usbusb device
A self-powered USB peripheral provides its own power, so is it okay to connect just D+, D- and GND to the host port, or do you always have to connect +5V as well, then wait for the negotiation phase for the device to tell the host that it is self-powered and therefore doesn't want any power, thank you very much?
The specific peripheral I'm looking at is based on an FTDI FT4232H USB-to-serial adapter, but I'm really hoping for a generic answer to the question.
If you are willing to sacrifice one of the hub ports, FTDI has devices that talk UART over USB and have a bunch of IO-lines on them that support bit banging, ready to use. Advantage is that is only a single chip and most operating systems have drivers available.
I believe it is this one: http://www.ftdichip.com/Products/ICs/FT232H.htm
That way you can use one of the hub's USB ports to connect to the chip that can be programmed to control the outputs.
From the datasheet:
Bit Bang Mode.
The 2 nd generation device has a new option referred to as “Bit Bang” mode. In Bit Bang mode, the eight UART interface control lines can be switched between UART interface mode and an 8 - bit Parallel IO port.
Would be a nice fit with your option #2
Connect the data lines from the USB port to the device, connect the ground of the external power supply to the USB ground and connect the external supply positive line to the device side USB supply only
Here is one example that shown the described connection scheme using a linear regulator as power source (limited to 1A that may not be enough) but you can use any 5v source.
In order to switch the power of the USB client when the plug is removed you can use a mosfet that turns on when the plug is inserted to the PC.
When there is no Vcc voltage in the base of the transistor the resistor R1 works as a pullup and keeps the mosfet off by keeping the gate high.
When the plug is inserted the PC side Vcc is applied to the base of the transistor so the current that flows through it creates a voltage drop across R1 and turns on the mosfet.
Note that the mosfet should be a logic level type that turns fully on with -5v Vgs.
Also the input should not exceed the max allowable Vgs (usually 20v but this is absolute max)
Another alternative is when a switching regulator is used that has a control pin, in that case the PC side Vcc supply can be used as a signal to turn the regulator on when the USB plug is connected.
The following schematic (I've modified the USB plugs slightly) shows such an example using a 1A switching regulator but the same principle applies to switching regulators of higher current.
Best Answer
The generic answer to this question is yes, the VBUS (+5V from cable) must be connected to the device even if it is self-powered. The reason is as follows:
To start the connect process on host side, the device must pull up D+ (in case of FS/HS mode), or D- (in case of LS device).
However, USB specifications have a mandatory requirement that no USB device should source any current on any interface pin unless it is connected to a cable, see section 7.1.5.1, which reads,
If a USB device doesn't have this control, one of data lines will be a source of current. Premature assertion of pull-ups were a source of problems for some legacy USB hosts. That's why this rule was instituted, and there is a special test for this in USB-IF certification program.
Therefore, the USB VBUS is an important "side-band" signal in USB connect protocol. As such, normal USB device ICs do have a separate input pin to sense the presence of USB host. Some IC manufacturers (e.g. FT232H, MCP2221, etc.) skip on this requirement, assuming that their chip will be solely used in bus-powered configuration, where the pull-up control requirement is automatically satisfied. However, when designing these chips into self-powered designs, some extra circuit efforts are needed to link the enabling of pull-ups with presence of VBUS on the USB port.
Regarding the USB connect "handshake" protocol, USB doesn't rely on current drawn from VBUS. The protocol is this: Host port must have VBUS active; VBUS is connected to device; device sees the VBUS and pulls-up 1.5k on one of D+/D- wires; host sees this connect, and after a 100ms delay asserts USB_RESET signaling (SE0 etc.).