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WebLight Design

Goal

To create a simple and inexpensive device that shows off WebUSB and helps validate the specification.

Overview

WebLight is a small circuit on a standalone printed circuit board that contains multicolored lights controllable by the computer it's plugged into. When plugged into a Type A USB receptable (the kind normally found on desktop and laptop PCs), it appears as a custom-class USB device that can be controlled by host software. Additionally, it responds to requests for WebUSB descriptors that indicate its willingness to communicate with certain web origins through the WebUSB API.

The user benefit of WebUSB is that hardware devices are even more plug-and-play than the traditional sense of that term: any WebUSB-compatible device will work without the requirement to install USB drivers.

Circuit

  • Influenced by Adafruit Trinket, V-USB design suggestions, and the DigiSpark.

  • We chose the "Level conversion on D+ and D-" option for solving the mismatch between 5v AVR/LED and 3.3v USB data lines. The reason we picked that approach rather than a lower-voltage AVR circuit is that it requires fewer and less-expensive components, and the disadvantages ("Not a clean solution," slight power wastage) wouldn't affect the end product in noticeable ways.

PCB

  • HASL (Hot Air Solder Leveling) is cheapest and easiest for manual soldering, but it usually contains lead (lead-free HASL is available).

  • ENIG (Electroless Nickel Immersion Gold) is expensive and looks nice. It's harder to deal with when manually soldering.

  • Most connectorless PCBs in production range from 2.4 to 2.6mm in thickness. This thickness is not available from short-run PCB manufacturers.

  • 2.0mm is reputed to work OK, and short-run PCB manufacturers support it.

  • The SparkFun EAGLE part for connectorless type-A says "If possible, ensure that your PCB is about 2.4mm thick to fit snugly."

  • 1.6mm is typical for prototyping PCBs. This works with a piece of paper taped on the back of the PCB, beneath the connector area. Otherwise it's a loose fit that is unreliable in some USB ports.

  • Typical width of connectorless PCBs is 12mm.

  • Length doesn't matter, except for ergonomics.

  • Making the PCB pluggable in either direction (upside-down or normal) requires extra diodes.

Microcontroller

  • Atmel AVR is inexpensive and has a fully open toolchain. I have experience with AVR.

  • Microchip PIC is usually the other contender. I don't have experience with PICs. While there are free compilers, the widely used ones are not open. For at least the past five years, the open-source SDCC has been aiming to support PIC16 and PIC18.

  • The ATTiny85 is a popular choice for small applications that bitbang USB. The SOIC8 version costs about 75 cents in quantity 100+.

  • The ATMega 'u2 and 'u4 series have built-in USB support, but they are relatively expensive (more than $3/unit in quantity).

LED

  • The WS2812B is the current hobbyist's choice for low-cost addressable RGB LED. It has substantial software support thanks in part to the Adafruit Neopixel brand. Brightness is adequate; maximum current is about 60mA at 5V. Disadvantages: the component has a reputation for being easily killed by voltage spikes, leading to best-practice recommendations to protect with large capacitors and resistors. The chip protocol is extremely timing-sensitive, meaning it will monopolize the microcontroller's execution for significant periods of time. Finally, the refresh rate is relatively low (400Hz), which makes it inappropriate for some applications (but isn't a consideration for this design).

  • The APA102C has a similar form factor to the WS2812B (5050 SMD) and similar luminosity and power characteristics. However, it uses an SPI-like protocol that is not sensitive to timing, making it ideal for an application like this where the microcontroller will be servicing timing-sensitive bitbanged USB requests. As of October 2015, is slightly more expensive than the WS2812 in quantity (12 cents vs. 10 cents), but because the maker and DIY lighting communities are embracing it, its price should come down soon.

Firmware

  • V-USB is the standard for AVR USB bitbanging. Its license terms are annoying but tolerable for this entirely open-source project.

  • Micronucleus is a bootloader based on V-USB. It has a smaller binary size, is reported to be faster, and claims better compatibility.

  • Having a bootloader is desirable during development, and likely to be useful to update production devices. It might turn out to be unnecessary for production devices, but today there is little cost to keeping it because we aren't bumping up against program-space limits.

  • light_ws2812 is a much lighterweight WS2812 library compared to Adafruit NeoPixel. (As noted above, WS2812 is not the long-term choice for this project.) This library also includes support for the APA102 chipset.

  • As shown in the Adafruit DotStar Arduino and Polulu libraries for APA102, there isn't a tremendous need for a large or resource-intensive software library for this LED.

  • TODO: describe V-USB and usbFunctionSetup()

WebUSB

  • TODO

  • TODO: describe challenge with WinUSB

USB VID/PID

Prototype Manufacturing

Production Manufacturing

  • TODO