AN588
Rev. 0.2
7
7. Ultra Low Power Wireless Sensor
The Si1012 Wireless MCU is used for the sensing and RF transmission functions of this reference design. In sleep
mode, the Si1012 supply current is approximately 50 nA. When the MCU is awoken from sleep mode using the
push button switch, it samples the current temperature using the on-chip temperature sensor, the remaining charge
in the thin film battery, and the amount of ambient light at the solar cell. The temperature and remaining charge are
transmitted over the RF connection once per button press. The light level is transmitted once a second for three
minutes.
One of the distinguishing features of the Si1012 which makes it excellent for energy harvesting applications is its
ultra low power consumption both in sleep and active modes. Figure 9 shows a simplified activity profile for the
active mode (RF packet transmission once a second). The peak current is 29 mA when the RF transmitter is
enabled and the average current over the three minute time interval is 51 µA (1 packet per second).
Figure 9. Activity Profile While Transmitting
The wireless sensor node uses a 919.84 MHz RF link based on the EZMAC
®
PRO software library. The RF link
data rate is 128 kbps using GFSK modulation (EZRadioPRO channel 19). The output power is +13 dB. The Sensor
Node will decrease the output power when the charge on the thin film battery drops below 75%. The printed
antenna on the Sensor Node is designed on a 2-layer PCB in order to achieve the lowest possible cost. The
expected range with the printed antenna is 100–300 feet (line of sight).
0.8 uA
19 mA
29 mA
Active Mode
TX
Active Mode
Rx
RTC Sleep
Active Mode
TX
Active Mode
Rx
RTC Sleep
1 mS
1 mS
998 mS
1 mS
1 mS
998 mS
