Hi Phil
I've attached a photo of a sketch, so I hope it's legible.
My computer is not set up for CAD at the moment.
The mosfets on my board are connected to the MSB of portb because initially I thought I would need the built in comparator. The code can easily accommodate your connection by shifting the drive byte right 4 bits.
I've included pull down resistors on the gates so that when the mcu is in reset, the pins won't float and cause the mosfets to potentially burn up.
I'm only driving one coil at a time. If I'm not mistaken, with heat as the limiting factor, increasing the current on a single coil is more efficient than driving 2 phases. So, if you're not doing microstepping, it might be possible to use a single current sensing resistor with the built in comparator and not have to graft on an external op amp. I haven't tried it, so I don't know if the switching transients would cause false triggering.
My mcu is running at 10 Mhz with an external crystal. My code isn't optimized yet, so the 8 Mhz rc oscillator might take too long to respond to the interrupts. I'll be glad to post my code if you're interested in trying it out. I'm a bit reluctant at the moment because it is still work in progress.
I'm using the image craft avr compiler. The eval version is limited to 4k after 45 days. I will look at avrgcc when I have time.
http://www.imagecraft.com/software/demos.html