The present invention provides a technique for operating an integrated
circuit comprising a plurality of circuit elements, with a plurality of
serial test scan chains, each being coupled to a different one of the
circuit elements. A scan chain selector is responsive to a specified scan
chain specifying value to select a corresponding one of the plurality of
test scan chains. A scan chain controller is also provided which has a
serial interface for receiving signals from outside of the integrated
circuit, the scan chain controller comprising an instruction decoder for
decoding scan chain controller instructions received from the serial
interface. In accordance with the present invention, the decoder is
responsive to a first scan chain controller instruction to specify a
pre-determined scan chain specifying value and a second scan chain
controller instruction for decoding by the decoder. The provision of such
a first scan chain controller instruction enables the efficiency of the
testing procedure to be improved.
