Sometime the program causes runtime errors that cause the chip to reboot and enter invalid state. ESP provides some diagnostic information when an error happens:
Exception (28):
epc1=0x40221a46 epc2=0x00000000 epc3=0x00000000 excvaddr=0x00000000 depc=0x00000000
ctx: sys
sp: 3ffff850 end: 3fffffb0 offset: 01a0
>>>stack>>>
3ffff9f0: 00000001 00000004 3ffe99b4 00000002
...
3fffffa0: 400201e9 efac3ebe ffffff01 55aa55aa
<<<stack<<<
See the list of exceptions Exception Causes (EXCCAUSE)
To upgrade your ESP8266 with the latest firmware, you need:
- The Flash Download Tool
- The official SDK from Espressif.
Typically you will need the NONOS SDK. For example ESP8266 NONOS SDK V1.5.4.
There is a really great article on the upgrade procedure.