Layouts of interpolated RO frequencies for several temperatures are shown in Fig.7 (package temperature was measured). These received results confirm that ring oscillators implemented in the device have different frequencies, so for future measurements an individual calibration process is required. The main approaches to this calibration process are described in literature. Some of them are based on physical thermal sensors integrated in devices (IOB diodes) [6]. Another calibration method determines ring oscillators frequencies in a controlled climate chamber, where also package temperature is measured by additional analog or digital temperature sensors [7]. A comparable method uses infra-red measurements of the device [8]. All these methods assume the temperature of the sensor is similar to that of the package. For each temperature, the frequencies are measured and the linear or polynomial regression is performed on data.