Determine what USB_VBUS should be connected for self-powered

myelin / modular-arm-usb

LPC11U14 USB test board by Phil Pearson and Philip Lindsay

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Determine what USB_VBUS should be connected for self-powered #5

Open follower opened 9 years ago

follower commented 9 years ago

"what usb_vbus should be connected to if battery powered but not connected to USB..." -- pjl

myelin commented 9 years ago

It looks like (from the LPC11U2x datasheet, 11.1) this needs to be a logic high while the USB interface is connected, but it shouldn't ever get above 3.6V unless VDD is at least 3.0V (min for USB operation).

Two options: use the resistive voltage divider suggested in the datasheet, or use a zener + resistor. (33k will draw 0.15 mA, so that's probably okay.)

follower commented 9 years ago

"For a bus-powered device, the VBUS signal does not need to be connected to the USB_VBUS pin (see Figure 28)."

So, it seems like if "USB_VBUS pin [is enabled] for GPIO" then it's okay not to have "USB_VBUS" connected to anything?

(Oh, I guess that's what's implied by making it MVP+. Nevermind. :) )