学习NodeMCU的低功耗休眠

http://blog.csdn.net/coolwaterld/article/details/45366701

Designing An Ultra Low Power Sensor Solution With ESP8266

https://github.com/EspressifSystems/low_power_voltage_measurement/wiki

https://github.com/EspressifSystems/low_power_voltage_measurement

http://bbs.espressif.com/viewtopic.php?t=883