Diagnostic method and apparatus for non-destructively observing latch data

A circuit for observing and restoring data of a plurality of shift registers without altering logical values of said data, said circuit comprising:

• a plurality of shift registers, wherein each of said shift registers comprises an input and an output;
• a first selector connected to said output of each of said shift registers;
• a second selector connected to said first selector and to said input of each of said shift registers;
• a control device connected to said first selector and said second selector,
• wherein said control device is adapted to control said first selector and said second selector in order to selectively connect a selected output of a selected shift register to a selected input of said selected shift register so as to form a wiring loop between said selected input and said selected output, and
• wherein said control device is further adapted to use said wiring loop to cause data to be transferred from said selected output to said selected input and to be circulated back through said selected shift register at least a single time such that said data returns to a same position within said selected shift register;
• and
• an observation wire connected to said wiring loop and said control device, wherein said observation wire is adapted to pass said data from said wiring loop to said control device as said data is also transferred from said selected output to said selected input,
• wherein said control device further comprises an input
• output port accessible from outside said circuit, and
• wherein said control device is further adapted to output, through said input/output port, said data such that said data is observable outside said circuit without altering logical values of said data within said selected shift register.