8©2015 Integrated Device Technology, Inc. Revision B, December 8, 2015
8535-21 Datasheet
Application Information
Recommendations for Unused Input and Output Pins
Inputs:
CLK Inputs
For applications not requiring the use of a clock input, it can be left
floating. Though not required, but for additional protection, a 1k
resistor can be tied from the CLK input to ground.
LVCMOS Control Pins
All control pins have internal pull-ups or pull-downs; additional
resistance is not required but can be added for additional protection.
A 1k resistor can be used.
Outputs:
LVPECL Outputs
All unused LVPECL outputs can be left floating. We recommend that
there is no trace attached. Both sides of the differential output pair
should either be left floating or terminated.
Termination for 3.3V LVPECL Outputs
The clock layout topology shown below is a typical termination for
LVPECL outputs. The two different layouts mentioned are
recommended only as guidelines.
The differential outputs are low impedance follower outputs that
generate ECL/LVPECL compatible outputs. Therefore, terminating
resistors (DC current path to ground) or current sources must be
used for functionality. These outputs are designed to drive 50
transmission lines. Matched impedance techniques should be used
to maximize operating frequency and minimize signal distortion.
Figures 2A and 2B show two different layouts which are
recommended only as guidelines. Other suitable clock layouts may
exist and it would be recommended that the board designers
simulate to guarantee compatibility across all printed circuit and clock
component process variations.
Figure 2A. 3.3V LVPECL Output Termination Figure 2B. 3.3V LVPECL Output Termination
R1
84
R2
84
3.3V
R3
125
R4
125
Z
o
= 50
Z
o
= 50
Input
3.3V
3.3V
+
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