AT90USB1287 - USB Regulator Question

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Hi Guys,

Currently working with the USB1287, as many of you know. So far so good, with the library device mode functional and the host mode not very far behind at all.

However, I have a question about the internal USB regulator in the USB1287.

The datasheet indicates that the regulator is used to regulate the voltage of the D+/D- pads, which must be within a 3.0-3.6V range at all times. The AVR can operate all the way up to 5V or so, hence the need for the regulator to ensure the correct voltage on the pins.

However, it would seem that the regulator is optional - it is enabled and disabled by a bit in a control register. I would expect that this is done so that applications using a rail voltage within the USB spec range can disable the regulator and save some current.

My question is thus: does disabling the internal USB pad regulator result in no operation (i.e., the control bit is to save power when the USB is not being used at all), or is it to save power when a USB spec voltage is used as the AVR's supply rail (and thus without the regulator the D+/D- pads assume rail voltage)?

- Dean :twisted:

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