I'm working on a design using a BF547 which uses the USB functionality, but the USB will only ever be put in to client mode (I've also seen this called 'device mode' or 'peripheral mode'). The BF547 datasheet says of USB_VBUS: "Pull high or low when unused", however we cannot clearly see whether 'unused' refers to being non-OTG mode or the entire USB being unused. I'm also aware that the USB_VBUS pin should not be exposed to 5V while VDDUSB is unpowered.
My questions are:
- If we are only using the USB in client mode, do we still need to connect the USB_VBUS pin to VBUS (or can we connect USB_VBUS to ground, avoiding the need for any protection circuitry relating to the 5V-while-unpowered problem?)
- If the VBUS connection is required, is it considered ok to not worry about the 5V-while-unpowered problem, given our device will have this condition briefly (for about a second) during startup; and after that it will only occur for long periods of time in unusual circumstances (e.g, battery goes completely flat and USB is active but in low-current draw mode for months).
Thanks in advance for any advice.