Xilinx LOGICORE UG144 - Manual (Page 103)

1-Gigabit Ethernet MAC v8.5 User Guide www.xilinx.com 103
UG144 April 24, 2009
Required Constraints R
-- DISCONTINUED PRODUCT --
INST "rgmii_rxd<?>" TNM = IN_RGMII;
INST "rgmii_rx_ctl" TNM = IN_RGMII;
TIMEGRP "DDR_RISING" = FFS;
TIMEGRP "DDR_FALLING" = FALLING FFS;
TIMEGRP "IN_RGMII" OFFSET = IN 1.1 ns VALID 2.2 ns BEFORE "rgmii_rxc" TIMEGRP "DDR_RISING";
TIMEGRP "IN_RGMII" OFFSET = IN -2.9 ns VALID 2.2 ns BEFORE "rgmii_rxc" TIMEGRP "DDR_FALLING"
The constraints defined in the preceding lines have a built in 10% tolerance: this allows all
combinations of the example designs to pass timing. However, these should be tightened
in a real customer design as per the following syntax. Manual adjustments to IDELAY
values or DCM phase shift requirements may be required to meet these constraints (see the
following family specific sections for details).
TIMEGRP "IN_RGMII" OFFSET = IN 1 ns VALID 2 ns BEFORE "rgmii_rxc" TIMEGRP "DDR_RISING";
TIMEGRP "IN_RGMII" OFFSET = IN -3 ns VALID 2 ns BEFORE "rgmii_rxc" TIMEGRP "DDR_FALLING"
Spartan-3, Spartan-3E, Spartan-3A, and Virtex-4 Devices
The RGMII design uses a DCM on the receiver clock domain for all devices except Virtex-
5. Phase-shifting is then applied to the DCM to align the resultant clock so that it will
correctly sample the 2 ns RGMII data valid window at the input flip-flops.
The fixed phase-shift is applied to the DCM using the following UCF syntax.
INST *rgmii_interface/rgmii_rxc_dcm CLKOUT_PHASE_SHIFT = FIXED;
INST *rgmii_interface/rgmii_rxc_dcm PHASE_SHIFT = 40;
The value of PHASE_SHIFT is preconfigured in the example designs to meet the setup and
hold constraints for the example RGMII pinout in the particular device. The setup/hold
timing which is achieved after place-and-route is reported in the data sheet section of the
TRCE report (created by the implement script).
For customers fixing their own pinout, the setup and hold figures reported in the TRCE
report can be used to initially setup the approximate DCM phase-shift.
Appendix C,
“Calculating DCM Phase-Shifting” describes a more accurate method for fixing the phase-
shift by using hardware measurement of a unique PCB design.
Virtex-5 Devices
The RGMII design uses IODELAY components on both the receiver and transmitter clock
domains for Virtex-5 devices. A fixed tap delay is applied to the rgmii_txc output clock
to move the rising edge of this clock to the centre of the output data window. For the
receiver clock, data and control signals, a fixed tap delay can be applied to either delay the
data and control signals or delay the clock so that the data/control are correctly sampled
by the rgmii_rxc clock at the IOB IDDR registers, meeting RGMII setup and hold timing.
The choice of delaying data/control or clock is dependant upon a number of factors, not
least being the required shift. There are trade-offs to be made with either choice. Delaying
the clock is clock-period specific as we move the clock to line up each edge with data from
the following edge. Delaying the data/control introduces more jitter which degrades the
overall setup/hold window. The interface timing report in the two cases is also quite
different. See
“Understanding Timing Reports for RGMII Setup/Hold timing.”
The following constraint shows an example of setting the delay value for two of these
IODELAY components. Data/Control bits can be adjusted individually to compensate for
any PCB routing skew, if desired.