R
Functional Description
FIXED Phase Shift prior to ISE 8.1i, Service Pack 3: See
Equation 3. The value corresponds to a phase shift range of
–180° to +180° degrees, which is different from the Spar-
tan-3 DCM design primitive and simulation model. Designs
created prior to ISE 8.1i, Service Pack 3 must be recom-
piled using the most recent ISE development software.
FIXED Phase Shift Mode
The FIXED phase shift mode shifts the DCM outputs by a
fixed amount (T ), controlled by the user-specified
PS
PHASE_SHIFT attribute. The PHASE_SHIFT value (shown
as P in Figure 44) must be an integer ranging from –255 to
+255. PHASE_SHIFT specifies a phase shift delay as a
PHASESHIFT
fraction of the T
The phase shift behavior is different
⎛
⎝
⎞
⎠
CLKIN.
----------------------------------------
tPS
=
• TCLKIN
Eq. 3
512
between ISE 8.1, Service Pack 3 and prior software ver-
sions, as described below.
When the PHASE_SHIFT value is zero, CLKFB and CLKIN
are in phase, the same as when the PS unit is disabled.
When the PHASE_SHIFT value is positive, the DCM out-
puts are shifted later in time with respect to CLKIN input.
When the attribute value is negative, the DCM outputs are
shifted earlier in time with respect to CLKIN.
Design Note
Prior to ISE 8.1i, Service Pack 3, the FIXED phase shift fea-
ture operated differently than the Spartan-3 DCM design
primitive and simulation model. Designs using software
prior to ISE 8.1i, Service Pack 3 require recompilation using
the latest ISE software release. The following Answer
Record contains additional information:
Figure 44b illustrates the relationship between CLKFB and
CLKIN in the Fixed Phase mode. In the Fixed Phase mode,
the PSEN, PSCLK, and PSINCDEC inputs are not used
and must be tied to GND.
http://www.xilinx.com/support/answers/23153.htm.
FIXED Phase Shift using ISE 8.1i, Service Pack 3 and
later: See Equation 2. The value corresponds to a phase
shift range of –360° to +360°, which matches behavior of
the Spartan-3 DCM design primitive and simulation model.
Equation 2 or Equation 3 applies only to FIXED phase shift
mode. The VARIABLE phase shift mode operates differ-
ently.
PHASESHIFT
⎛
⎝
⎞
⎠
----------------------------------------
tPS
=
• TCLKIN
Eq. 2
256
a. CLKOUT_PHASE_SHIFT = NONE
CLKIN
CLKFB
(via CLK0 or CLK2X feedback)
b. CLKOUT_PHASE_SHIFT = FIXED
CLKIN
0
–255
+255
Shift Range over all P Values:
P
256
* T
CLKIN
CLKFB
(via CLK0 or CLK2X feedback)
DS312-2_61_021606
Figure 44: NONE and FIXED Phase Shifter Waveforms (ISE 8.1i, Service Pack 3 and later)
56
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DS312-2 (v3.8) August 26, 2009
Product Specification
XILINX [ XILINX, INC ]
