USB Support

This camera project is going to have USB support, but not the sort of USB that your personal camera has. As discussed earlier, the camera processor module will communicate with the host computer via WiFi, so there is no need for USB there. But I'm not talking about peripheral USB ports anyhow. In fact, I want to hang USB peripherals off the processor module.

Motivations

There are two specific uses I have in mind for USB support. The first is to control filter wheels. There are a variety of store-bought filter wheels available, and most (if not all) can be controlled by USB. These are typically controlled by the host computer, but that host will be some distance away from the telescope, so they really have to connect to the processor module via a USB type-A connector.

I would also like to be able to attach a USB camera to the processor module. Why? Because I would like to build and test the processor board before the sensor board, and it would be very handy to control store-bought USB cameras during testing. I'm thinking specifically of my Nikon-D80 camera, although there are other USB astronomy cameras as well. Also, the former might be great for taking context shots while the main telescope is shooting, and the latter may be useful as an auto-guider. In either case, the extra camera is the USB device and the processor will need another USB type-A connector.

Implementation

So I need two USB ports and both need to at least support being USB hosts. It turns out that the Overo board supports one USB 2.0 host port and one USB OTG port. The OTG port (USB On The Go) should be able to choose to be a host when connected to peripherals, so that will work out fine. Also, the power management chip on the Overo CPU board already includes all the power management needed for the OTG port to work, there is little for me to do there other then provide a socket and a couple simple discrete resisters/capacitors.

The USB 2.0 port is a little more complex. The power management chip doesn't help with this port, but the Overo CPU board does include an SMSC PHY to handle most of the USB protocol. It still requires an external power management chip, and so I've placed a TPS61202 to manage power. The USB PHY provides the signals to select the power mode, so this won't be a software issue.

And finally, there are sockets. The USB 2.0 port gets a mini-A socket, and the USB OTG gets a mini-AB. This by the way preserves the option of having this device act as a USB target. I can't imagine why I would want to do that, but it's literally free.

The schematic sheet for the USB interface is processor_usb.sch, with a matching symbol processor_usb-1.sym.