Powering USB's Vcc pin

[asp87]

Hi,

I have a PIC microcontroller connected to Beaglebone Black (similar to Raspberry Pi). The micro as well Beaglebone Black both have its own power supply.

Is it necessary to connect the Vcc pin of USB socket to +5V? If we leave it unconnected and just use these three pins- Rx, Tx and GND, it should work correctly, right?

 

[FvM]

The USB standard specifies that self-powered devices shall monitor VUSB to detect attachement and detachment. An USB device designed in concordance with the standard thus won't operate without VUSB. Your self-elaborated PIC device possibly will.

 

[asp87]

Hi,

Thanks for the reply. I'm very new to USB communication.

1. I'm using PIC18f4550, a 5V PIC device and the Vusb pin is grounded using a 470nF capacitor. Since you said attachment/detachment monitoring is through Vusb pin, how can monitoring be possible in this case?
2. While connecting two self powered devices, do we interconnect the +5V lines? This sounds bit dangerous. The interconnection should only be for Rx, Tx and ground lines, right?

 

[FvM]

I wrote VUSB but meaned VBUS.

VUSB is the internal driver supply of the PIC USB interface. It's not grounded, but connected to a bypass capacitor.

PIC18 has specific no pin to connect VBUS, a GPIO pin has to be used in a self-powered design. See the paragraph "USB Power Modes" in PIC data sheet.