Determining a length of the instruction register of an unidentified device on a scan chain

A program storage medium, comprising:

• a processor-readable device configured with instructions for determining a total length of at least one instruction register of one or more unidentified devices in a scan chain of a plurality of devices having respective instruction registers, wherein execution of the instructions by one or more processors causes the one or more processors to perform operations including,
• determining from a first data shift of the scan chain whether each of the plurality of devices is an identified device that has a respective identification register with a recognized value in a database;
• determining an overall length of the instruction registers of the plurality of devices in the scan chain from a first instruction shift of the scan chain;
• for each pair of sub-sequences of unidentified ones of the plurality of devices in the scan chain, wherein a first and second sub-sequence of the pair are separated in the scan chain by a third sub-sequence of at least one identified device, determining an actual position within the overall length for the respective instruction register of an identified device of the third sub-sequence in response to, for each of at least one trial position within the overall length, a second data shift of the scan chain obtaining the recognized value of the respective identification register of the identified device following a second instruction shift of the scan chain that attempts to set the respective instruction register of the identified device at the trial position;
• and
• determining the total length of the instruction register of every unidentified device in each of the sub-sequences of the unidentified devices as a function of the overall length of the instruction registers of the plurality of devices, the actual position of the identified device for each pair of the sub-sequences of the unidentified devices, and a length from the database of the respective instruction register of each identified device.

A system for determining a total length of at least one instruction register, comprising:

• a plurality of devices each having a respective instruction register, wherein the plurality of devices are coupled to form a scan chain;
• and
• a scan controller coupled to the scan chain, the scan controller adapted to determine from a first data shift of the scan chain whether each of the plurality of devices is an identified device that has a respective identification register with a recognized value, the scan controller further adapted, for each of a plurality of sub-sequences of unidentified ones of the plurality of devices, to determine a total length of the instruction register of every unidentified device in the sub-sequence, including determining an overall length of the instruction registers of the plurality of devices from a first instruction shift of the scan chain, and determining, for each pair of a first and second sub-sequence of unidentified devices that are separated in the scan chain by a third sub-sequence of at least one identified device, an actual position within the overall length for the respective instruction register of an identified device of the third sub-sequence, the scan controller further adapted to determine the actual position within the overall length for the respective instruction register of the identified device using each of at least one trial position, wherein the trial position is determined to be the actual position in response to a second data shift of the scan chain obtaining the recognized value of the respective identification register of the identified device following a second instruction shift of the scan chain that attempts to set the respective instruction registers of the plurality of devices to select the respective identification register for the identified device and to select a respective bypass register for devices in the plurality of devices other than the identified device.