Choice of component bit width for nonlinear neuron implementation on FPGA

Purpose. Investigation of the relationship between input data error of a neuron intended for use in an artificial neural network implemented on FPGA, and computational error, as well as development of a methodology for selecting the bit width of neuron components aimed at reducing hardware costs while maintaining computational accuracy consistent with the accuracy of the input data. Methods. The study employed methods of digital circuit design based on the VHDL hardware description language, error analysis of computations relative to a floating-point reference model, as well as device synthesis and FPGA resource utilization estimation methods integrated into Xilinx ISE. Mathematical statistics techniques, including the construction of regression models describing the dependence of accuracy and hardware costs on input data bit width, were applied to process the experimental results. Results. A method has been proposed for estimating the bit width of the processing unit, enabling its precision to be matched with the inherent error level of the input data. The impact of the bit width of input data and weight coefficients on computational accuracy and the amount of FPGA hardware resources consumed by the implemented neuron was investigated. Based on the VHDL description of the device, a parameterized model was developed that enables coordinated adjustment of the neuron’s internal component bit widths as the bit width of input signals is varied. To assess the effect of bit width on computational accuracy, a floating-point-based reference model was used. For each bit-width configuration, comparative computations of the device’s output were performed, and the resulting error was quantified. The influence of bit width on FPGA resource utilization — specifically the number of LUTs and flip-flops (FFs) — was also analyzed. The proposed methodology was validated on the Xilinx Spartan-3E XC3S500E (xc3s500e-4pq208) FPGA platform using the ISE Design Suite 14.7 environment. Multiple versions of the digital neuron were implemented, with input data bit widths ranging from 4 to 12 bits (including the sign bit). For each variant, the operating clock frequency, utilized FPGA resources, and computational accuracy were recorded. As a case study using 12-bit input data, an experimental evaluation determined that a sigmoid function lookup table with 8,192 entries achieves an optimal trade-off between computational accuracy (maximum relative error — 0.12%) and hardware cost (occupying only 1% of the FPGA’s available resources). Conclusion. This paper presents a description of a neuron circuit with a sigmoid activation function, implemented in the VHDL hardware description language and suitable for integration into neural network solutions on FieldProgrammable Gate Arrays (FPGAs). The device accepts signed integer input values of fixed bit width, computes the weighted sum of inputs and bias, and generates the neuron’s output using a precomputed lookup table stored in block RAM. The operation, scaling, and optimization of the module are described in detail. The proposed method enables determination of the optimal bit width for the processing unit, ensuring that computational error remains consistent with the error level of the input data while minimizing hardware resource consumption. The obtained relationships can be utilized during the design phase to select parameters for digital processing modules in real-time systems and embedded devices.