如果你想要使用AT指令，你应当先对flash烧录AT固件，然后将波特率设置为115200。
If you want to use AT command, you should download AT firmware into flash first and then set the baud rate as 115200.
如果 IoT 固件先被烧录入flash，则默认波特率应当是74880，而且AT指令无效。
If the IoT firmware has been pre-downloaded into flash before, the default baud rate should be 74880. and AT command cannot take effect.
如何自定义波特率？How to change baud rate?
http://bbs.espressif.com/viewtopic.php?f=16&t=10&p=23&hilit=baudrate+115200#p23