Deskew_Adjust Attribute; Dfs_Frequency_Mode Attribute; Dll_Frequency_Mode Attribute; Duty_Cycle_Correction Attribute - Xilinx Virtex-5 FPGA User Manual

Chapter 2: Clock Management Technology

DESKEW_ADJUST Attribute

The DESKEW_ADJUST attribute affects the amount of delay in the feedback path. The
possible values are SYSTEM_SYNCHRONOUS, SOURCE_SYNCHRONOUS,
0, 1, 2, 3, ..., or 31. The default value is SYSTEM_SYNCHRONOUS.
For most designs, the default value is appropriate. In a source-synchronous design, set this
attribute to SOURCE_SYNCHRONOUS. The remaining values should only be used after
consulting with Xilinx. For more information, consult the
Setting"section.

DFS_FREQUENCY_MODE Attribute

The DFS_FREQUENCY_MODE attribute specifies the frequency mode of the digital
frequency synthesizer (DFS). The possible values are Low and High. The default value is
Low. The frequency ranges for both frequency modes are specified in the Virtex-5 FPGA
Data Sheet. DFS_FREQUENCY_MODE determines the frequency range of CLKIN, CLKFX,
and CLKFX180.

DLL_FREQUENCY_MODE Attribute

The DLL_FREQUENCY_MODE attribute specifies either the High or Low frequency
mode of the delay-locked loop (DLL). The default value is Low. The frequency ranges for
both frequency modes are specified in the Virtex-5 FPGA Data Sheet.

DUTY_CYCLE_CORRECTION Attribute

The DUTY_CYCLE_CORRECTION attribute controls the duty cycle correction of the 1x
clock outputs: CLK0, CLK90, CLK180, and CLK270. The possible values are TRUE and
FALSE. The default value is TRUE. When set to TRUE, the 1x clock outputs are duty cycle
corrected to be within specified limits. See the Virtex-5 FPGA Data Sheet for details. It is
strongly recommended to always set the DUTY_CYCLE_CORRECTION attribute to
TRUE. Setting this attribute to FALSE does not necessarily produce output clocks with the
same duty cycle as the source clock.

DCM_PERFORMANCE_MODE Attribute

The DCM_PERFORMANCE_MODE attribute allows the choice of optimizing the DCM
either for high frequency and low jitter or for low frequency and a wide phase-shift range.
The attribute values are MAX_SPEED and MAX_RANGE. The default value is
MAX_SPEED. When set to MAX_SPEED, the DCM is optimized to produce high
frequency clocks with low jitter. However, the phase-shift range is smaller than when
MAX_RANGE is selected. When set to MAX_RANGE, the DCM is optimized to produce
low-frequency clocks with a wider phase-shift range. The DCM_PERFORMANCE_MODE
affects the following specifications: DCM input and output frequency range, phase-shift
range, output jitter, DCM_TAP, CLKIN_CLKFB_PHASE, CLKOUT_PHASE, and duty-
cycle precision. The Virtex-5 FPGA Data Sheet specifies these values.
For most cases, the DCM_PERFORMANCE_MODE attribute should be set to
MAX_SPEED (default). Consider changing to MAX_RANGE only in the following
situations:
•
•
60
The frequency needs to be below the low-frequency limit of the MAX_SPEED setting.
A greater absolute phase-shift range is required.
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"Source-Synchronous
Virtex-5 FPGA User Guide
UG190 (v5.0) June 19, 2009