58 www.xilinx.com Virtex-5 FPGA User Guide
UG190 (v5.0) June 19, 2009
Chapter 2: Clock Management Technology
DCM Attributes
A handful of DCM attributes govern the DCM functionality. Table 2-6 summarizes all the
applicable DCM attributes. This section provides a detailed description of each attribute.
For more information on applying these attributes in UCF, VHDL, or Verilog code, refer to
the Constraints Guide at:
http://www.support.xilinx.com/support/software_manuals.htm
.
CLKDV_DIVIDE Attribute
The CLKDV_DIVIDE attribute controls the CLKDV frequency. The source clock frequency
is divided by the value of this attribute. The possible values for CLKDV_DIVIDE are: 1.5,
2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6, 6.5, 7, 7.5, 8, 9, 10, 11, 12, 13, 14, 15, or 16. The default value is 2.
In the low frequency mode, any CLKDV_DIVIDE value produces a CLKDV output with a
50/50 duty-cycle. In the high frequency mode, the CLKDV_DIVIDE value must be set to
an integer value to produce a CLKDV output with a 50/50 duty-cycle. For non-integer
CLKDV_DIVIDE values, the CLKDV output duty cycle is shown in Table 2-5.
CLKFX_MULTIPLY and CLKFX_DIVIDE Attribute
The CLKFX_MULTIPLY attribute sets the multiply value (M) of the CLKFX output. The
CLKFX_DIVIDE attribute sets the divisor (D) value of the CLKFX output. Both control the
CLKFX output making the CLKFX frequency equal the effective CLKIN (source clock)
frequency multiplied by M/D. The possible values for M are any integer from two to 33.
The possible values for D are any integer from 1 to 32. The default settings are M = 4 and
D=1.
CLKIN_PERIOD Attribute
The CLKIN_PERIOD attribute specifies the source clock period (in nanoseconds). The
default value is 0.0 ns. Setting this attribute to the input period values produces the best
results.
Table 2-5: Non-Integer CLKDV_DIVIDE
CLKDV_DIVIDE Value
CLKDV Duty Cycle in
High Frequency Mode
(High Pulse/Low Pulse Value)
1.5 1/3
2.5 2/5
3.5 3/7
4.5 4/9
5.5 5/11
6.5 6/13
7.5 7/15
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