RAM-like test structure superimposed over rows of macrocells with added differential pass transistors in a CPU

摘要

A test structure is added to a microprocessor. The test structure is a RAM-like array of scan-clock word lines which selects a row of macrocells to be read or written. Perpendicular to the scan-clock word lines and the rows of macrocells are scan-data bit lines. Each testable macrocell has true and complement signal nodes that are connected to a pair of scan- data bit lines through a pair of n-channel pass transistors. The gates of the pass transistors are controlled by the scan-clock word line. The true and complement signal nodes are the cross- coupled inverters or gates in a latch. The latch is written or loaded by driving opposite data values onto the pair of scan-data bit lines when the pass transistors are activated by the scan-clock word line. The macrocells have random widths and thus do not form regular columns, so the columns of scan-data bit lines must be expanded to accommodate the various macrocell widths. Non- storage macrocells such as logic gates and buffers can be read but not written using the pass transistors connected to true and complement nodes in the macrocell. Reading causes a small voltage difference to be generated on the scan-data bit lines which is sensed by a sense amplifier. Only two n-channel transistors are added to a macrocell to make the macrocell testable. Thus testing is added with minimal area, cost, and delay to the macrocell.