting second serial scan data; a clock-input node for receiving clock pulses; serial shift means, responsive to said clock pulses, the serial shift means having an input operatively coupled to receive the first serial scan data from the scan-in node and an output for outputting a time delayed version of the first serial scan data received at the flip flop input in synchronism with said clock pulses; a controllable scan-out selector having a first input coupled to the scan-in node, a second input coupled to the output of the serial shift means, a control terminal, and an output coupled to the scan-out node for selectively outputting to the scan-out node a selected one of first and second signals respectively applied to the first and second scan-out selector inputs in response to a scan-out control signal supplied to the scan-out selector control terminal; and scan-out selector control means, operatively coupled to the scan-in node and to the controllable scan-out selector, for receiving the bypass control signal from the scan-in node and for outputting the received bypass control signal to the control terminal of the controllable scan-out selector as said scan-out control signal.

Claim 12

A variable length, serial scan string having a plurality of serial scan cells coupled in series to define said serial scan string, each of the serial scan cells comprising: a scan-in node for receiving first serial scan data, the first serial scan data including a bypass control signal; a scan-out node for outputting second serial scan data; a clock-input node for receiving clock pulses; serial shift means, responsive to said clock pulses, the serial shift means having an input operatively coupled to receive the first serial scan data from the scan-in node and an output for outputting a time delayed version of the first serial scan data received at the flip flop input in synchronism with said clock pulses; a controllable scan-out selector having a first input coupled to the scan-in node, a second input coupled to the output of the serial shift means, a control terminal, and an output coupled to the scan-out node for selectively outputting to the scan-out node a selected one of first and second signals respectively applied to the first and second scan-out selector inputs in response to a scan-out control signal supplied to the scan-out selector control terminal; and scan-out selector control means, operatively coupled to the scan-in node and to the controllable scan-out selector, for receiving the bypass control signal from the scan-in node and for outputting the received bypass control signal to the control terminal of the controllable scan-out selector as said scan-out control signal.

Claim 15

The method of claim 11 wherein: said step of providing includes providing each of at least two of the serial scan cells with a corresponding programmable bypass means; said step of shifting includes simultaneously shifting at least two predefined bypass control signals through the serial scan string each to a corresponding one of the at least two serial scan ells having programmable bypass means said step of programming includes simultaneously programming the at least two programmable bypass means with the predefined bypass control signals shifted thereto.

Claim 16

A serial scan string having variable shift length, the serial scan string comprising a plurality of serial scan cells coupled in series, each of the cells including a serial shift means for shifting data serially through the cell, at least one of the serial scan cells including: programmable bypass means for electively bypassing the serial shifting operation of the corresponding shift means of the at least one serial scan cell in response to a bypass control signal programmed into the programmable bypass means; serial transfer means for serially shifting a predefined bypass control signal through the serial scan string to the programmable bypass means; and programming means for programming the programmable bypass means in accordance with the predefined bypass control signal shifted thereto.

Claim 17

A serial scan string having variable shift length, the serial scan string comprising a plurality of serial scan cells coupled in series, each of the cells including a serial shift means for shifting data serially through the cell, at least one of the serial scan cells including: programmable bypass means for electively bypassing the serial shifting operation of the corresponding shift means of the at least one serial scan cell in response to a bypass control signal programmed into the programmable bypass means; serial transfer means for serially shifting a predefined bypass control signal through the serial scan string to the programmable bypass means; and programming means for programming the programmable bypass means in accordance with the predefined bypass control signal shifted thereto.