Soil contamination may occur in the upcoming decommissioning activities of the TRIGA2000 Reactor. Measurement of contaminant radioactivity, which can be performed using single-channel spectroscopy, is required in soil decontamination processes. This research develops a timer/counter system for single-channel spectroscopy using a microcontroller. The performance of the ATMega328P microcontroller Timer/Counter on Arduino has been tested for single-channel spectroscopy. Microcontroller's Timer/Counter 1 is used as a counter while Timer/Counter 2 is used as a timer. Tests include the linearity test, comparative test, and chi-square test. The test results show that the ATMega328P microcontroller Timer/Counter works well and can be used as the end of a single-channel spectroscopic system.