Many people around have this problem: http://www.esp8266.com/viewtopic.php?f=6&t=2791
Well. Read carefully if you develop under windows:
If you have experienced problems while flashing with esptool and other tools (ESP8266Flasher.exe, flash_download_tool_v1.2_150512.exe, etc), this might give you a clue.
If you are trying to flash the ESP8266 (ESP12 module for example) and your flashing operation hangs in the middle of the procedure, pay attention to this. It can save you a lot of time…
Depending on which USB to TTL converter you are using, the system might not work correctly. There must be something about buffering or drivers or something in Windows that makes a strong difference whether you are using one converter or another.
Validated USB to TTL (these are known to work)
– TTL-232R-3V3-PCB from FTDI
– Silicon Labs CP210x USB to UART bridges
Known that does not work:
– Prolific based USB to TTL converters
If you have a Prolific USB to UART bridge, you might try to make it work by changing the esptool.py by reducing the packet size of the flash transmission
ESP_RAM_BLOCK = 0x180
ESP_FLASH_BLOCK = 0x40
Juan Ramón Vadillo