Guest mattlokes Posted February 23, 2011 Report Posted February 23, 2011 Hi guys does anyone know what the USB current limit of the Vega is when the device is in host mode? Im trying to work out how much current my usb HDD is pulling!
Guest MaxiP Posted February 23, 2011 Report Posted February 23, 2011 From a quick google.... "Low power bus powered functions draw all its power from the VBUS and cannot draw any more than one unit load. The USB specification defines a unit load as 100mA. Low power bus powered functions must also be designed to work down to a VBUS voltage of 4.40V and up to a maximum voltage of 5.25V measured at the upsteam plug of the device. For many 3.3V devices, LDO regulators are mandatory. High power bus powered functions will draw all its power from the bus and cannot draw more than one unit load until it has been configured, after which it can then drain 5 unit loads (500mA Max) provided it asked for this in its descriptor. High power bus functions must be able to be detected and enumerated at a minimum 4.40V. When operating at a full unit load, a minimum VBUS of 4.75 V is specified with a maximum of 5.25V. Once again, these measurements are taken at the upstream plug." So max would be 5.25V or 500mA
Guest simonta Posted February 23, 2011 Report Posted February 23, 2011 (edited) So max would be 5.25V or 500mA Not quite. You can't have volts or amps. You always have both! 500ma is the max load regardless of the voltage, but that's also a maximum per device, not per hub. If the Vega hub is powered (don't know if it is), then the maximum load on the hub is 1600ma. 3x500ma for each device and 100ma for the hub. If it's bus powered, then the max is 400ma for all devices as the hub takes one unit for itself. If the drive tries to draw more than the hub provides, it simply won't work, the hub will shut down the port. Hubs are designed not to fry. The only answer then is to use a powered hub as an extension to the built in one so the device draws it's power from the extension hub, not the host. Finally, to answer the question, you can't figure it out. You can measure it (which would need you to get an ammeter in series with the UBS power line - not easy), or read the manual! Edited February 23, 2011 by simonta
Guest mattlokes Posted February 23, 2011 Report Posted February 23, 2011 Thanks for the answers :huh: i knew it would be either 500mA or 100mA i just didn't want to get the ammeter out and the usb breakout boards. Going on a long haul flight and i want to be able to power the USB hdd off of another battery rather than the vegas usb power so that it will last longer. Cheers for your input :)
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