Datasheet

73M1903 Data Sheet DS_1903_032

42 Rev. 2.1

Appendix B – Crystal Oscillator

The crystal oscillator is designed to operate over wide choice of crystals (from 9 MHz to 27 MHz). The

crystal oscillator output is 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 through 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

mux

count ctrl

overflow

Counter

Pdvsr

Pdvsr +1

Sequence

Rst

Sequence

Counter

Fref

Pseq[7:0]

Prst[2:0]

Figure 17: NCO Block Diagram

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).