**4.1 Optimization of the SD-card communication speed**

The ESP32 features an SD bus interface that allows communication to SD-Cards in 1-bit and 4-bit modes. Read and write speed was compared in both modes with the performance of the SPI bus. In addition, a SanDisk Extreme 32GB (Speedclass 10, UHS Speed Class 3, max. Transfer speed 160 MB/s) was compared with a SanDisk Ultra 32GB (Speedclass 10, UHS Speed Class 1, max. Transfer speed 98 MB/s) during the three different communication scenarios. The Arduino sample programs "SD\_Test" [18] for SPI and "SDMMC\_Test" [19] for 1-bit and 4-bit data transfer were used to control the SD-Card, transferring data and measuring the transfer time. The sample programs are included when installing the ESP32 board manager into the Arduino IDE. All codes were executed on a DFRobot FireBeetle ESP32. **Table 5** shows the GPIO-Pin connections of the setup, respectively.
