## 4 Experimental Results
\begin{table}
\begin{tabular}{c c} \hline \hline Core & ARM 32-bit Cortex-M4 \\ MCU & STM32F407 VG76 \\ Floating-point Unit & enable \\ Flash Memory & 1 MByte \\ RAM & 192 KBytes \\ Frequency & Up to 168 MHz \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Specification of STM32F4-Discovery embedded board. the hardware. In the current implementation stage, the cell area after synthesis tends to be large. This increase in size is because the combinational and sequential logics are used without a separate storage device, such as RAM, for coefficients. The Xilinx Vivado uses FPGA resources, which was placed on the chip beforehand, with routing when synthesizing and implementing RTL on Zynq FPGA. Utilization and area calculations may not be accurate due to different specifications and resources for each FPGA. Therefore, in this evaluation, the cell area was measured after RTL synthesis of the custom I\({}^{2}\)S module using Synopsys Design Compiler. Figure 7a shows the change in the cell area of the custom I\({}^{2}\)S module while changing the number of coefficients, which was made according to the length of the Hann window and the maximum value of the number of the power of 2, according to the quantization precision. Figure 7b is the smallest FPGA tool synthesis result when the coefficient window length is 10 and the quantization degree is \(-5\). Figure 7c is the largest case when the coefficient window length is 80 and the quantization degree is \(-10\). Since the Hann window coefficient has a value from 0 to 1, the power of 2 is also negative. The number of coefficients was changed from 10 to 80 corresponding to the window size from 0.625 to 5 ms. Quantization degree represents the number of powers of 2 in the negative direction used to express decimal coefficients, and it was changed from \(-5\) to \(-10\). As a result, the length and quantization degree of the coefficient (window) increased as the value of the synthesized cell area increased. To minimize the additional hardware usage, the coefficient length and quantization degree should be lower. However, if the window coefficient length factor is too low, the size of the window becomes smaller and the number of FFT operations performed in the MCU software, required to recognize a single word, increases. Alternatively, if the quantization degree is lowered and coarse-grained, it may affect the difference between the original Hann window function result. Considering this trade-off, it is important to determine a factor suitable for the application. In this paper, the Hann window array implemented in RTL in hardware has a length of 80, which corresponds to a window size of 5 ms at 16 kHz. This dimension matches those of the input layer of the neural network and the spectrogram-converted audio data sampling frequency. The quantization degree uses a value of \(-5\); thus, the maximum resolution of the quantization is \(2^{-5}\). Table 2 compares the changes in the execution time for the functions, the energy consumption of the embedded board, and the area after the synthesis of the RTL implemented in the FPGA board when the window process is excluded from the TinyML application using the proposed structure. In the row corresponding to the windowing function, a 0.219 ms decrease occurred, apart from the necessary time consumption for  
Figure 6: Function proportions of the TinyML application on the MCU edge device. profiles. Furthermore, the execution time for other preprocessing functions also decreased slightly. Additionally, the energy consumption was reduced by 3.27%, as compared to the conventional case. Energy consumption was measured using the Microchip Power Debugger current visualizer. However, for the proposed structure, the hardware area of the I\({}^{2}\)S module increased by 93.39%. The size of the window function for the audio data sequence affects the I\({}^{2}\)S module's area increase. As the size of the window increases, the number of coefficients to be stored in the window function inevitably also increases.