Sometimes you will see a message like
ets Jan 8 2013,rst cause:2, boot mode:(3,6) load 0x4010f000, len 3460, room 16 tail 4 chksum 0xcc load 0x3fff20b8, len 40, room 4 tail 4 chksum 0xc9 csum 0xc9 v0007e100 ~ld 7
on the ESP8266 serial line.
rst cause: 2 means that the ESP was restarted from the firmware using
ESP.restart();
Typically such a restart is intentional. Look for ESP.restart() calls in your firmware. It’s not straightforward to identify which ESP.restart() call caused the reset. I recommend to insert Serial.println() statements describing the reset cause before every call to ESP.restart(), for example:
Serial.println("Resetting due to Wifi not connecting...");
ESP.restart();