Spartan-3E FPGA Family: Functional Description
DS312 (v4.1) July 19, 2013
Product Specification
55
FIXED Phase Shift Mode
The FIXED phase shift mode shifts the DCM outputs by a
fixed amount (TPS), controlled by the user-specified
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
fraction of the TCLKIN. The phase shift behavior is different
between ISE 8.1, Service Pack 3 and prior software
versions, as described below.
Design Note
Prior to ISE 8.1i, Service Pack 3, the FIXED phase shift
feature 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:
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.
Eq 2
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
Spartan-3 DCM design primitive and simulation model.
Designs created prior to ISE 8.1i, Service Pack 3 must be
recompiled using the most recent ISE development
software.
Eq 3
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
outputs 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.
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.
mode. The VARIABLE phase shift mode operates
differently.
t
PS
PHASESHIFT
256
----------------------------------------
T
CLKIN
=
t
PS
PHASESHIFT
512
----------------------------------------
T
CLKIN
=
X-Ref Target - Figure 44
Figure 44: NONE and FIXED Phase Shifter Waveforms (ISE 8.1i, Service Pack 3 and later)
DS312-2_61_021606
CLKIN
CLKFB
* TCLKIN
P
256
b. CLKOUT_PHASE_SHIFT = FIXED
Shift Range over all P Values:
–255
+255
0
CLKIN
CLKFB
a. CLKOUT_PHASE_SHIFT = NONE
(via CLK0 or CLK2X feedback)