The microcontroller measuring converter (MMC) as the main element of the wireless soil humidity and temperature sensor (WSS) is considered. In order to reduce energy consumption and simplify the hardware of the WSS, we propose to measure the soil temperature with a sensor built-in the microcontroller (MCU). Measurements should be performed after the MCU has exited sleep mode, at which MCU temperature point is equal to the soil temperature where the WSS is located. The disadvantage of a sensor built-in the MCU is low accuracy (absolute error ± 10 ℃). The purpose of the study is improving the accuracy of temperature measurements with a sensor built-in MCU. The hardware part of the MMC is implemented on the ATmega328P MCU, which is part of the Arduino Nano. A capacitive sensor is used to measure soil moisture. The software of the experimental MMC comprises three modules: converting the soil moisture sensor capacitance into code; converting the temperature sensor voltage into code using the MCU’s built-in Analog to Digital Converter; measuring the MCU’s temperature with the DS18B20 reference sensor. A solution for selecting a reference voltage is proposed, in which the absolute measurement error in the range of 0–25 ℃ with a resolution of 1 ℃ is reduced by 8 times and amounts to ± 1 ℃ MMC resolution can be increased to 0.5 ℃. The proposed method for improving the accuracy of soil temperature measurement with a sensor built-in the MCU is recommended to use in the manufacture of WSSs required for irrigation monitoring and control systems in precision agriculture.