Access USB Devices on the Web

If I said plainly and simply "USB", there is a good chance that you will immediately think of keyboards, mice, audio, video, and storage devices. You're right, but you'll find other kinds of Universal Serial Bus (USB) devices out there.

These non-standardized USB devices require hardware vendors to write platform-specific drivers and SDKs in order for you (the developer) to take advantage of them. Sadly this platform-specific code has historically prevented these devices from being used by the Web. And that's one of the reasons the WebUSB API has been created: to provide a way to expose USB device services to the Web. With this API, hardware manufacturers will be able to build cross-platform JavaScript SDKs for their devices.

But most importantly this will make USB safer and easier to use by bringing it to the Web.

Let's see the behavior you could expect with the WebUSB API:

1. Buy a USB device.
2. Plug it into your computer. A notification appears right away, with the right website to go to for this device.
3. Click the notification. The website is there and ready to use!
4. Click to connect and a USB device chooser shows up in Chrome where you can pick your device.

Tada!

What would this procedure be like without the WebUSB API?

1. Install a platform-specific application.
2. If it's even supported on my operating system, verify that I've downloaded the right thing.
3. Install the thing. If you're lucky, you'll get no scary OS prompts or popups warning you about installing drivers/applications from the internet. If you're unlucky, the installed drivers or applications malfunction and harm your computer. (Remember, the web is built to contain malfunctioning websites).
4. If you only use the feature once, the code stays on your computer until you think to remove it. (On the Web, the space for unused is eventually reclaimed.)

# Before I start

This article assumes you have some basic knowledge of how USB works. If not, I recommend reading USB in a NutShell. For background information about USB, check out the official USB specifications.

The WebUSB API is available in Chrome 61.

# Available for origin trials

In order to get as much feedback as possible from developers using the WebUSB API in the field, we've previously added this feature in Chrome 54 and Chrome 57 as an origin trial.

The latest trial has successfully ended in September 2017.

# Privacy and security

# HTTPS only

Because of this feature's power, it only works on secure contexts. This means you'll need to build with TLS in mind.

# User gesture required

As a security precaution, navigator.usb.requestDevice() may only be called through a user gesture such as a touch or mouse click.

# Permissions Policy

A Permissions Policy is a mechanism that allows developers to selectively enable and disable various browser features and APIs. It can be defined via an HTTP header and/or an iframe "allow" attribute.

You can define a Permissions Policy that controls whether the usb attribute is exposed on the Navigator object, or in other words if you allow WebUSB.

Below is an example of a header policy where WebUSB is not allowed:

Feature-Policy: fullscreen "*"; usb "none"; payment "self" https://payment.example.com

Below is another example of a container policy where USB is allowed:

<iframe allowpaymentrequest allow="usb; fullscreen"></iframe>

# Let's start coding

The WebUSB API relies heavily on JavaScript Promises. If you're not familiar with them, check out this great Promises tutorial. One more thing, () => {} are simply ECMAScript 2015 Arrow functions.

# Get access to USB devices

You can either prompt the user to select a single connected USB device using navigator.usb.requestDevice() or call navigator.usb.getDevices() to get a list of all connected USB devices the website has been granted access to.

The navigator.usb.requestDevice() function takes a mandatory JavaScript object that defines filters. These filters are used to match any USB device with the given vendor (vendorId) and, optionally, product (productId) identifiers. The classCode, protocolCode, serialNumber, and subclassCode keys can also be defined there as well.

For instance, here's how to get access to a connected Arduino device configured to allow the origin.

navigator.usb.requestDevice({ filters: [{ vendorId: 0x2341 }] })
.then(device => {
  console.log(device.productName);      // "Arduino Micro"
  console.log(device.manufacturerName); // "Arduino LLC"
})
.catch(error => { console.error(error); });

Before you ask, I didn't magically come up with this 0x2341 hexadecimal number. I simply searched for the word "Arduino" in this List of USB ID's.

The USB device returned in the fulfilled promise above has some basic, yet important information about the device such as the supported USB version, maximum packet size, vendor, and product IDs, the number of possible configurations the device can have. Basically it contains all fields in the device USB Descriptor.

// Get all connected USB devices the website has been granted access to.
navigator.usb.getDevices().then(devices => {
  devices.forEach(device => {
    console.log(device.productName);      // "Arduino Micro"
    console.log(device.manufacturerName); // "Arduino LLC"
  });
})

By the way, if a USB device announces its support for WebUSB, as well as defining a landing page URL, Chrome will show a persistent notification when the USB device is plugged in. Clicking this notification will open the landing page.

# Talk to an Arduino USB board

Okay, now let's see how easy it is to communicate from a WebUSB compatible Arduino board over the USB port. Check out instructions at https://github.com/webusb/arduino to WebUSB-enable your sketches.

Don't worry, I'll cover all the WebUSB device methods mentioned below later in this article.

let device;

navigator.usb.requestDevice({ filters: [{ vendorId: 0x2341 }] })
.then(selectedDevice => {
    device = selectedDevice;
    return device.open(); // Begin a session.
  })
.then(() => device.selectConfiguration(1)) // Select configuration #1 for the device.
.then(() => device.claimInterface(2)) // Request exclusive control over interface #2.
.then(() => device.controlTransferOut({
    requestType: 'class',
    recipient: 'interface',
    request: 0x22,
    value: 0x01,
    index: 0x02})) // Ready to receive data
.then(() => device.transferIn(5, 64)) // Waiting for 64 bytes of data from endpoint #5.
.then(result => {
  const decoder = new TextDecoder();
  console.log('Received: ' + decoder.decode(result.data));
})
.catch(error => { console.error(error); });

Keep in mind that the WebUSB library I'm using is just implementing one example protocol (based on the standard USB serial protocol) and that manufacturers can create any set and types of endpoints they wish. Control transfers are especially nice for small configuration commands as they get bus priority and have a well defined structure.

And here's the sketch that has been uploaded to the Arduino board.

// Third-party WebUSB Arduino library
#include <WebUSB.h>

WebUSB WebUSBSerial(1 /* https:// */, "webusb.github.io/arduino/demos");

#define Serial WebUSBSerial

void setup() {
  Serial.begin(9600);
  while (!Serial) {
    ; // Wait for serial port to connect.
  }
  Serial.write("WebUSB FTW!");
  Serial.flush();
}

void loop() {
  // Nothing here for now.
}

The third-party WebUSB Arduino library used in the sample code above does basically two things:

• The device acts as a WebUSB device enabling Chrome to read the landing page URL.
• It exposes a WebUSB Serial API that you may use to override the default one.

Look at the JavaScript code again. Once I get the device picked by the user, device.open() runs all platform-specific steps to start a session with the USB device. Then, I have to select an available USB Configuration with device.selectConfiguration(). Remember that a configuration specifies how the device is powered, its maximum power consumption and its number of interfaces. Speaking of interfaces, I also need to request exclusive access with device.claimInterface() since data can only be transferred to an interface or associated endpoints when the interface is claimed. Finally calling device.controlTransferOut() is needed to set up the Arduino device with the appropriate commands to communicate through the WebUSB Serial API.

From there, device.transferIn() performs a bulk transfer onto the device to inform it that the host is ready to receive bulk data. Then, the promise is fulfilled with a result object containing a DataView data that has to be parsed appropriately.

If you're familiar with USB, all of this should look pretty familiar.

# I want more

The WebUSB API lets you interact with the all USB transfer/endpoint types:

• CONTROL transfers, used to send or receive configuration or command parameters to a USB device, are handled with controlTransferIn(setup, length) and controlTransferOut(setup, data).
• INTERRUPT transfers, used for a small amount of time sensitive data, are handled with the same methods as BULK transfers with transferIn(endpointNumber, length) and transferOut(endpointNumber, data).
• ISOCHRONOUS transfers, used for streams of data like video and sound, are handled with isochronousTransferIn(endpointNumber, packetLengths) and isochronousTransferOut(endpointNumber, data, packetLengths).
• BULK transfers, used to transfer a large amount of non-time-sensitive data in a reliable way, are handled with transferIn(endpointNumber, length) and transferOut(endpointNumber, data).

You may also want to have a look at Mike Tsao's WebLight project which provides a ground-up example of building a USB-controlled LED device designed for the WebUSB API (not using an Arduino here). You'll find hardware, software, and firmware.

# Revoke access to a USB device

The website can clean up permissions to access a USB device it no longer needs by calling forget() on the USBDevice instance. For example, for an educational web application used on a shared computer with many devices, a large number of accumulated user-generated permissions creates a poor user experience.

// Voluntarily revoke access to this USB device.
await device.forget();

As forget() is available in Chrome 101 or later, check if this feature is supported with the following:

if ("usb" in navigator && "forget" in USBDevice.prototype) {
  // forget() is supported.
}

# Limits on transfer size

Some operating systems impose limits on how much data can be part of pending USB transactions. Splitting your data into smaller transactions and only submitting a few at a time helps avoid those limitations. It also reduces the amount of memory used and allows your application to report progress as the transfers complete.

Because multiple transfers submitted to an endpoint always execute in order, it is possible to improve throughput by submitting multiple queued chunks to avoid latency between USB transfers. Every time a chunk is fully transmitted it will notify your code that it should provide more data as documented in the helper function example below.

const BULK_TRANSFER_SIZE = 16 * 1024; // 16KB
const MAX_NUMBER_TRANSFERS = 3;

async function sendRawPayload(device, endpointNumber, data) {
  let i = 0;
  let pendingTransfers = [];
  let remainingBytes = data.byteLength;
  while (remainingBytes > 0) {
    const chunk = data.subarray(
      i * BULK_TRANSFER_SIZE,
      (i + 1) * BULK_TRANSFER_SIZE
    );
    // If we've reached max number of transfers, let's wait.
    if (pendingTransfers.length == MAX_NUMBER_TRANSFERS) {
      await pendingTransfers.shift();
    }
    // Submit transfers that will be executed in order.
    pendingTransfers.push(device.transferOut(endpointNumber, chunk));
    remainingBytes -= chunk.byteLength;
    i++;
  }
  // And wait for last remaining transfers to complete.
  await Promise.all(pendingTransfers);
}

# Tips

Debugging USB in Chrome is easier with the internal page about://device-log where you can see all USB device related events in one single place.

The internal page about://usb-internals also comes in handy and allows you to simulate connection and disconnection of virtual WebUSB devices. This is be useful for doing UI testing without for real hardware.

On most Linux systems, USB devices are mapped with read-only permissions by default. To allow Chrome to open a USB device, you will need to add a new udev rule. Create a file at /etc/udev/rules.d/50-yourdevicename.rules with the following content:

SUBSYSTEM=="usb", ATTR{idVendor}=="[yourdevicevendor]", MODE="0664", GROUP="plugdev"

where [yourdevicevendor] is 2341 if your device is an Arduino for instance. ATTR{idProduct} can also be added for a more specific rule. Make sure your user is a member of the plugdev group. Then, just reconnect your device.

# Resources

• Stack Overflow: https://stackoverflow.com/questions/tagged/webusb
• WebUSB API Spec: http://wicg.github.io/webusb/
• Chrome Feature Status: https://www.chromestatus.com/feature/5651917954875392
• Spec Issues: https://github.com/WICG/webusb/issues
• Implementation Bugs: http://crbug.com?q=component:Blink>USB
• WebUSB ❤ ️Arduino: https://github.com/webusb/arduino
• IRC: #webusb on W3C's IRC
• WICG Mailing list: https://lists.w3.org/Archives/Public/public-wicg/
• WebLight project: https://github.com/sowbug/weblight

Send a tweet to @ChromiumDev using the hashtag #WebUSB and let us know where and how you're using it.

# Acknowledgements

Thanks to Joe Medley for reviewing this article.