Low Cost Convolutional Neural Network Accelerator Based on Bi-Directional Filtering and Bit-Width Reduction

This paper presents a low-area and energy-efficient hardware accelerator for the convolutional neural networks (CNNs). Based on the multiply-accumulate-based architecture, three design techniques are proposed to reduce the hardware cost of the convolutional computations. First, to reduce the computational cost of convolutions, an adaptive bit-width reduction combined with near-zero skipping is proposed based on differential input method (DIM). The DIM-based design technique can reduce 62.5% of operation bit-width and improve 17.0% of activation sparsity with almost ignorable CNN accuracy degradation. Second, it has been found that adopting a bi-directional filtering window in a CNN accelerator can considerably reduce the energy for data movement with a much smaller number of memory accesses. To expedite the bi-directional filtering operations, we also propose a bi-directional first-input-first-output (bi-FIFO). With SRAM bit-cell layout manner, the proposed bi-FIFO facilitates fast data re-distribution with area and energy efficiency. To verify the effectiveness of the proposed techniques, the AlexNet accelerator has been designed. The numerical results show that the proposed adaptive bit-width reduction scheme achieves 34.6% and 58.2% of area and energy savings, respectively. The bi-FIFO-based accelerator also achieves 32.8% improved processing time.