FS8108

Page 6

Jan 2003

Functional Description

Programmable Input Frequency Divider

The VCO input to the FIN pin is divided by the programmable divider and then internally

output to the phase/frequency detector (PFD) as f

divider consists of a ÷ 64/65 (P/P+1) dual-modulus prescaler and a 18-bit (N) counter,

which is further comprised of a 6-bit swallow (A) counter, and a 12-bit main (B) counter.

The total divide ratio, M, is related to values for P, A, and B through the relation

with

The minimum programmable divisor for continuous counting is given by

and the valid total divide ratio range for the input

divider is

Programmable Reference Frequency Divider

The crystal oscillator output is divided by the programmable divider and then internally

output to the PFD as f

÷ 8 (S) prescaler and a 13-bit reference (R) counter. The total divide ratio, T, is related to

values for S and R through the relation

The usable divisior range of the reference counter is

and therefore, the

valid total divide ratio range for the reference divider is

(in steps of 8.)

Serial Input Data Format

The divide ratios for the input and reference dividers are input using a 19-bit serial inter-

face consisting of separate clock (CLK), data (DATA), and latch enable (LE) lines. The

format of the serial data is shown in Fig. 1. The data on the DATA line is written to the

shift register on the rising edge of the CLK signal and is input with MSB first, and the last

bit is used as the latch select control bit. The data on the DATA line should be changed on

the falling edge of CLK, and LE should be held low while data is being written to the shift

register. Data is transferred from the shift register to one of the frequency divider latches

when LE is set high. When the latch select control bit is set low, data is loaded to the 18-

bit N-counter latch, and when the latch select control bit is set high, the 2 MSBs are

MP

1

+

B A

–

()

×

+

×

PB

+

×

==

≥

P

–

()

×

64

63

×

4032,

==

M

4032 to 262143.

=

TS R

×

8

×

==

R

5 to 8191,

=

T

40 to 65528

=