General Standards Corporation PCIe-24DSI32, Reference Manual

Page: 26 / 51

PCIe-24DSI32
_____________________________________________________________________________
General Standards Corporation Ph:(256)880-8787 FAX:(256)880-8788 Email: solutions@GeneralStandards.com
3-13
The following example illustrates one of several methods for determining Nvco and Nref :
(Calculating the optimum values for integers Nvco and Nref can often be simplified by
converting all variables, including Fref , into products of their prime factors). Other approaches
include calculating all possible values of Nvco and Nref, and then sorting the results for the
frequency closest to the required frequency.
EXAMPLE: Required sample rate Fsamp is 15.360 kHz . Fref = 32.768 MHz :
1. Insert the Fref and required Fsamp values into Equation 3-3, expressing both in Hertz.
2. Convert Fref and Fsamp into their prime factors, and simplify the fraction by canceling all factors
that are duplicated in the numerator and denominator
3. Select a value for DIVISOR that adjusts the value of the fraction to between 0.61 and 1.68 (20MHz
to 55MHz) /32.768MHz, with unity being ideal. In this case, use DIVISOR = 4:
4. Multiply both numerator and denominator by an integer that produces the lowest possible in-range
values for Nvco and Nref , which must be 30 or greater. In this case, multiply by 2:
5. Final Results:
Nvco = 48; Nref = 50; DIVISOR = 4
.
To confirm the results, first use Equation 3-2 to verify that the rate generator frequency Fgen is
within the specified range of 20-55 MHz. If Fgen is out of range, select another DIVISOR value
in Step-3:
Finally, use Equation 3-1 to verify the sample rate Fsamp :
24
25 Nvco
Nref
=
(2
10
* 3 * 5) * (2
9
) * DIVISOR
2
18
* 5
3
Nvco
Nref
=
6 * DIVISOR
25
=
.
15360 * 512 * DIVISOR
32768000
Nvco
Nref
=
.
.
Nvco
Nref
=
Fgen
Fref
*
48
50
=
32,768,000 Hz
*
*
=
31,457,280 Hz.
Fgen
512 * DIVISOR
= Fsamp
31,457,280 Hz
512 * 4
= =
15,360 Hz .
48
50 Nvco
Nref
=
.