This paper presents a new technique of radio frequency (RF) signal strength detection with a received signal strength indicator (RSSI) circuit which can be deployed in an internet-of-things (IoT) network. The proposed RSSI circuit is based on a direct conversion of RF to digital code indicating the signal strength. The direct conversion is achieved by the repeated switching of a rectifier‚Äôs output voltage using an ultra-low power comparator. We use a 5-bit programmable feedback circuit to correct detection inaccuracies. The RSSI circuit is implemented in a 65-nm CMOS process and consumes 15nW power. It has a linear dynamic range of 26dB and exhibits an error of (+/-)0.5dB with a wide bandwidth of 500MHz. We also present a detailed analysis of the proposed technique and verify it with simulation and measurement results. The high detection accuracy with ultra-low power consumption of our RSSI circuit is favourable for IoT applications including localization, beamforming, hardware security and other low-power applications.