[fprint] Improving the etes603 driver

[Jason Gerard DeRose]

Patrick,

Another update: following your advice that tuning is where I should 
start, I made some big progress today, so thanks for the tip!

I started by looking at what values your automatic tuning is setting for 
the hardware I'm testing on.

When I first use the device, the initial tuning seems to consistently 
get set at:

gain=0x23, dcoffset=0x32, vrt=0x09, vrb=0x0F

If I then repeatedly bring up the device by doing enrollments in fairly 
quick succession, the automatic tuning values seem to stabilize at:

gain=0x23, dcoffset=0x32, vrt=0x08, vrb=0x0E

I'm guessing the small difference in vrt and vrb is probably the result 
of the hardware warming up.

Then I went through many iterations of manually setting values for vrt 
and vrb to see if I could find values that would produce better results.

Eventually I came up with these values, which now have the driver 
working quite will on my hardware anyway:

gain=0x23, dcoffset=0x32, vrt=0x14, vrb=0x16

I'm not sure these vrt, vrb values are necessarily "perfect" just yet, 
but they are close enough to make the etes603 quite usable.

So on this particular hardware, your current automatic tuning seems to 
set vrt a fair amount lower than it needs to be, and vrb somewhat lower 
than it should be.

Which of course raises the question of how to correctly automatically 
tune the revision of hardware I'm working with without breaking the 
tuning that worked correctly on the revision of hardware you originally 
developed the driver against.

Out of curiosity, do you know what automatic tuning values you would 
typically get when you originally developed the driver?

Thanks for all your help!
-Jason