## 4 System Integration and Performance  
The circuit has been implemented in a 0.18 \(\upmu\)m CMOS process with 0.25 mm\({}^{2}\) total area, as can be seen in the chip microphotograph in Figure 8. The main blocks, including the digital and analog blocks of solar energy harvester and sensor interface circuits, have been specified separately. When operating at 1 MHz, the whole circuit consumes almost 2.1 \(\upmu\)W; while the average power consumption of the energy harvester is less than 350 nW. The wireless transceiver sends the measurement results for both temperature and H\({}_{2}\) concentration to a base station every 15 seconds. Table 4 presents measurement and simulation results for power consumption of the energy harvester and sensor interface circuits in different system operation modes. These operation modes have been defined according to the remaining charge of the battery, which is estimated accurately by detecting the battery voltage between 900 mV and 1.1 V during wireless data transmission, and the battery is discharged by a high current, close to 5 CA. Battery discharge capacity is determined according to the discharge curve of the battery shown in Figure 4. By using highly resistive poly resistors, total layout area has been decreased compared to [9]; however, as these embedded resistors are sensitive to process variations, absolute values are important for all resistors, including resistors that have been used in the beta multiplier current source, and oscillator are trimmed initially. If the battery gets discharged, a lower battery voltage is detected during 5 CA discharge and the system is switched to a lower clock frequency to decrease the average power consumption of the whole system. In S\({}_{\text{L2}}\) and S\({}_{\text{L1}}\) modes, the operating frequency is decreased to 500 kHz and 250 kHz, respectively. In S\({}_{\text{L0}}\) mode, when the remaining charge of the battery is less than 25%, the circuit consumes less than 0.6 \(\upmu\)W by operating at 125 kHz frequency instead of 1 MHz and using lower bias currents. The average power consumption of the energy harvester drops to less than 110 nW in S\({}_{\text{L0}}\) mode. In addition, measurement results are sent every 120 seconds instead of every 15 seconds in S\({}_{\text{L3}}\) mode, to further reduce the total average power consumption of the whole sensor. Figure 8: Chip microphotograph of the whole microsystem. In order to estimate total power consumption of the sensor, the average power consumption of sensor biasing circuit and wireless transceiver should be calculated. Pd nanowires should be biased with a 7 \(\mu\)A bias current for 10 seconds, before measuring H\({}_{2}\) concentration. TZ1053 consumes 5 \(\mu\)A during standby and consumes 3.3 mA during a period of 20 ms to send a sample with the minimum payload size of 55 bytes [3]. In S\({}_{\mathrm{L3}}\) mode, samples are sent every 15 seconds, and the average current consumption of Pd nanowire sensors and wireless transmission are 4.67 \(\mu\)A and 9.4 \(\mu\)A, respectively. By sending the samples every 120 seconds in S\({}_{\mathrm{L0}}\) mode, these values will be reduced to 0.58 \(\mu\)A and 5.6 \(\mu\)A, respectively. So, the total average current consumption of the whole sensor is less than 16 \(\mu\)A, operating at its highest performance in S\({}_{\mathrm{L3}}\) mode, and is reduced to less than 7 \(\mu\)A, operating in S\({}_{\mathrm{L0}}\) mode. Four nanowire solar cells presented in [1] have been connected in series to provide an appropriate V\({}_{\mathrm{mpp}}\) voltage, close to the V\({}_{\mathrm{EOC}}\) of the V6 HR NiMH battery. The power delivered to the battery by the PV module can be simulated using the equivalent circuit model of the PV module. This PV module, with total area of 28 mm\({}^{2}\), can provide a maximum power of 2.88 mW at its V\({}_{\mathrm{mpp}}\) under AM1.5 illumination level [1].