I am designing a small battery operated device that must operate without a USB connection, but at times connect to USB for both communication and battery charging. I would like to use 2.5V for Vddd. I am concerned about the signaling voltage levels on D+ and D-. My research suggests that these need to have a 3V differential between them. But how is this possible when Vddd is only 2.5V? I don't see facility anywhere to use the 5V VBUS for *strictly* USB purposes, because D+ and D- pins are specially designed to use Vddd and not a Vddio pin.
Meanwhile, in the PSoC5 LP datasheet I find on page 73 table 11-14, cases quoting Vddd=1.8V. This implies the task can be done with Vddd=2.5V. But how is Vohusb in the same table produced to be between 2.8V and 3.6V? I have seen mention of a 3.3V regulator, but if that's the solution, I would worry that it's only a step-down regulator without step-up capability from 1.8V or 2.5V up to 3.3V.
I've been searching for a while for an appnote or other advice on how to do PSoC5 LP usb communication with Vddd below 3V, such as my desired 2.5V or the previously table implied 1.8V. But I can't find anything.
Is there advice out there somewhere? Or can you please offer advice? Is there a simple solution here?
It does occur to me to make Vddd automagically float up from 2.5V (battery power) to 3.3V (when USB connected), and then use a separate Vddio=2.5V pin for interfacing with my 2.5V-only peripheral. Perhaps involve an ideal-diode chip. But this gets a little complicated. I'd rather not have to do that if there is a more simple solution.
Finally, I would also like for my software to know when the USB is connected. For this purpose, I am considering using the VBUS to GPIO circuit on page 16 of the USBFS_v2_50.pdf datasheet. I realize this uses the resistor divider to reduce 5V to 3.3V, so I would use two 30K resistors to reduce 5V to 2.5V. Yes, I know this is simple and understand it. I came to this point, however, by considering configuring the USBFS component to do "Enable VBUS Monitoring". I believe that's intended for self-powered devices. Maybe I should just use it as GPIO and be done with it, But the whole "Enable VBUS Monitoring" and self-powered vs bus-powered question sort of bleeds over into this subject as well.
Thanks very much in advance for your help.