2.2.1. Hardware

For the micro controller unit (MCU), the STM32F103 model developed by STMicroelectronics, USA was used. The response time of MCU was set up at 100 ms and the operation voltage was 3.7 V. The MCU pre-scaler was set up at 3 and the period was 400 based on 72 MHz main clock frequency. Based on this, the sensing period was calculated and it was set at 16 us by using I2C communication.

For the capacitance measurement function, the MPR121QR2 sensor (Freescale Inc., Austin, TX, USA) was used; it converts capacitance to digital values. This sensor was used in our previous study [35] testing the feasibility of our capacitive insole sensor and showed a high correlation (R<sup>2</sup> > 0.90, *p* < 0.05) with F-scan (Tekscan Inc., Boston, MA, USA). The MPR121QR2 sensor has a resolution of 0.01 pF and the measurement range covers 10 pF to 2000 pF. The data were sampled at 100 Hz [36]. Normalization was performed for all 10 channels of the insole sensor before signal processing. The standard deviation values between the data collected from the 10 channels were lower than 0.03 pF.

The HM-13 chip was used for Bluetooth communication and the baud rate was set to 115,200, the non-parity bit was 0, stop bit was 1.

The printed circuit board (PCB) was developed containing all sensors as presented in Figure 4a,b shows the MCU, MPR121QR2, and Bluetooth communication part of the developed PCB schematics.
