XC3S100E Datasheet, PDF (52/193 Pages) Xilinx, Inc – DC and Switching Characteristics

R
Functional Description
ponent is enabled by setting the attribute to either the mode and the Variable Phase mode, respectively. These
FIXED or VARIABLE values, which select the Fixed Phase two modes are described in the sections that follow.
Table 29: PS Attributes
Attribute
CLKOUT_PHASE_SHIFT
PHASE_SHIFT
Description
Disables the PS component or chooses between
Fixed Phase and Variable Phase modes.
Determines size and direction of initial fine phase
shift.
Values
NONE, FIXED, VARIABLE
Integers from –255 to +255
Determining the Fine Phase Shift
The user controls the phase shift of CLKFB relative to
CLKIN by setting and/or adjusting the value of the
PHASE_SHIFT attribute. This value must be an integer
ranging from –255 to +255. This corresponds to a phase
shift range of –180 to +180 degrees, which is different from
the original Spartan-3 DCM. The PS component uses this
value to calculate the desired fine phase shift (T_PS) as a
fraction of the CLKIN period (T_CLKIN). Given values for
PHASE_SHIFT and T_CLKIN, it is possible to calculate T_PS
as follows:
T_PS = (PHASE_SHIFT/512) * T_CLKIN (4)
Both the Fixed Phase and Variable Phase operating modes
employ this calculation. If the PHASE_SHIFT value is zero,
then CLKFB and CLKIN are in phase, the same as when the
PS component is disabled. When the PHASE_SHIFT value
is positive, the CLKFB signal is shifted later in time with
respect to CLKIN. If the attribute value is negative, the
CLKFB signal is shifted earlier in time with respect to
CLKIN.
The Fixed Phase Mode
This mode fixes the desired fine phase shift to a fraction of
the T_CLKIN, as determined by Equation (4) and its
user-selected PHASE_SHIFT value P. The set of wave-
forms in Figure 41b 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.
In Figure 41:
• P represents the integer value ranging from –255 to
+255 to which the PHASE_SHIFT attribute is assigned.
(P = approximately -90 as shown)
• N is an integer value ranging from (P – 255) to
(+255 – P) that represents the net phase shift effect
from a series of increment and/or decrement
operations.
• N = {Total number of increments} – {Total number of
decrements} provided the user does not try to
increment past + 255 or decrement past -255. A
positive value for N indicates a net increment; a
negative value indicates a net decrement.
DS312-2 (v1.1) March 21, 2005
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Advance Product Specification