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Khadas VIM3 development guide

The Khadas VIM3 is an ARM64-based single board computer. It is possible to run Fuchsia on the VIM3. This guide shows Fuchsia contributors how to install Fuchsia on a VIM3 and do other common development tasks.

See Appendix: Feature support for details on which VIM3 features Fuchsia supports.

Audience

This guide assumes you're comfortable with:

Tinkering with electronics and hardware.
Building Fuchsia from source and other CLI workflows.

Install Fuchsia on a Khadas VIM3 board

See Troubleshooting and Appendix: Support if you have any trouble completing the installation process.

Prerequisites

You'll need all of the following hardware and software:

A Khadas VIM3 single-board computer. Googlers should request a board through the Fuchsia Ops team.

Caution: Fuchsia is primarily focused on supporting the Pro model VIM3. The Basic model may work but is less of a priority.
A desktop or laptop computer that's running Linux and has 2 USB ports available. This computer is called the host.

Caution: A macOS host may work but these instructions have not been validated with macOS. Building Fuchsia on a remote Linux computer and then attempting to flash Fuchsia onto the VIM3 with a local macOS host is known to not work.
A power supply of at least 24W to your host. The VIM3 can draw that much power when DVFS is enabled.
A working Fuchsia development environment on your host. In other words, you should be able to build Fuchsia from its source code on your host.
A USB to TTL serial cable.
A USB-C to USB-A cable that supports both data and power delivery. The USB-C side is for the VIM3. The other side is for your host.

Caution: Other USB cable types, like USB-C to USB-C, may work, but appear to have power delivery issues more frequently than USB-C to USB-A. See Troubleshooting: Bootloops.

The following is optional:

A heatsink. This enables running 2 CPU cores on the VIM3 at full speed without reaching 80°C, the critical temperature beyond which cores are throttled down.

See the VIM3 collection in the Khadas shop for examples of compatible accessories.

Build Fuchsia

If you don't already have an in-tree environment set up, you should start the process now because it can take a while to complete:

Download the Fuchsia source code.
Configure and build Fuchsia.
- When configuring the build, use fx set core.vim3.

You'll use the Fuchsia development environment to build the Fuchsia image for VIM3 and run an in-tree CLI tool for flashing the Fuchsia image onto the VIM3.

Set up the hardware

Set up the VIM3 to communicate with your host:

Connect the VIM3 and your host to each other with the USB-C to USB-* cable. The white LED on the VIM3 should turn on.

Caution: Don't put a USB hub between the VIM3 and your host. The hub may make it harder for your VIM3 and host to detect and communicate with each other.

This connection is used to power and flash the VIM3 with fastboot.
Connect the serial cable wires to the VIM3's GPIOs:
- GND to pin 17.
- TX (out from VIM3) to pin 18.
- RX (into VIM3) to pin 19.
- Don't connect the power wire of your serial cable to any VIM3 GPIO. The VIM3 is getting power through the USB cable.
Tip: Pins 1, 20, 21, and 40 are labeled on the circuit board.
Caution: In general the colors for TX and RX wires are not standardized. For example your RX wire may be blue or green. If your host is unable to see the serial logs, power off the VIM3 board and swap the TX and RX connections. Some USB-to-serial adapters have their wires improperly labeled.

See Serial Debugging Tool for an example image of how your serial wires should be connected to the VIM3.
Connect the USB end of the serial cable to your host.

Verify the serial connection

Make sure that you can view the serial logs:

Open Fuchsia's serial console:
```
fx serial
```
Tip: If fx serial detects multiple USB devices and you don't know which one to use, try disconnecting the serial cable from your host, running ls /dev/ttyUSB*, then re-connecting the serial cable and running the command again. If you see no difference when running ls /dev/ttyUSB* try ls /dev/tty* or ls /dev/* instead.
Press the reset button on the VIM3. The reset button is the one with the R printed next to it on the circuit board. See VIM3/3L Hardware for a diagram. In your serial console you should see human-readable logs.

See Troubleshooting: Bootloops if your VIM3 seems to keep rebooting.

Erase the eMMC

Before you can install Fuchsia, you need to get the VIM3 firmware and software to a known-good state. The first step is to erase the eMMC.

Press the reset button on your VIM3.
Right after you press the reset button, start repeatedly pressing the Space key as your VIM3 boots up. Make sure that your cursor is focused on your serial console. The bootloader process should pause and your serial console should show a kvim3# prompt. Your serial console is now providing you access to the U-Boot shell.
Run the following command in the U-Boot shell:
```
store init 3
```
Your serial console logs should verify that the eMMC was correctly erased.

See Erase eMMC for more details.

Update the Android image on the VIM3

Now you need to get the VIM3 firmware and software to a known-good state:

Click the following URL to download an Android image that is known to work well with subsequent Fuchsia installations: https://dl.khadas.com/firmware/vim3/android/VIM3_Pie_V211220.7z
Extract the compressed archive file (VIM3_Pie_V211220.7z). After the extraction you should have a VIM3_Pie_V211220 directory with an update.img file in it.
Follow the instructions in Install OS into eMMC. When running aml-burn-tool the value for the -i flag should be the path to your update.img file. Your command should look similar to this:
```
aml-burn-tool -b VIM3 -i ~/Downloads/VIM3_Pie_V211220/update.img
```
Tip: The TST Mode workflow is probably the easiest and fastest way to get your VIM3 into Upgrade Mode.
If the white and red LEDs on your VIM3 are off and the blue LED is on, it means that your VIM3 is in sleep mode. Try putting your VIM3 back into Upgrade Mode and then pressing the reset button again.

At this point the white LED on your VIM3 should be on and you should see logs in your serial console after you press the reset button on your VIM3.

Update the bootloader

This section explains how to flash a prebuilt version of Fuchsia's modified U-Boot onto the VIM3. See the following link if you would prefer to build the modified U-Boot from source: https://third-party-mirror.googlesource.com/u-boot/+/refs/heads/vim3

Access the U-Boot shell again by pressing the reset button and then repeatedly pressing the Space key in your serial console. When your serial console shows the kvim3# prompt, you're in the U-Boot shell.
In your U-Boot shell run the following command:
```
fastboot
```
You should see the following logs in your serial console:
```
g_dnl_register: g_dnl_driver.name = usb_dnl_fastboot

USB RESET
SPEED ENUM

USB RESET
SPEED ENUM
```
If you see the first line (g_dnl_register: g_dnl_driver.name = usb_dnl_fastboot) but not the lines after that, try using a different USB-C to USB-* cable and make sure that it supports both data and power delivery.
Open a new terminal window in your host and run the following commands:
```
cd ~/fuchsia/prebuilt/third_party/fastboot
./fastboot flashing unlock
./fastboot flashing unlock_critical
./fastboot flash bootloader ~/fuchsia/prebuilt/third_party/firmware/vim3/u-boot.bin.unsigned
./fastboot reboot
```
Important: When working with Fuchsia, remember to use the in-tree version of fastboot at ~/fuchsia/prebuild/third_party/fastboot/fastboot. The fastboot protocol allows arbitrary vendor protocol extensions and Fuchsia may rely on this functionality in the future.

Flash Fuchsia into the eMMC (one-time only)

Use this workflow only the first time you flash Fuchsia onto the VIM3. If you've already got Fuchsia running on the VIM3, use the Update your Fuchsia image workflow because it's faster.

If you just ran the ./fastboot reboot command from the last section then your VIM3 should already be in fastboot mode. You can check your fx serial logs to confirm. Otherwise press the reset button and then repeatedly press the F key in your fx serial console until you see USB RESET and SPEED ENUM again.

Caution: You have to press the F key now to enter fastboot mode. Previously you pressed the Space key.
From a separate terminal on your host run the following command:
```
fx flash
```

Your VIM3 is now running Fuchsia!

Update your Fuchsia image

Complete these steps when you already have Fuchsia running on your VIM3 and want to update the Fuchsia image.

Run the following command from a terminal on your host:
```
fx serve
```
Leave this command running.
Make some changes in your in-tree Fuchsia checkout and build the changes.
Open a new terminal window and perform an OTA update of the Fuchsia image on your VIM3:
```
fx ota
```

Reduce VIM3 noise by disabling its fan

If the VIM3 fan's loud noise is bothering you, you can disable it with any of the following workflows:

Add --args vim3_mcu_fan_default_level=0 to your fx set invocation.
Add vim3_mcu_fan_default_level=0 to ~/fuchsia/local/args.gn if you use persistent local build arguments.

Ensure that VIM3 tryjobs run

Include the following line in your commit message to ensure that VIM3 tryjobs run:

Cq-Include-Trybots: luci.turquoise.global.try:bringup.vim3-debug,core.vim3-debug,core.vim3-vg-debug

Troubleshooting

This section explains workarounds for common issues.

Troubleshooting: Bootloops

Problem:

You're looking at the VIM3 serial logs. The logs suggest that the VIM3 keeps rebooting.

Root cause:

Unknown. There appears to be an underlying bug in Khada's power delivery implementation. Fuchsia addressed the bug in issue 122113.

Workaround 1:

Un-plug and re-plug the USB cable. Repeat 2-3 times if necessary.

Workaround 2:

Always use a USB-C to USB-A cable.

Troubleshooting: Hardware mismatch

Problem:

This error when flashing fuchsia: Hardware mismatch! Trying to flash images built for vim3 but have 0

Solution:

Go back to Update the Android image step.

Appendix: Fix a bricked VIM3

Do these steps if you've bricked your VIM3 and need to "factory reset" it:

Erase the eMMC.
Update the Android image.
Update the bootloader.
Flash Fuchsia into the eMMC (one-time only).

Appendix: Support

For issues that seem related to VIM3 hardware or firmware, try the VIM3 official docs and Khadas VIM3 official forum.
For issues that seem related to Fuchsia, try the Fuchsia mailing lists and chat rooms.

Appendix: Feature support

Fuchsia currently supports these features of the VIM3:

UART Serial Debugger
Paving over ethernet and USB
Storage (eMMC)
HDMI Display and Framebuffer
GPU (Mali) and Vulkan graphics
Ethernet
SDIO
I2C
GPIO
Temperature Sensors and DVFS
RTC
Clock
Fan
NNA
USB-C in peripheral mode
USB-A
Audio¹

addition to the transducers, external hardware including DACs/ADCs need to be added and integrated via the GPIO header to be able to playback and capture audio this way.

These features are under development and may not be supported:

Video decoder
SPI

The following features are not supported, but might be added by future contributions:

SPI Flash
USB-C in host mode
Power management and PMIC
Wake on LAN
UART BT

These features are not supported and are unlikely to be added:

Video encoding (due to non-public firmware)
Trusted Execution Environment / secure boot

Appendix: Update the boot splash screen

To update the boot splash screen to be the Fuchsia logo, run the following command from a host terminal while the VIM3 is in fastboot mode:

~/fuchsia/prebuilt/third_party/fastboot/fastboot flash logo \
    ~/fuchsia/zircon/kernel/target/arm64/board/vim3/firmware/logo.img

VIM3 does not include transducers like speakers and microphones, in ↩