Some Thoughts on the Processor Module

Back in my post on Broad Strokes, I described how I plan to divide the camera into a sensor module and a processor module. Since then I've rambled on about the sensor, but haven't said much about the processor. So let me run through my current thinking on the processor module.

I've settled on using a Gumstix Overo Air as a processing core. Gumstix as a company is very open source friendly, and all their products already have Linux ported and ready to go. Also, they publish full schematics for their various accessory boards. I can use these as design hints for the actual accessory board that I will have to make for the camera.

The Overo Air includes the processor, 256Meg SDRAM, 256Meg flash, a microSD card slot, and Wireless (802.11g) support. These alone make the board a reasonably complete Linux server. Load Linux on this board and I've got a lot of stuff for free. The Overo also includes connectors that carry many important processor signals out to an accessory board. This is where the application specific functionality gets added, so that's where my development effort needs to go.

Networking

The Overo Air COM board includes wifi already, so it is possible to declare victory right there. However, I've decided to throw in ethernet support. Although I expect the wireless support to be sufficient in most cases (and has the obvious advantage of being wireless), I think it prudent to include a wired network interface for a little extra speed and stability. Wifi in my home can sometimes flake on me when the neighbor turns on their routers; and besides, the wired link should be faster. So I will include ethernet support for extra stability, speed and flexibility.

Some Gumstix boards use the LAN9221 ethernet interface chip. This is a pretty nice chip as it includes the MAC and PHY for 100baseT in a single package. There are also known Linux drivers for this chip. All I need to do is add a socket, some magnetics and a few other discretes, and it's done. In fact, sockets are available with the magnetics and LEDs built in.

USB

I'm actually interested in USB host support. I have found optical filter changers that use USB as the source of power and control. I would like to be able to control these devices from the processor module, so the processor will need at least one USB Type A socket. The Type-A socket is the kind that you find on your computer and into which you plug your peripherals. So definitely I will include USB host support. And conveniently, the Overo includes most of the electronics for the USB host; beyond the socket and a few discretes, the problem is largely solved out of the box.

What about USB peripheral support? That would mean including a Type-B socket (like what you would find on a USB peripheral) and would allow one to connect to a host computer via USB. I've decided against it. It wouldn't take much to add the support (the Overo has the necessary signals) but with already two forms of networking supported, I don't think even more are required. The only possible advantage I might see is the ability to mimic other USB cameras and thus gain compatibility with some existing software.

I'm still wavering on the idea of also adding a USB connection to act as a serial console for the processor. This would require some extra parts (probably an FTDI FT232 or the like) to transparently connect a standard UART to USB. This would be darned handy when initially bringing up the board, but useless otherwise. So I'm undecided. Perhaps the thing to do is design it in, but leave the parts unpopulated if I decide not to do it.

User Interface

Most existing astronomy cameras have no user interface. Or, rather, all the user interface is on a host computer that drives the camera and collects the images. But my camera system will have the capacity for some autonomy. It would be completely silly to expect to do image post processing in the camera module (that is a job for when you get home) but there are some telescope operations that are best done without a computer getting in the way. I'm thinking specifically of focus adjustment and mount/tracker alignment. There may be other things as well, if the user interface support them.

A small LCD display with a touch screen seems perfect. A 480x272 display should be big enough to display a small start field (or large planet) along with some simple controls, or more complicated menus. A touch screen will allow all the input to be through drawn buttons. This should be plenty for doing many camera manipulations, and it is infinitely flexible, a highly desirable feature for open source projects.

So I will include the Samsung LTE430WQ LCD touch screen display. This is the device recommended by Gumstix (they sell them as accessories) so I'm confident that there will be available drivers. I'll be able to find reference schematics and other support, so not a lot of risk.

Motors

One of the tasks for the processor module is to provide optical tracking. To do this, it will need to be able to control the stepper motors of the mount, and for that it will need motor drivers. The Overo does not directly include stepper motor controls, but they should be relatively easy to implement in an accessory FPGA, along with some external circuitry to safely power the motors. This will have to be somewhat configurable in order to be able to handle different mounts. That can probably be handled by jumpers/pots to select voltages, and software to adjust step timings and calibrate angles.

To get full motion control, the controller will need to be able to drive at least 2 motors: R.A and declination. I also plan on adding two more motor drivers, one for possible focus control, and another for random mechanical accessories that I haven't thought of yet.

Power

And of course all of this needs power. The processor module will receive DC power from an external source, probably a rechargeable 12V battery. Besides taking power for itself, the processor will create the right voltages and power up the sensor module, the stepper motors and and USB peripheral. I will also need some sort of power control circuitry to allow a clean start/stop of the various units. This part could get interesting as I am no master of this subject. All I know for certain is that the processor module will take in battery power and turn it into clean, filtered supply voltages for the rest of the system.

Summary

The processor board clearly hosts a lot of the complexity of the Icarus Astronomy Camera. Fortunately, the Gumstix Overo bundles up a lot of the pieces. It provides the processor and the system basics, and gives me a head start on most of the peripherals. A lot of the remaining bits can be handled using off the shelf components, and many implementation questions can be answered by reference schematics. Incidentally, I had considered adding a kitchen sink, but concluded that functionality was already covered elsewhere.