The SD-Dev carrier board

The Signaloid SD-Dev is a compact carrier board and development system designed for testing and characterizing the Signaloid C0-microSD.

SD-Dev port
Figure 1: SD-Dev port diagram.

Peripheral mode

When you connect the Signaloid SD-Dev to a host computer via USB, it acts as a generic USB-to-SD adapter, exposing one microSD and one full-size SD slot to the host.

When using the SD-Dev without an onboard Raspberry Pi compute module, you must connect it to your host computer via the PWR+D port.

Standalone mode

You can attach a Raspberry Pi CM4 or CM5 to the carrier board, producing a compact single-board computer (SBC) with hot-swappable FPGA modules in a standalone configuration. In this mode, the carrier board provides power measurement capabilities for the on-board FPGA modules and allows you to power cycle them programmatically.

When using the SD-Dev with an onboard Raspberry Pi compute module, power it via the PWR port so that you can use the P0 and P1 ports to connect peripherals to the Raspberry Pi compute module.

Getting started with the Signaloid SD-Dev

To get started with the SD-Dev, view the SD-Dev quickstart guide.

Carrier-Board Specifications

  • Compact design with the following I/O:
    • 2 USB Type-C downstream ports
    • 1 USB Type-C upstream port
    • 1 USB Type-C upstream power-only port
    • 1 Micro HDMI port
    • JST connectors for SPI and I²C peripherals
  • One full-size SD slot and one microSD slot for Signaloid C0-microSD boards or regular storage cards
    • SD card detection
    • Programmable power cycling
    • Power measurement
    • Open-top microSD socket allows you to probe the Signaloid C0-microSD debug pads
  • Dual function (peripheral and standalone modes)
  • Board outline of 57 x 57 mm

Flashing the eMMC of a Raspberry Pi compute module.

To flash the onboard eMMC of your Compute Module using the Signaloid SD-Dev board, please refer to the official Raspberry Pi documentation. Note that the SD-Dev board does not include a disable eMMC Boot jumper. Instead, prior to connecting the SD-Dev to the host computer, press and hold the onboard push button and continue with the flashing process.

Raspberry Pi compute module as a USB Serial gadget

When using the SD-Dev in standalone mode, with a Raspberry Pi compute module attached, you can configure the Raspberry Pi to act as a USB serial gadget, i.e. access it through its USB serial port. Optionally, you can also attach a console to that serial port.

To do that:

  1. Edit the /boot/firmware/config.txt file:
    • Comment out the line otg_mode=1.
    • Add this dtoverlay=dwc2,dr_mode=peripheral at the end of the file.
  2. Create the /etc/modules-load.d/otg.conf file with the following content: 
     dwc2
 g_serial
  3. Reboot
  4. Connect your SD-Dev to your host machine using the PWR+D USB-C port.
  5. Access the serial port from your host machine using tools like screen on Linux/Mac, or minicom on Windows.

At this point your Raspberry Pi will create a /dev/ttyGS0 serial port, which you can read from and write to from both the Raspberry Pi and the host machine.

To attach a console to ttyGS0, so that you can have access to a full Raspberry Pi terminal from your host machine:

  1. Symlink the getty service: 
     sudo ln -s /lib/systemd/system/[email protected] /etc/systemd/system/getty.target.wants/[email protected]
  2. Enable and start the service: 
     sudo systemctl enable --now [email protected]

Raspberry Pi compute module as a USB Ethernet gadget

When using the SD-Dev in standalone mode, with a Raspberry Pi compute module attached, you can configure the Raspberry Pi to act as a USB Ethernet gadget, i.e. attach it to a host machine using USB-C and access it via SSH.

To do that:

  1. Edit the /boot/firmware/config.txt file:
    • Comment out the line otg_mode=1.
    • Add this dtoverlay=dwc2,dr_mode=peripheral at the end of the file.
  2. Create the /etc/modules-load.d/otg.conf file with the following content: 
     dwc2
 g_ether
  3. Reboot
  4. Connect your SD-Dev to your host machine using the PWR+D USB-C port.
  5. Add an ethernet connection: 
     sudo nmcli con add type ethernet con-name usb-otg ifname usb0
  6. Activate the connection: 
     sudo nmcli con up usb-otg
  7. Identify your Raspberry Pi IP address: 
     ip addr show dev usb0
  8. Connect to your Raspberry Pi from your host machine via SSH using the IP from the previous step.

If everything works correctly, you can automate the connection activation by creating a systemd service:

  1. Create the /lib/systemd/system/usb_otg.service file with the following content: 
     [Unit]
 Description=USB Ethernet gadget
 After=NetworkManager.service
 Wants=NetworkManager.service
	
 [Service]
 Type=oneshot
 RemainAfterExit=yes
 ExecStart=/bin/sh -c 'nmcli con up usb-otg'
	
 [Install]
 WantedBy=sysinit.target
  2. Enable and start the service: 
     sudo systemctl enable --now usb_otg.service