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©2016 Integrated Device Technology, Inc

Revision B June 28, 2016

831752 Data Sheet

Recommendations for Unused Input Pins

Inputs:

LVCMOS Control Pins

All control pins have internal pullups or pulldowns; additional

resistance is not required but can be added for additional protection.

A 1k

 resistor can be used.

CLK/nCLK Inputs

For applications not requiring the use of the differential input, both

CLK and nCLK can be left floating. Though not required, but for

additional protection, a 1k

 resistor can be tied from CLK to ground.

FCLK/nFCLK Pins

In case FCLK/nFCLK are unused, one of the following two

configurations can be used: either FCLK/nFCLK are configured as

an output (DIR_SEL = 1) and left floating, or FCLK/nFCLK are

configured as an input (DIR_SEL = 0). In this case 1k pulldown is

required on FCLK and 1k Pullup is required on nFCLK.

Wiring the Differential Input to Accept Single-Ended Levels

Figure 2 shows how a differential input can be wired to accept single

ended levels. The reference voltage V1= VDD/2 is generated by the

bias resistors R1 and R2. The bypass capacitor (C1) is used to help

filter noise on the DC bias. This bias circuit should be located as

close to the input pin as possible. The ratio of R1 and R2 might need

to be adjusted to position the V1in the center of the input voltage

swing. For example, if the input clock swing is 2.5V and VDD = 3.3V,

R1 and R2 value should be adjusted to set V1 at 1.25V. The values

below are for when both the single ended swing and VDD are at the

same voltage. This configuration requires that the sum of the output

impedance of the driver (Ro) and the series resistance (Rs) equals

the transmission line impedance. In addition, matched termination at

the input will attenuate the signal in half. This can be done in one of

two ways. First, R3 and R4 in parallel should equal the transmission

line impedance. For most 50

 applications, R3 and R4 can be

100

. The values of the resistors can be increased to reduce the

loading for slower and weaker LVCMOS driver. When using

single-ended signaling, the noise rejection benefits of differential

signaling are reduced. Even though the differential input can handle

full rail LVCMOS signaling, it is recommended that the amplitude be

reduced. The datasheet specifies a lower differential amplitude,

however this only applies to differential signals. For single-ended

applications, the swing can be larger, however VIL cannot be less

than -0.3V and VIH cannot be more than VDD + 0.3V. Though some

of the recommended components might not be used, the pads

should be placed in the layout. They can be utilized for debugging

purposes. The datasheet specifications are characterized and

guaranteed by using a differential signal.

Figure 2. Recommended Schematic for Wiring a Differential Input to Accept Single-ended Levels