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ICS251
FIELD PROGRAMMABLE SS VERSACLOCK SYNTHESIZER
EPROM CLOCK SYNTHESIZER
IDT FIELD PROGRAMMABLE SS VERSACLOCK SYNTHESIZER
2
ICS251
REV D 030212
Pin Assignment
8-pin (150 mil) SOIC
Output Clock Selection Table
Pin Descriptions
External Components
The ICS251 requires a minimum number of external
components for proper operation.
Series Termination Resistor
Clock output traces over one inch should use series
termination. To series terminate a 50
Ω trace (a commonly
used trace impedance), place a 33
Ω resistor in series with
the clock line, as close to the clock output pin as possible.
The nominal impedance of the clock output is 20
Ω.
Decoupling Capacitor
As with any high-performance mixed-signal IC, the ICS251
must be isolated from system power supply noise to perform
optimally.
A decoupling capacitor of 0.01F must be connected
between VDD and the PCB ground plane.
Crystal Load Capacitors
The device crystal connections should include pads for
small capacitors from X1 to ground and from X2 to ground.
These capacitors are used to adjust the stray capacitance of
the board to match the nominally required crystal load
capacitance. Because load capacitance can only be
increased in this trimming process, it is important to keep
stray capacitance to a minimum by using very short PCB
traces (and no vias) been the crystal and device. Crystal
capacitors must be connected from each of the pins X1 and
X2 to ground.
The value (in pF) of these crystal caps should equal (CL -6
pF)*2. In this equation, CL= crystal load capacitance in pF.
Example: For a crystal with a 16 pF load capacitance, each
crystal capacitor would be 20 pF [(16-6) x 2] = 20.
PCB Layout Recommendations
For optimum device performance and lowest output phase
noise, the following guidelines should be observed.
1) The 0.01F decoupling capacitor should be mounted on
the component side of the board as close to the VDD pin as
possible. No vias should be used between the decoupling
capacitor and VDD pin. The PCB trace to VDD pin should
be kept as short as possible, as should the PCB trace to the
ground via. Distance of the ferrite bead and bulk decoupling
from the device is less critical.
X1 / I C L K
VD D
G N D
S0
S1
CL K
X2
1
2
3
4
8
7
6
5
PD T S
S1
S0
CLK (MHz)
Spread
Percentage
0
User
Configurable
User
Configurable
01
User
Configurable
User
Configurable
10
User
Configurable
User
Configurable
11
User
Configurable
User
Configurable
Pin
Number
Pin
Name
Pin
Type
Pin Description
1
S0
Input
Select pin 0 for frequency selection on CLK. Internal pull-up resistor.
2
VDD
Power
Connect to +3.3 V.
3
X1/ICLK
XI
Connect this pin to a crystal or external clock input.
4
X2
XO
Connect this pin to a crystal, or float for clock input.
5
CLK
Output
Clock output. Weak internal pull-down when tri-state.
6
S1
Input
Select pin 1 for frequency selection on CLK. Internal pull-up resistor.
7
GND
Power
Connect this to Ground.
8PDTS
Input
Powers down entire chip. Tri-states CLK outputs when low. No internal pull-up
resistor. The pin must be tied either directly or through the external resistor to
VDD or GND. External resistor value must be less than 15kOhm.