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If you don't have a USB-to-TTL serial adapter available, but you have a Raspberry Pi, you can use this procedure for flashing your device.

Tutorial (a little outdated)

Summarizing the process and needed connections based on the above blog, please refer there for a more detailed explanation 😉

Preparation of the Raspberry Pi

Install esptool (pip needed)
sudo pip install esptool
Prepare serial connection.
Note: Use the correct serial connection as per your version of the Raspberry Pi.

Device Serial Connection

RaspberryPi 1/2/3 (BT disabled or miniuart) /dev/ttyAMA0

RaspberryPi 3 /dev/ttyS0

sudo systemctl stop serial-getty@ttyS0.service

sudo systemctl disable serial-getty@ttyS0.service

sudo cp /boot/cmdline.txt /boot/cmdline.bak (This backups the file ;))

sudo nano /boot/cmdline.txt -> delete "console=serial0,115200" option
I don’t know if this is necessary, but I did it as it was mentioned here:
https://spellfoundry.com/2016/05/29/configuring-gpio-serial-port-raspbian-jessie-including-pi-3/

sudo nano /boot/config.txt

-> add the line (at the bottom): enable_uart=1

Please note following overlays setting port to /dev/ttyAMA0
- pi3-miniuart-bt
- pi3-disable-bt
Reboot your Pi

Device	Serial Connection
RaspberryPi 1/2/3 (BT disabled or miniuart)	/dev/ttyAMA0
RaspberryPi 3	/dev/ttyS0

Connections

Connect GND, RX->TX and TX->RX as shown below.

If you power the device with the 3.3V power from the RPi it will cause it to reboot when connecting. You can still enter flash mode if you power up the RPi with the 3.3v connected and pressing the connecting GPIO0 to ground (e.g., pressing the button on the Sonoff Basic) simultaneously. It's recommended to use either an external power supply for this or use the 5V GPIO and a 3.3V Voltage Regulator.
(issue #4807)

Do not connect 3.3V yet!

https://www.sigmdel.ca/michel/ha/sonoff/flashing_sonoff_en.html

(For RaspberryPi 3 (40 GPIO pins) GND is the last pin on the left, the 39th pin.)

This is how it looks in real-life 😆

Flashing the firmware

Erase flash memory: Connect 3.3V while button pressed for about 10 seconds to enter flash-mode. Then release and run:

# if using Raspberry Pi 1/2 use
# esptool.py --port /dev/ttyAMA0 erase_flash

# if using Raspberry Pi 3 use
esptool.py --port /dev/ttyS0 erase_flash

You should see output like this:

➜  ~ esptool.py --port /dev/ttyAMA0 erase_flash
esptool.py v2.5.1
Serial port /dev/ttyAMA0
Connecting....
Detecting chip type... ESP8266
Chip is ESP8285
Features: WiFi, Embedded Flash
MAC: dc:4f:22:ab:5e:dc
Uploading stub...
Running stub...
Stub running...
Erasing flash (this may take a while)...
Chip erase completed successfully in 3.3s
Hard resetting via RTS pin...
➜  ~

Flash .bin, which you can download from the release-section:

Again connect 3.3V while pressing button for 10 seconds. Then release and run:

# if using Raspberry Pi 1/2 use
# esptool.py --port /dev/ttyAMA0 write_flash -fm dout 0x0 /path/to/downloaded/tasmota.bin

# if using Raspberry Pi 3 use
esptool.py --port /dev/ttyS0 write_flash -fm dout 0x0 /path/to/downloaded/tasmota.bin

You should see output like this:

➜  ~ esptool.py --port /dev/ttyAMA0 write_flash -fm dout 0x0 tasmota.bin
esptool.py v2.5.1
Serial port /dev/ttyAMA0
Connecting.....
Detecting chip type... ESP8266
Chip is ESP8285
Features: WiFi, Embedded Flash
MAC: dc:4f:22:ab:5e:dc
Uploading stub...
Running stub...
Stub running...
Configuring flash size...
Auto-detected Flash size: 1MB
Compressed 456000 bytes to 312426...
Wrote 456000 bytes (312426 compressed) at 0x00000000 in 29.2 seconds (effective 124.8 kbit/s)...
Hash of data verified.

Leaving...
Hard resetting via RTS pin...
➜  ~

Setup Wi-Fi:
- Connect with an external device to the sonoff-xxxx Wi-Fi AP and configure over the browser.
- Alternative: Reboot your device after flashing (toggle power wire) and connect to the serial console via miniterm:
```
# if using Raspberry Pi 1/2 use
# miniterm.py /dev/ttyAMA0 115200 -e

# if using Raspberry Pi 3 use
miniterm.py /dev/ttyS0 115200 -e
```
  You will see some output, after pressing enter.
  
  Now just type in, your SSID, press enter and then type the password…
  
  Backlog ssid yourSSID; password yourPassword

That’s it 😃

Sources:
Flashing commands
https://spellfoundry.com/2016/05/29/configuring-gpio-serial-port-raspbian-jessie-including-pi-3/

Wiki has moved to link on the left.