XC3S100E_06 Datasheet, PDF (56/231 Pages) Xilinx, Inc – Configurable Logic Block (CLB)

Functional Description
R
FIXED Phase Shift Mode
The FIXED phase shift mode shifts the DCM outputs by a
fixed amount (T_PS), 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 T_CLKIN. The phase shift behavior is different
between ISE 8.1, Service Pack 3 and prior software ver-
sions, 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.
Please read the following Answer Record for
additional information.
Answer Record #23153
www.xilinx.com/xlnx/xil_ans_display.jsp?getPagePath
=23153
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.
t_PS
=
⎛
⎝
P_----H_-----A_-----S_-2--E_-5---S6_----H_-----I_--F_----T_{--⎠⎞}
•
T_CLKIN
Eq. 2
a. CLKOUT_PHASE_SHIFT = NONE
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.
t_PS
=
⎛
⎝
P_----H_-----A_-----S_-5--E_-1---S2_----H_-----I_--F_----T_{--⎠⎞}
•
T_CLKIN
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 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.
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.
Equation 2 or Equation 3 applies only to FIXED phase shift
mode. The VARIABLE phase shift mode operates differ-
ently.
CLKIN
CLKFB
(via CLK0 or CLK2X feedback)
b. CLKOUT_PHASE_SHIFT = FIXED
CLKIN
Shift Range over all P Values:
–255
0
P
256 ^{* TCLKIN}
+255
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
www.xilinx.com
DS312-2 (v3.4) November 9, 2006
Product Specification