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DCM Attributes
phase-shift increment (or decrement) will increase (or decrease) the phase shift by a period
of 1/256 x CLKIN period.
When set to DIRECT, the DCM output can be phase-shifted in variable mode in the
positive range with respect to CLKIN. Each phase-shift increment/decrement will
increase/decrease the phase shift by one DCM_TAP (see the Virtex-4 Data Sheet).
The starting phase in the VARIABLE_POSITIVE and VARIABLE_CENTER modes is
determined by the phase-shift value. The starting phase in the DIRECT mode is always
zero, regardless of the value specified by the PHASE_SHIFT attribute. Thus, the
PHASE_SHIFT attribute should be set to zero when DIRECT mode is used. A non-zero
phase-shift value for DIRECT mode can be loaded to the DCM using Dynamic
Reconfiguration Ports in the Virtex-4 Configuration Guide.
CLK_FEEDBACK Attribute
The CLK_FEEDBACK attribute determines the type of feedback applied to the CLKFB.
The possible values are 1X or NONE. The default value is 1X. When set to 1X, CLKFB pin
must be driven by CLK0. When set to NONE leave the CLKFB pin unconnected.
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 “Source-Synchronous
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-4 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-4 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-4 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.
Virtex-4 User Guide
UG070 (v2.0) January 4, 2007
www.xilinx.com
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