An open source hardware accessory that supports log capture, simplifies CLI interaction, and facilitates firmware updates for USB Type-C devices

Mar 16, 2023

Project update 3 of 5

Design & Testing Progress, Serial Over USB Type-C in the Wild, and a Design Example

by Dmitrii Votintcev

Our campaign is a success!! Thank you for all the support. We will be shipping USB-Cereal in the coming months! In the meantime we have made good progress:

The campaign progress is going well and in agreement with our schedule.

Serial Over USB Type-C In the Wild

USB-Cereal’s main goal is to provide a simple, hassle-free interface for serial communication with the device under development. Such an interface sounds like a good idea, and the industry tends to agree. In the field we have spotted a few known manufacturers who use the same or similar methods to pull logs from closed-case devices:

Having a serial console is indispensable for a fast development cycle and to debug low-level issues.

Couldn’t agree more! It is super useful to have UART readily available for various development and exploratory tasks.

Please let us know if your device uses UART on USB Type-C SBU pins—or if you know of any production devices that we might have missed—and we’ll feature it in future update!

Design Example

Some of the common design challenges around USB Type-C receptacles are related to the physical aspects of the port, especially considering the real-world use cases:

There are various white papers on the subject, including:

We invite you to have a look so you can make your USB Type-C designs as robust as possible. If such failure cases are not taken into account, they can cause regulatory hurdles or dysfunctional devices, either of which are likely to result in unwanted consequences for product manufacturers.

For the purposes of this update, however, let’s focus on the robust serial-link-over-SBU pins. Below is a proposed design that would help mitigate common failure modes:

A few off-the-shelf USB Type-C application-specific ICs can be used to protect the controller and other peripherals attached to the USB Type-C’s signal lines. I like TI’s TPD series of ICs for this purpose. It implements both ESD diodes and fast-acting (80 ns) overvoltage protection with a proper disconnect. The voltage threshold for the SBU-line OVP is set at just below 5 V (Vovp), so a downstream device might need further protection. Here it is likely appropriate to use simple current-limiting resistors to limit the power into the downstream device in case of an overvoltage event landing somewhere between Vcc_downstream and Vovp.

Please feel free to shoot us any comments or questions! We hope the engineering community adopts USB Type-C for development purposes with the simple USB-Cereal strategy!

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