Dave Madison's "Minimalist" Shield Makes the Teensy 4.0's USB Host Capabilities Easier to Access

Maker Dave Madison has put together a "Minimalist USB Host Shield," designed to take advantage of the Teensy 4.0's built-in capacity to act as a USB Host — creating an add-on that is "as physically small as possible."

"One of the awesome features of [the Teensy 4.0] is its built-in USB Host capability," Madison explains. "This allows the microcontroller to talk to other USB devices such as human interface devices (mice, keyboards, joysticks), MIDI controllers, and even other development boards over USB serial. While some previous boards such as the Teensy 3.6 had this feature, this is the first Teensy board to include USB host functionality in a small format package."

While the functionality is built-in, however, not all of the hardware is present — and it's not just the USB Type-A socket that's missing: There's no current limiter, meaning that hot-plugging a USB device could brown-out the Teensy — or draw too much power and overload the upstream USB port.

"To solve these problems, the design adds a current limit switch with a soft start feature," Madison says. "This slowly ramps up the current for the Teensy's host port, and allows the use of a large (100µF) capacitor to handle smaller surges. The current limit switch also allows devices to be 'hot plugged,' meaning connected and disconnected while the microcontroller is running. As a bonus, the specific IC I used also provides some electrostatic discharge (ESD) protection on the USB data pins."

The compact PCB is designed to mimic the footprint of the Teensy 4.0 board itself, plus the USB Type-A port required to accept a USB device. There's also a second row of general-purpose input/output (GPIO) headers, which allows a Teensy-and-shield sandwich to be connected to a breadboard, alongside two mounting holes.

"Connected USB devices can be interfaced with using the USBHost_t36 library," Madison writes. "So far I’ve tested the shield with a Logitech mouse, a MIDI keyboard, and a handful of controllers (PS3, PS4, Xbox 360, Xbox One, etc.). All performed great and without issue. The shield has no digital logic and thus no special configuration is required when programming. It's entirely plug and play with any existing Teensy 4.0 USB host programs."

More details on the project are available on Madison's Parts Not Included website, while the design files, Gerbers, and bill of materials have been published to GitHub under the a public-domain license.