73M1903C-EVM Maxim Integrated Products, 73M1903C-EVM Datasheet - Page 42

• Manufacturer Part Number: 73M1903C-EVM
• Description: BOARD DEMO 73M1903C WORLDWIDE
• Manufacturer: Maxim Integrated Products

Specifications of 73M1903C-EVM

• Main Purpose: Interface, Analog Front End (AFE)
• Utilized Ic / Part: *
• Primary Attributes: *
• Secondary Attributes: *
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Embedded: -

73M1903C Data Sheet
DS_1903C_033
Appendix B
Crystal Oscillator
The crystal oscillator is designed to operate over wide choice of crystals (from 4.9 MHz to 27 MHz). The
crystal oscillator output is the input to an NCO based pre-scaler (divider) prior to being passed onto an
on-chip PLL. The intent of the pre-scaler is to convert the crystal oscillator frequency, Fxtal, to a
convenient frequency to be used as a reference frequency, Fref, for the PLL. A set of three numbers–
Pdvsr (5 bit), Prst (3 bit) and Pseq (8 bit) must be entered thru the serial port as follows:
Pdvsr = Integer [Fref/Fxtal];
Prst = Denominator of the ratio (Fref/Fxtal) minus 1 when it is expressed as a ratio of two smallest
integers = Nnco1/Dnco1;
Pseq = Divide Sequence
Fxtal
overflow
Fref
Counter
Pdvsr
count ctrl
mux
Pdvsr +1
Sequence
Sequence
Register
Counter
Rst
Prst[2:0]
Pseq[7:0]
Figure 20: NCO Block Diagram
Please note that in all cases, pre-scaler should be designed such that pre-scaler output frequency, Fref,
is in the range of 2 ~ 4 MHz.
In the first example below, the exact divide ratio required is Fxtal/Fref = 15.625 =125/8. If a divide
sequence of {÷16,÷16,÷15,÷16,÷16,÷15,÷16,÷15} is repeated, the effective divide ratio would be exactly
15.625. Consequently, Pdvsr of 15, the length of the repeating pattern, Prst = 8 –1 =7, and the pattern,
{1,1,0,1,1,0,1,0}, where 0 means Pdvsr, or ÷15, and 1 means Pdvsr +1, or ÷16 must be entered as below.
Example 1: Fxtal = 27 MHz, Fref = 1.728 MHz.
Pdvsr = Integer [Fxtal/Fref] = 15 =0Fh
Prst[2:0] = 8 – 1 = 7 from Fxtal/Fref = 15.625 =125/8;
Pseq = ÷16,÷16,÷15,÷16,÷16,÷15,÷16,÷15 => {1,1,0,1,1,0,1,0} =DAh.
In the second example, Fxtal/Fref =4.0. This is a constant divide by 4. Thus, Pdvsr is 4, Prst = 1 –
1 =0 and Pseq = {x,x,x,x,x,x,x,x).
42
Rev. 5.0