Hi Bruce. I had to read your post a couple of times
to get it but I think that is really clever.
Part of my problem with the 18bit period value is
that I have way too much resolution at the slow
speeds, with values like 200,000 ticks, and not
enough resolution with high speed values like 370
ticks.

If I use ranges this evens out. It also saves me
a lot of math calcs as I can do the bulk of the range
work when I *detect* the pulse rate, letting me use
the better resolution from sampling period over 4
to 16 pulses as the speeds increase.

Vehicle specs:
10kph = 10Hz = 200,000 ticks (min speed)
400kph = 5400Hz = 370 ticks (max speed)

So rather than using my original plan of sampling
over 16 pulses only for higher speeds, I will do the
sampling in 4 specific ranges;

speed range low = period of one pulse
speed range 2 = period of 4 pulses
speed range 3 = period of 8 pulses
speed range 4 = period of 16 pulses

So, as you suggested I can then use a
12 bit stored data consisting of:
10 bits (actual data value, cropped to 10bit)
2 bits (range indicator)

This should be simple to implement as there
is no real data processing done on the fly,
just storage. When it is uploaded to the PC
later I can do the conversion from period
data in four specific ranges to actual speed,
as it needs a set calibration applied by the
PC anyway to account for the vehicle gearing.

I will have to do a little bit of homework re
the exact range choices and resolution but
I think this is a good way to go about it.

Darn it, just done a couple of hours number
crunching and I don't think it will work.
I really don't think the data will fit in 12
bits with the resolution I need.
Technically, 12 bits is 4096, which should
give me 400kph at 0.1kph resolution. But my
problem is a 10:1 difference from vehicle
to vehicle re the freq per kph. I really need
that 0.1kph resolution to 4000kph to give
the same resolution for all vehicle types.
Darn!:o)
-Roman

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