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Functional Description
The DCM supports three major functions:
Digital Clock Managers (DCMs)
•
Clock-skew Elimination: Clock skew describes the
Differences from the Spartan-3 Architecture
extent to which clock signals may, under normal
circumstances, deviate from zero-phase alignment. It
occurs when slight differences in path delays cause the
clock signal to arrive at different points on the die at
different times. This clock skew can increase set-up
and hold time requirements as well as clock-to-out
time, which may be undesirable in applications
operating at a high frequency, when timing is critical.
The DCM eliminates clock skew by aligning the output
clock signal it generates with another version of the
clock signal that is fed back. As a result, the two clock
signals establish a zero-phase relationship. This
effectively cancels out clock distribution delays that
might lie in the signal path leading from the clock
•
•
•
Spartan-3E FPGAs have two, four, or eight DCMs,
depending on device size.
The Spartan-3E DCM has a maximum phase shift
range of 180°. The Spartan-3 DCM range is 360°.
The Spartan-3E DLL supports lower input frequencies,
down to 5 MHz. Spartan-3 DLLs supports down to 24
MHz.
Overview
Spartan-3E Digital Clock Managers (DCMs) provide flexi-
ble, complete control over clock frequency, phase shift and
skew. To accomplish this, the DCM employs a Delay-Locked
Loop (DLL), a fully digital control system that uses feedback
to maintain clock signal characteristics with a high degree of
precision despite normal variations in operating tempera-
ture and voltage. This section provides a fundamental
description of the DCM. See XAPP462: "Using Digital Clock
Managers (DCMs) in Spartan-3 Series FPGAs" for further
information.
output of the DCM to its feedback input.
•
•
Frequency Synthesis: Provided with an input signal,
the DCM can generate a wide range of different output
clock frequencies. This is accomplished by either
multiplying and/or dividing the frequency of the input
clock signal by any of several different factors.
Phase Shifting: The DCM provides the ability to shift
the phase of all its output clock signals with respect to
its input clock signal.
The XC3S100E FPGA has two DCMs, one at the top and
one at the bottom of the device. The XC3S250E and
XC3S500E FPGAs each include four DCMs, two at the top
and two at the bottom. The XC3S1200E and XC3S1600E
FPGAs contain eight DCMs with two on each edge (see
also Figure 42). The DCM in Spartan-3E FPGAs is sur-
rounded by CLBs within the logic array and is no longer
located at the top and bottom of a column of block RAM as
in the Spartan-3 architecture,. The Digital Clock Manager is
instantiated into a design as the “DCM” primitive.
The DCM has four functional components: the
Delay-Locked Loop (DLL), the Digital Frequency Synthe-
sizer (DFS), the Phase Shifter (PS), and the Status Logic.
Each component has its associated signals, as shown in
Figure 37.
DCM
PSINCDEC
Phase
Shifter
PSEN
PSCLK
PSDONE
Clock
CLK0
Distribution
CLKIN
CLKFB
CLK90
Delay
CLK180
CLK270
CLK2X
CLK2X180
CLKDV
CLKFX
DFS
DLL
CLKFX180
LOCKED
Status
Logic
RST
8
STATUS [7:0]
DS099-2_07_040103
Figure 37: DCM Functional Blocks and Associated Signals
DS312-2 (v1.1) March 21, 2005
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39
Advance Product Specification
XILINX [ XILINX, INC ]
