## 3 System Implementation
### Solar Energy Harvester
As can be seen in Figure 4, if discharge capacity is less than 20%, V\({}_{\text{bat}}\) is higher than 1.1 V during this period; however, if discharge capacity is higher than 20%, V\({}_{\text{bat}}\) drops from 1.1 V down to 900 mV, depending on the remaining charge of the battery. In our circuit, battery voltage is detected during this period to accurately estimate the energy stored in the battery. Depending on the detected V\({}_{\text{bat}}\), the power-performance of the integrated electronic circuits and operation duty cycle of the wireless transceiver are reconfigured to guarantee autonomous  
Figure 4: Typical discharge curves of the target NiMH microbattery at room temperature [2]. operation of the sensor. If V\({}_{\text{bat}}\) is high enough, the sensor can operate at its highest performance: solar energy harvester and sensor interface circuits work at their highest speed, and measurement results are sent to the base station every 10 seconds. If V\({}_{\text{bat}}\) is not high enough, the sensor interface circuit and wireless transceiver are activated with a lower duty cycle to minimize total power consumption of the sensor. If V\({}_{\text{bat}}\) is close to V\({}_{\text{EOD}}\), these blocks should get deactivated temporarily to avoid full discharge of the battery. The voltage level detector (LD) in Figure 5 is used to determine the battery voltage level [6]. After circuit startup, battery voltage is checked to make sure that it is more than the end of discharge threshold voltage (V\({}_{\text{EOD}}\)) and can power up the circuit. If V\({}_{\text{bat}}\) is less than V\({}_{\text{EOD}}\), it means that the battery is fully discharged and cannot provide enough power for the electronic circuits. In this situation, the PV module continuously charges the battery by keeping the switch closed. As soon as V\({}_{\text{bat}}\) passes V\({}_{\text{EOD}}\), LD starts its normal operation, comparing V\({}_{\text{bat}}\) with specified threshold voltages in a timely manner and updating the V\({}_{\text{bat\_level}}\) as a result. In order to generate a bandgap reference voltage, 25 substrate PNP transistors have been used as Q\({}_{1}\) and Q\({}_{2}\) in a common-centroid layout [6]. These transistors are biased with a 100 nA current source to generate V\({}_{\text{BE1}}\) and V\({}_{\text{BE2}}\) in non-overlapping \(\Phi_{1}\) and \(\Phi_{2}\) phases, and the SC circuit sums up V\({}_{\text{BE1}}\) and (V\({}_{\text{BE1}}\)\(-\)V\({}_{\text{BE2}}\)) with appropriate coefficients. When V\({}_{\text{bat}}\) reaches V\({}_{\text{EOC}}\), the switch between the PV module and the battery is turned off to avoid overcharge of the battery. This SC circuit detects when V\({}_{\text{bat}}\) passes the V\({}_{\text{L}}\) specified in Equation (1) by setting the V\({}_{\text{bat\_level}}\) output. By using a variable \(\gamma\)C capacitor, different voltage levels between V\({}_{\text{EOD}}\) and V\({}_{\text{EOC}}\) can be detected to estimate the remaining charge of the battery. In Equation (1), \(\alpha\), \(\beta\) and \(\gamma\) coefficients are the ratios of tunable \(\alpha\)C, \(\beta\)C and \(\gamma\)C capacitors. These variable capacitors have been implemented using a matrix of 10 fF metal-insulator-metal (MIM) capacitors that have been used as unity capacitors. \[V_{L}=[\alpha\times(V_{BE1}-V_{BE2})+\ \beta\times V_{BE1}]/(\gamma)=V_{ref}/(\gamma) \tag{1}\]  
As V\({}_{\text{BE1}}\) is complementary to absolute temperature (CTAT) and (V\({}_{\text{BE1}}\)\(-\)V\({}_{\text{BE2}}\)) is proportional to absolute temperature (PTAT), different CTAT, PTAT or temperature-independent reference voltages (V\({}_{\text{ref}}\)) can be built by proper selection of \(\alpha\)C and \(\beta\)C capacitors. After that, by modifying the \(\gamma\)C  
Figure 5: Circuit diagram of voltage level detector. capacitor, target voltage levels can be detected.