Self-testing multi-processor die with internal compare points

A method for testing a microprocessor die comprising the steps of:

• connecting an external tester to external pins on the die;
• asserting with the external tester an external test pin on the die to initiate a self-test mode by the die;
• simultaneously resetting a plurality of substantially-identical processor cores on the die in response to the external test pin;
• (a) pulsing a processor clock to the processor cores, the processor cores each generating an output which is not applied to external pins of the die;
• (b) comparing the output generated from each processor core and signaling an error when at least one of the processor cores does not match the output from the other processor cores;
• (c) setting an error bit for the processor core having the output that does not match the outputs from the other processor cores;
• fetching additional instructions and repeating steps (a) to (c) until a test sequence is complete;
• applying the error bits for each processor core to external pins on the die;
• reading with the external tester the error bits for each processor core and marking the die as functional or non-functional in response to the error bits read;
• whereby the die is self-tested by internally comparing outputs from multiple processor cores.

A multi-processor die comprising:

• a plurality of central processing unit (CPU) cores, each core having a pipeline for processing instructions and a scan test chain serially connecting storage nodes within the core;
• a shared cache for supplying instructions and operands to the plurality of cores;
• a self-test unit, receiving serial outputs from the scan test chains in each core, the self-test unit including:
• compare means for comparing a serial output from a first core to the serial outputs from other cores;
• voting means, coupled to the compare means, for determining a correct serial output, the correct serial output having a value matching a value of a serial output on another core;
• error means, coupled to the voting means, for signaling an error when a serial output does not match the correct serial output;
• wherein an external tester does not compare the serial output to an expected value but the voting means determined the correct serial output as a matching value, and
• wherein the shared cache occupies a larger area of the die than the area occupied by a processor core, each processor core further comprising a local cache, coupled to the shared cache, for storing instructions and operands for use by a local processor core.