Multi-mode trace signal selection for post-silicon debug

Trace buffers are used during post-silicon debug to increase the visibility to the internal signals of a chip via online tracing of a few state elements within a capture window. Due to the small bandwidth of the trace buffer, only a few state elements can be selected or tracing in order to restore the states of the remaining state elements as many as possible. In this work, we show that the quality of restoration corresponding to a set of trace signals selected for a single operating mode, may significantly degrade over the remaining operating modes of a design; an operating mode refers to specific values taken by control signals such as signals for mode selection and scan enable. This is the first work to study the multi-mode trace signal selection problem in order to maximize the restoration over all the operating modes of a design. We propose algorithmic strategies for this problem as well as a procedure to reduce the number of modes by merging the ones with “similar” restoration maps; merging improves the runtime scalability of our multi-mode trace selection algorithm with increase in the number of modes, without much loss in the solution quality.