ACME: An Energy-Efficient Approximate Bus Encoding for I²C

In ultra low power systems with many peripherals, off-chip serial interconnects contribute significantly to the total energy budget. Leveraging the error-resilience characteristics of many embedded applications, the approximate computing paradigm has been applied to serial bus encodings to reduce interconnect consumption. However, the power model considered in previous works was purely capacitive. Accordingly, the objective of these approximate encodings was simply to reduce the transition count. While this works well for most bus standards, one notable exception is represented by (IC)-C-2, whose open-drain physical connection makes the static energy consumed by logic-0 values on the bus extremely relevant. In this work, we propose ACME, the first approximate serial bus encoding targeting specifically (IC)-C-2 connections. With a simple encoding/decoding scheme, ACME concurrently reduces both the static and dynamic energy on the bus by maximizing the number of logic-1 values in codewords, while simultaneously reducing transitions. Using an accurate bus model and realistic capacitance and resistance values selected according to the (IC)-C-2 standard, we show that our encoding outperforms state-of-the-art solutions and reduces the total energy consumption on the bus by 57% on average, with an error smaller than 0.1 %.