An ECG signal processor (ESP) is proposed for ambulatory arrhythmia monitoring systems. The ESP consists of three heterogeneous processors and performs filtering, data compression, ECG classification, and encryption. A data reduction scheme, consisting of skeleton and Huffman coding, are employed to reduce the on-chip memory capacity and memory access power. Clock gating and voltage scaling are also applied to reduce the power consumption. The ESP consumes 1.26-μW at 0.7V, while providing real time signal processing.