Can an Acroname hub create a data-only USB connection to a phone?

2021 April 19

A primary feature of Acroname hubs is the ability to turn a USB port on and off. Looking more closely, Acroname hubs also have the more granular ability to disable power/VBUS on a given port, independently from the USB D+/D- data lines on the same port. The logical thought is that this creates a scenario where a data-only / non-charging connection to the device or phone is possible - quite useful for mobile device labs or applications where long-term charging or over-charging is not desired.

In practice, a data-only USB connection is not usually possible with commonly available or shipping phones or other USB devices.

In order to save power, mobile devices typically disable their USB engines until power from the host (usually +5V on VBUS) is present. The presence of power from the host or hub indicates that a host connection is available and the device or phone will then re-enable its USB data processing engines. This behavior is rooted in the phone firmware and is the expected behavior of most commercially available mobile devices.

When the Acroname hub is instructed to disable VBUS/power to the mobile device, but keep the data lines connected, the mobile device will interpret the absence of VBUS to mean that the host has been disconnected - and that there is no host present. Under this condition, the mobile device will still have a then subsequently also shut down it’s USB engine and data connection even though data lines are still electrically and physically connected between the hub and the device.

By issuing custom firmware, some device manufacturers have the ability to bypass this behavior so that a data-only connection can be established, but this is usually not an option for test system designers who do not work directly with or for the device manufacturer.

These behaviors are typical of USB devices or phones using connectors prior to USB-C. Native USB Type C devices use a different criteria to determine if a host is connected, so a different set of challenges and mechanisms are involved when connecting and/or powering these devices.

One workaround to help solve the overcharging condition is to instruct the downstream-facing port on the hub to report a lower charging power capability. This is possible by disabling Charging Downstream Port (CDP) mode, which indicates support for high current charging. Disabling CDP mode for any port causes that port to report support for Standard Downstream Port (SDP) mode, which usually results in the phone drawing lower current from the hub.

Keep in mind that although switching between CDP and SDP modes can be performed on any port individually, a change to the CDP/SDP settings does require saving settings and a reset / power cycle of the hub. Some customers have implemented this same CDP/SDP mode switch by using a secondary set of USB-C-Switches behind the hub in order to dynamically switch between CDP and SDP modes; thus avoiding having to reset the hub when switching CDP/SDP modes. One example is illustrated in Acroname’s blog post about eliminating USB device overcharge.

Acroname has developed additional technologies to help minimize overcharge conditions and we can share some of those details with you and your team at your request. If you have additional questions, or would like to discuss working with Acroname on your lab or scaled USB charging solution, please reach out to Acroname’s technical team any time at support@acroname.com or via phone at +1 720-564-0373.

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