Just to share my finding about deep sleep mode. As we know now GPIO16 and RESET has to be connected together for ESP8266 to wake from deep sleep mode. I guess during wakeup, RTC will generate a pulse on GPIO16 so as to reset the chip. Below is the waveform observed on GPIO16/RST
The falling edge actually only reaches 1.96V then it climbs immediately, with effective pulse width only 5.2us.
It seems ESP8266 is very easy to get resetted?