Xilinx 1000BASE-X - Manual (Page 174)

174 www.xilinx.com Ethernet 1000BASE-X PCS/PMA or SGMII v9.1
UG155 March 24, 2008
Chapter 12: Constraining the Core R
GMII Input Setup/Hold Timing
Input GMII timing specification
Figure 12-3 and Table 12-2 illustrate the setup and hold time window for the input GMII
signals. These are the worst-case data valid window presented to the FPGA device pins.
Observe that there is, in total, a 2 ns data valid window of guaranteed data which is
presented across the GMII input bus. This must be correctly sampled by the FPGA devices.
Virtex-II, and Virtex-II Pro devices
Figure 5-14 illustrates the GMII input logic which is provided by the example design for
the Virtex-II and Virtex-II Pro family. Although not illustrated, these families have input
delay elements (which are always of a fixed delay). These are also automatically inserted
by the Xilinx tools and are set to provide a zero-hold time.
These input delays will automatically meet input setup and hold timing on the GMII
without any specific constraints.
Spartan-3, Spartan-3E, and Spartan-3A devices
Figure 5-15 illustrates the GMII input logic which is provided by the example design for
the Spartan-3 class family. A DCM must be used on the gmii_tx_clk clock path as
illustrated. Phase-shifting is then applied to the DCM to align the resultant clock so that it
will correctly sample the 2ns GMII data valid window at the input flip-flops.
The fixed phase shift is applied to the DCM using the following UCF syntax.
INST "gmii_tx_dcm" CLKOUT_PHASE_SHIFT = FIXED;
INST "gmii_tx_dcm" PHASE_SHIFT = -20;
INST "gmii_tx_dcm" DESKEW_ADJUST = 0;
The value of PHASE_SHIFT is preconfigured in the example designs to meet the setup and
hold constraints for the example GMII pinout in the particular device. The setup/hold
Figure 12-3: Input GMII timing
Table 12-2: Input GMII Timing
Symbol Min Max Units
t
SETUP
2.00 - ns
t
HOLD
0.00 - ns
t
SETUP
t
HOLD
GMII_TXD[7:0],
GMII_TX_EN,
GMII_TX_ER
GMII_TX_CLK