USB – Understanding USB Backwards-Compatibility

usb

Is the USB 3.0 (and latest 3.2) specification only guaranteed to be
backwards compatible to USB 2.0?

Some Arduino microcontrollers follow the original low-speed 1.5 Mb/s
USB standard, so equivalently, are new smartphones and laptops not
guaranteed to work with them? A specific example product which
does not support low-speed USB would help.

Best Answer

Latest USB (3.0-3.1-3.2) specifications guarantee to include USB 2.0 as a "side-band" channel, so USB 2.0 runs on independent set of wires (legacy D+ and D-). This is the requirement of the standard. The USB 2.0, in turn, guarantees backward compatibility with FS (12 Mbps signal rate) and LS (1.5 Mbps rate) devices.

Low-speed and FS USB devices aren't going to disappear soon, because the cost (and energy consumption) of Super-Speed protocol controllers is prohibitive, and require thick ugly stiff cables which are not ergonomic.

All new laptops/PCs/smartphones use processors and chipsets with USB based on Intel's xHCI controller specifications, with IP provided mostly by Synopsys, and modern xHCI IP includes all USB 2.0 functionality, and therefore supports all legacy modes.

ADDENDUM: USB 3.2 specifications, in Section 3 "Architectural Overview", explicitly states:

USB 3.2 is a dual-bus architecture that provides backward compatibility with USB 2.0. One bus is a USB 2.0 bus (see Universal Serial Bus Specification, Revision 2.0) and the other is an Enhanced SuperSpeed bus (see Section 3.1). USB 3.2 specifically adds dual-lane support.

As one can see, it explicitly includes the USB 2.0 Specifications. They, in turn, read in Section 4.2.2

There are three data rates:

• The USB high-speed signaling bit rate is 480 Mb/s.

• The USB full-speed signaling bit rate is 12 Mb/s.

• A limited capability low-speed signaling mode is also defined at 1.5 Mb/s.

USB 2.0 host controllers and hubs provide capabilities so that full-speed and low-speed data can be transmitted at high-speed between the host controller and the hub, but transmitted between the hub and the device at full-speed or low-speed. This capability minimizes the impact that full-speed and low-speed devices have upon the bandwidth available for high-speed devices.

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