R
Functional Description
clock distribution network, the clock signal returns to the
DLL via a feedback line called CLKFB. The control block
inside the DLL measures the phase error between CLKFB
and CLKIN. This phase error is a measure of the clock skew
that the clock distribution network introduces. The control
block activates the appropriate number of delay steps to
cancel out the clock skew. When the DLL phase-aligns the
CLK0 signal with the CLKIN signal, it asserts the LOCKED
output, indicating a lock on to the CLKIN signal.
DLL Attributes and Related Functions
The DLL unit has a variety of associated attributes as
described in Table 29. Each attribute is described in detail in
the sections that follow.
Table 29: DLL Attributes
Attribute
Description
Chooses either the CLK0 or CLK2X output to NONE, 1X, 2X
Values
CLK_FEEDBACK
CLKIN_DIVIDE_BY_2
CLKDV_DIVIDE
drive the CLKFB input
Halves the frequency of the CLKIN signal just FALSE, TRUE
as it enters the DCM
Selects the constant used to divide the CLKIN 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, 5.5, 6.0,
input frequency to generate the CLKDV
6.5, 7.0, 7.5, 8, 9, 10, 11, 12, 13, 14,
output frequency
15, and 16
CLKIN_PERIOD
Additional information that allows the DLL to Floating-point value representing the
operate with the most efficient lock time and
the best jitter tolerance
CLKIN period in nanoseconds
DLL Clock Input Connections
For best results, an external clock source enters the FPGA
via a Global Clock Input (GCLK). Each specific DCM has
four possible direct, optimal GCLK inputs that feed the
DCM’s CLKIN input, as shown in Table 30. Table 30 also
provides the specific pin numbers by package for each
GCLK input. The two additional DCM’s on the XC3S1200E
and XC3S1600E have similar optimal connections from the
left-edge LHCLK and the right-edge RHCLK inputs, as
described in Table 31 and Table 32.
•
The DCM supports differential clock inputs (for
example, LVDS, LVPECL_25) via a pair of GCLK inputs
that feed an internal single-ended signal to the DCM’s
CLKIN input.
Design Note
Avoid using global clock input GCLK1 as it is always shared
with the M2 mode select pin. Global clock inputs GCLK0,
GCLK2, GCLK3, GCLK12, GCLK13, GCLK14, and
GCLK15 have shared functionality in some configuration
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DS312-2 (v3.8) August 26, 2009
Product Specification
XILINX [ XILINX, INC ]
