ICS843034-01 Datasheet, PDF (3/22 Pages) Integrated Circuit Systems – FEMTOCLOCKS™ MULTI-RATE LVPECL FREQUENCY SYNTHESIZER

Integrated
Circuit
Systems, Inc.
PRELIMINARY
ICS843034-01
FEMTOCLOCKS™
MULTI-RATE LVPECL FREQUENCY SYNTHESIZER
FUNCTIONAL DESCRIPTION
NOTE: The functional description that follows describes op-
eration using a 25MHz crystal. Valid PLL loop divider values
for different crystal or input frequencies are defined in the In-
put Frequency Characteristics, Table 5, NOTE 1.
The ICS843034-01 features a fully integrated PLL and there-
fore requires no external components for setting the loop band-
width. A fundamental crystal is used as the input to the on-
chip oscillator. The output of the oscillator is fed into the phase
detector. A 25MHz crystal provides a 25MHz phase detector
reference frequency. The VCO of the PLL operates over a
range of 490MHz to 640MHz. The output of the M divider is
also applied to the phase detector.
The phase detector and the M divider force the VCO output fre-
quency to be M times the reference frequency by adjusting the
VCO control voltage. Note that for some values of M (either too
high or too low), the PLL will not achieve lock. The output of the
VCO is scaled by a divider prior to being sent to each of the LVPECL
output buffers. The divider provides a 50% output duty cycle.
The ICS843034-01 supports either serial or parallel program-
ming modes to program the M feedback divider and N output
divider. The input divider P can only be changed using the P_DIV
pin. It cannot be changed from the default ÷1 setting using the
serial interface. Figure 1 shows the timing diagram for each mode.
In parallel mode, the nP_LOAD input is initially LOW. The data
on the M, NA, and NB inputs are passed directly to the M di-
vider and both N output dividers. On the LOW-to-HIGH transi-
tion of the nP_LOAD input, the data is latched and the M and N
dividers remain loaded until the next LOW transition on
nP_LOAD or until a serial event occurs. As a result, the M and
Nx bits can be hardwired to set the M divider and Nx output
divider to a specific default state that will automatically occur
during power-up. The TEST output is LOW when operating in
the parallel input mode. The relationship between the VCO fre-
quency, the crystal frequency and the M divider is defined as
follows: fVCO = fxtal x M
P
The M value and the required values of M0 through M5 are shown
in Table 3B to program the VCO Frequency Function Table.
Valid M values for which the PLL will achieve lock for a 25MHz
reference are defined as 20 ≤ M ≤ 25. The frequency out is de-
fined as follows: FOUT = fVCO = fxtal x M
N
NxP
Serial operation occurs when nP_LOAD is HIGH and S_LOAD
is LOW. The shift register is loaded by sampling the S_DATA
bits with the rising edge of S_CLOCK. The contents of the
shift register are loaded into the M divider and Nx output di-
vider when S_LOAD transitions from LOW-to-HIGH. The M
divide and Nx output divide values are latched on the HIGH-
to-LOW transition of S_LOAD. If S_LOAD is held HIGH, data
at the S_DATA input is passed directly to the M divider and Nx
output divider on each rising edge of S_CLOCK. The serial
mode can be used to program the M and Nx bits and test bits
T1 and T0. The internal registers T0 and T1 determine the state
of the TEST output as follows:
T1 T0
00
01
10
11
TEST Output
LOW
S_Data, Shift Register Output
Output of M divider
CMOS Fout A0
S_CLOCK
S_DATA
S_LOAD
nP_LOAD
M0:M8, P_DIV,
NA0:NA2, NB0:NB2
nP_LOAD
S_LOAD
843034AY-01
SERIAL LOADING
T 1 T0 NB2 NB1 NB0 NA2 NA1 NA0 M8 M7 M6 M5 M4 M3 M2 M1 M 0
tt
SH
t
S
PARALLEL LOADING
M, N, P
tt
SH
Time
FIGURE 1. PARALLEL & SERIAL LOAD OPERATIONS
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REV. C NOVEMBER 28, 2005