73M1903-DB-CTR21 Maxim Integrated Products, 73M1903-DB-CTR21 Datasheet - Page 46
73M1903-DB-CTR21
Manufacturer Part Number
73M1903-DB-CTR21
Description
MCU, MPU & DSP Development Tools 73M1903 DEMO BOARD CTR21
Manufacturer
Maxim Integrated Products
Datasheet
1.73M1903-EVM-600.pdf
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73M1903 Data Sheet
Example 3:
Crystal Frequency = 27 MHz; Desired Sampling Rate, Fs = 7.2 kHz
Step 1. First compute the required VCO frequency, Fvco, corresponding to
Fs = 2.4 kHz x 3 = 7.2 kHz.
Fvco = 2 x 2304 x Fs = 2 x 2304 x 2.4 kHz x 3 = 33.1776 MHz.
Step 2. Express the required VCO frequency divided by the Crystal Frequency as a ratio of two integers.
This is initially given by:
After a few rounds of simplification this reduces to:
The two ratios are not unique and many other possibilities exist. But for this particular application, they
are found to be the best set of choices within the constraints of Prst and Nrst allowed. (Nnco1, Nnco2
must be less than or equal to 8.)
The ratio, Nnco1/Dnco1 = 8/125, is used to form a divide ratio for the NCO in prescaler and Nnco2/Dnco2
=5/96 for the NCO in the PLL.
Pre-scaler NCO: From Nnco1/Dnco1 = 8/125,
Pdvsr = Integer [ Dnco1/Nnco1 ] = 15;
Prst[2:0] = Nnco1 – 1 = 7;
Dnco1/Nnco1 = 125/8 = 15.625 suggests a divide sequence of {÷16,÷16,÷15,÷16,÷16,÷15,÷16,÷15}, or
Pseq = {1,1,0,1,1,0,1,0} = DAh.
PLL NCO: From Nnco2/Dnco2 = 5/96,
Ndvsr = Integer [ Dnco2/Nnco2 ] = 19;
Nrst[2:0] = Nnco2 – 1 = 4;
Dnco2/Nnco2 = 19.2 suggests a divide sequence of {÷19, ÷19, ÷19, ÷19, ÷20}, or
Nseq = {x,x,x,0,0,0,0,1} = x1h.
46
Fvco
=
Fvco
/
Fxtal
/
Fxtal
=
(
Dnco2
Nnco1
Dnco1
Nnco2
=
125
8
2
)
•
•
(
2304
=
27
96
•
5
MHz
2.4kHz
)
125
96
8
5
•
3
.
DS_1903_032
Rev. 2.1
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