Electronic – Product Development using ARM mBED OS

I've been blogging about the Linux board I've been working on starting here and continuing here. I started thinking I would use a Cortex A8, but eventually settled on the Atmel AT91SAM9G20.

To try to answer your other questions:

• You can see the components I used in my schematic, but I don't know of a more general explanation of why each is necessary.
• The processor is only about 10-20% of the total cost of parts. Assembly is another $4-40, depending on the quantity built. I suspect the profit margin is 30-50%.
• Depending on the package, the OMAP3517 BGA package has either a 1 mm or 0.65 mm ball pitch. Generally, below 0.8 mm pitch, you need to X-ray at least some fraction of the finished boards to check for errors. (Just for the record, the OMAP3517 1 mm packages aren't actually available yet.)

If you have other, more specific questions, I'd be glad to try to answer them.

You can follow the recipe at http://mbed.org/cookbook/Motor which uses the Motor interface for driving a standard DC motor with PWM and an H-Bridge.

If, instead of using the L293D dual H bridge driver, you want to use a transistor to drive the motor. You choose the transistor mainly by looking for a switching transistor which has a current and voltage rating that exceed the current and voltage ratings of the motor (so it depends on the motor) - for high currents you may need a heatsink for the transistor.

For a bipolar junction transistor (BJT) you'll need a resistor to limit base current. Most driver circuits incorporate a diode to prevent harm to the transistor from back-EMF from the motor windings.

The connections to a BJT are base, emitter, and collector

You can find out which leg is which by looking at the data sheet for the specific transistor you have chosen. For example

You can instead use a MOSFET, these can be more efficient (lower voltage drop when 'on')

Some rather good instructions can be found here