73M1903C データシートの表示（PDF） - Teridian Semiconductor Corporation
部品番号
コンポーネント説明
メーカー
73M1903C Datasheet PDF : 46 Pages
| |||
73M1903C
Modem Analog Front End
DATA SHEET
APPENDIX B
Crystal Oscillator
The crystal oscillator is designed to operate over wide choice of crystals (from 4.9MHz to 27MHz). 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 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
Pdvsr +1
mux count ctrl
Sequence
Register
Pseq[7:0]
Sequence
Counter
Rst
Prst[2:0]
Figure 18: 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 ~ 4MHz.
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 = 27MHz, Fref = 1.728MHz.
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).
Page: 40 of 46
© 2005-2008 TERIDIAN Semiconductor Corporation
Rev 4.3
Share Link: 