# Gadget Mode to setup a serial connection over USB
Prepare Raspberry Pi OS connection over a microUSB serial cable

Based upon Adafruit guide: https://learn.adafruit.com/turning-your-raspberry-pi-zero-into-a-usb-gadget/serial-gadget:
The author discovered the guide was out of date, consulted the [2025 Raspberry Pi whitepaper on OTG](https://pip-assets.raspberrypi.com/categories/685-app-notes-guides-whitepapers/documents/RP-009276-WP/Using-OTG-mode-on-Raspberry-Pi-SBCs), found additional errors, and was finally able to correct them thanks to this blog https://www.isticktoit.net/?p=1383

## Important :
- **Using the Gadget Mode will make the usb port unable to connect to a mouse or keyboard!!!** You will need to disable it make them work again.
- You will want to disable the serial console on RX/TX in `raspi-config` if you are trying to use that serial port for communicating with other devices.


## Procedure

Make sure you have your USB cable plugged into the left port **USB** and not the right port marked **PWR**.
1. Open a connection to the device via ssh or Raspberry Pi Connect
1. Become root (the superuser)
```
sudo su
```
1. Enable the DWC version 2 USB peripheral device tree overlay by appending it to the end after the `[all]` category (which is at the end)
```
echo "dtoverlay=dwc2" >> /boot/firmware/config.txt
```
1. We add the required module `dwc2` for loading at startup
```
echo "dwc2" >> /etc/modules
```
1. Check that we can load the libcomposite module
```
modprobe libcomposite
```
1. Put a copy of this repository in root's home directory using git
```
apt install -y git
git clone https://gitea.cs.ru.is/RU-V207/rover-pi.git
```
1. Go to Scripts and test if the `gadget.sh` runs without errors and hopefully a Serial port appers in Device Manager or equivalent
 ```
cd rover-pi/Scripts/
./gadget.sh
```
1. Install the systemd
```
cp serial-ether-gadget.service /lib/systemd/system/.
```
1. Enable the service
```
systemctl enable serial-ether-gadget.service
```
1. Enable the getty (which connects a login terminal to the serial port)
```
systemctl enable serial-getty@ttyGS0.service
```
1. Reboot and see if the serial port appears

# Connecting to the serial terminal
You will need a serial terminal program (which used to be very very common) in order to communicate over your serial port

## Windows
- Putty:  https://www.chiark.greenend.org.uk/~sgtatham/putty/latest.html
- MobaXterm: https://mobaxterm.mobatek.net/

## Linux
If using a Linux VM, you will need to pass the Pi USB connection there. 
This is similar to the procedure for accessing an Arduino.
1.  Start the Linux VM if you haven't already
1. Menu: Devices > USB
1. Look for "Silicon Labs CP210x USB to UART Bridge" and click on it.
1. check if port /dev/ttyUSB0 is found. The Pi will most often be accessible over this port.
    ```
    ls -l /dev/ttyU*
    ```
1. The user must be in the dialout group to access it.  Use the Linux command: `uid` -to check if you are in the group. If not in the dialout group, use:
    ```
    sudo usermod -a -G dialout <username>
	```
1. You will use the GNU `screen` application in Linux to access the Pi.  You can install it with:
    ```
    sudo apt install screen
	```
1. You should now gain access to the Pi with:
	```
  ` sudo screen /dev/ttyUSB0 115200
    ```
1. Press enter and you will be prompted to log in to the Pi with default credentials. User: pi - PW: raspberry

# Disabling the OTG mode to use a keyboard and mouse again

**WARNING: Untested**
1. Comment out the dwg2 line in `/boot/firmware/config.txt` using a `#` in front of the line
   ```
   [all]
   #dtoverlay=dwg2
   ```
1. Comment out the dwg2 line in `/etc/modules` using a `#` in front of the line
   ```
   #dwg2
   ```
1. Disable the gadget service
    ```
	systemctl disable serial-ether-gadget.service
	```
1. Disable the getty
    ```
	systemctl disable serial-getty@ttyGS0.service
	```

# Connecting over the USB ethernet to ssh into your Pi

I have not tested this procedure because the serial console and normal network connection is sufficient for me --foley

https://www.raspberrypi.com/news/usb-gadget-mode-in-raspberry-pi-os-ssh-over-usb/


# Untested
 Here are items that bear investigating but haven't been tested yet
 
 - [Modify Imager manifest to install otg](https://github.com/raspberrypi/rpi-imager/tree/main/doc/local_json)
 
