DS-LFSR: a BIST TPG for low switching activity

A test pattern generator (TPG) for built-in self-test (BIST), which can reduce switching activity during test application, is proposed. The proposed TPG, called dual-speed LFSR (DS-LFSR), consists of two linear feedback shift registers (LFSRs), a slow LFSR and a normal-speed LFSR. The slow LFSR is driven by a slow clock whose speed is 1/dth that of the normal clock, which drives the normal-speed LFSR. The use of DS-LFSR reduces the frequency of transitions at the circuit inputs driven by the slow LFSR, leading to a reduction in switching activity during test application. A procedure is presented to design a DS-LFSR so as to achieve high fault coverage by ensuring that patterns generated by it are unique and uniformly distributed. A new gain function and a method to compute its value for each circuit input are proposed to select inputs to be driven by the slow LFSR. Also, a procedure to increase the number of inputs driven by the slow LFSR by combining compatible inputs is presented to further decrease the switching activity. Finally, DS-LFSRs are designed for the ISCAS85 and ISCAS89 benchmark circuits and shown to provide a 13% to 70% reduction in the numbers of load-capacitance weighted transitions with no loss of fault coverage (for stuck-at as well as transition delay faults) and at very slight area overheads