A testable CMOS asynchronous counter

A testable design for an asynchronous n-bit CMOS counter is presented, with test inputs that provide full coverage for stuck-at and stuck-open faults. The test time is O(n) where the counter outputs are not observable, compared to O(n ²) for a synchronous counter. Three control signals are required for the testable counter as opposed to one reset signal for the base counter. The testable counter incorporates a scan path, utilizing the state storage in the counter cells, whereby the counter is converted into an n-bit master-slave asynchronous shift register with the counter´s request input being used as the shift-register input. The only observable outputs are acknowledge and carry-out signals. The counter utilizes two-cycle (transition) signaling and guarantees that new output values are available before acknowledge is toggled. Two 16-b counters, one base design and one scan-based design, were fabricated on the same chip (2.0-μm n-well CMOS) through MOSIS. Four parts were received, all of which passed the test suites developed