--- Olin Lathrop <olin_piclist@embedinc.com> wrote:

> How are you going to measure or synthesize these
> frequencies to that
> accuracy unless you've got your own atomic clock? 
> The latest cesmium clocks
> are good to about 47 bits, so I guess this is at
> least theoretically
> possible.

There is absolute accuracy and relative accuracy, both
have value.  Our reference is not an atomic clock, but
it's absolute accuracy is 5e-7 Hz, which translates to
1.2e-4 Hz at the output.  And the reference can be
calibrated.

> But that's the point, I'm not sure.  It may be that
> high precision is
> required for the solution approach you present, but
> I'm questioning whether
> there may not be other solution approaches at a
> higher level.

The math is dictated by the hardware, the hardware is
dictate by the performance requirements.  I'm sorry it
is not practical to explain it all in a post, but
there is just no way around the need for precise math.
 Those 48 bit and 20 bit numbers are going directly
into IC's that won't do their job correctly if they
are off by even one bit.

> In any case, wide multiplies can be done by
> stringing together numerous 8
> bit multiplies.  This works just like doing long
> multiplication by hand with
> decimal digits.  The basic multiplier in your head
> only works on two 0-9
> digits at a time but you can use that basic
> operation multiple times to
> multiply large numbers.  For example, suppose each
> upper case letter below
> is a digit in a decimal number:
> 
>   ABC * DE = E*C + E*B*10 + E*A*100 + D*C*10 +
> D*B*100 + D*A*1000
> 
> The same mechanism can be used to harness the 8x8
> multiplier inside the 18F
> by using bytes instead of decimal digits.  The
> equation above still works if
> the letters represented bytes and fixed multipliers
> were powers of 256
> instead of 10.

Thanks, I will have to use this technique.  It has
been awhile since I wrote any assembly.

Clay


	
		
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