ESP8266 or ESP32 SoC can work as the WiFi communication module for other microcontrollers. Espressif and other developers prepared the ready-to-use firmware with the AT-command interpreter. This firmware can be downloaded from the web and flashed into ESP8266 or ESP32 memory with a flash tool.

To use the ESP8266 or ESP32 chip as a modem (Figure 1) first download the appropriate AT-command firmware.

1. Download the latest ESP Flash Download Tool (v3.9.5 at the time of writing) from ^[1].
2. Download the latest AT release from ^[2]

The newest version of Espressif firmware is not compatible with ESP8266 SoCs. In the case of using ESP8266-based boards, download older AiThinker firmware available on GitHub ^[3].

• Detect ESP8266 module parameters. Start the ESP Flash Download Tool (“ESPFlashDownloadTool_v3.9.5”), set the COM port corresponding to your programmer, and then click the START button to detect the specs of the board. After detection, you should see something like this (Figure 2):

The firmware can come in different versions. It can be a set of binary files which must be uploaded at specific memory addresses or as a combined single binary file. Note, that a single file is prepared for a specific flash memory size.

• Gather information. Make a note of the flash memory size. In this example, we have a 32 Mbit flash.
• Load the correct size of the combined AT binary firmware file (“.bin”) and set the offset as 0×0, you should see something like this (Figure 3):

There are also other flashing tools like NodeMcu Flasher ^[4]. While using a single binary file other flashing tools can be used like esp8266 flasher ^[5], Tasmotizer ^[6] or others.

• Then, click the START button and wait until the flashing process is over.

If necessary, to restore the original firmware:

• Detect ESP8266 module parameters. Start the ESP Flash Download Tool (“ESPFlashDownloadTool_v3.6.4”), set the COM port corresponding to your programmer, then click the START button in order to detect the specs of the board. After detection, you should see something like this (Figure 4):

• From the downloaded AT firmware folder, open the “readme.txt” file containing the information for flashing the firmware. Inside the file, there should be a “BOOT MODE” section, as follows:

# BOOT MODE
## download
### Flash size 8Mbit: 512KB+512KB
    boot_v1.2+.bin              0x00000
    user1.1024.new.2.bin        0x01000
    esp_init_data_default.bin   0xfc000 (optional)
    blank.bin                   0x7e000 & 0xfe000

### Flash size 16Mbit: 512KB+512KB
    boot_v1.5.bin               0x00000
    user1.1024.new.2.bin        0x01000
    esp_init_data_default.bin   0x1fc000 (optional)
    blank.bin                   0x7e000 & 0x1fe000

### Flash size 16Mbit-C1: 1024KB+1024KB
    boot_v1.2+.bin              0x00000
    user1.2048.new.5.bin        0x01000
    esp_init_data_default.bin   0x1fc000 (optional)
    blank.bin                   0xfe000 & 0x1fe000

### Flash size 32Mbit: 512KB+512KB
    boot_v1.2+.bin              0x00000
    user1.1024.new.2.bin        0x01000
    esp_init_data_default.bin   0x3fc000 (optional)
    blank.bin                   0x7e000 & 0x3fe000

### Flash size 32Mbit-C1: 1024KB+1024KB
    boot_v1.2+.bin              0x00000
    user1.2048.new.5.bin        0x01000
    esp_init_data_default.bin   0x3fc000 (optional)
    blank.bin                   0xfe000 & 0x3fe000

• Indicate – correct for your ESP8266 flash size – firmware files & addresses. The firmware is broken down into several files. They need to be provided to the ESP Flash Download Tool, together with the corresponding addresses found in the readme.txt file above. For our ESP8266 example it should look like this (Figure 5)

• Then, click the START button and wait until the flashing process is over.

After uploading AT firmware and connecting the module to the PC, we can use ESP8266 as a modem with simple AT commands.

We can connect ESP8266 to PC with TTL-Serial-to-USB adapter, or we can use any microcontroller with a serial interface. The default baud rate settings are 115200,N,8,1. Next from any terminal type command:

AT

and press enter. If you get OK, the ESP8266 module is ready to use. Let’s try out some other commands. For example, let’s figure out exactly what firmware version we’re dealing with. To do that, we’ll use the following command:

AT+GMR

As a Wifi device ESP8266 can connect to the network in such modes:

• mode 1 – client mode – the ESP8266 connecting to an existing wireless network,
• mode 2 – access point mode (AP) – other wireless network devices can be connected to the ESP8266,
• mode 3 – dual mode (router) – the ESP8266 act as an access point and connect at the same time to an existing wireless network.

By default, the ESP8266’s stock firmware is set to AP mode. If you’d like to confirm that, send the following command:

AT+CWMODE?

You should get this response: +CWMODE:2, where 2 corresponds to AP mode. To switch ESP8266 to client device mode, we use the following command:

AT+CWMODE=1

Now we can scan the airwaves for all WiFi access points in range. To do that, we send:

AT+CWLAP

Then the ESP8266 will return a list of all the access points in range. In with each line will be item consisting of the security level of the access point, the network name, the signal strength, MAC address, and wireless channel used. Possible security levels of the access point <0–4> mean:

• 0 – open,
• 1 – WEP,
• 2 – WPA_PSK,
• 3 – WPA2_PSK,
• 4 – WPA_WPA2_PSK.

Now we can connect to the available access point using proper “ssid_name” and “correct_password” with the command:

AT+CWJAP="ssid_name","corect_password"

If everything is OK, the ESP8266 will answer:

WIFI CONNECTED
WIFI GOT IP
OK

It means that ESP8266 is connected to the chosen AP and got a proper IP address. To check what the assigned address is we send the command:

AT+CIFSR

To set up ESp8266 to behave both as a WiFi client as well as a WiFi Access point.

AT+CWMODE=3