Xilinx 1000BASE-X - Manual (Page 162)

162 www.xilinx.com Ethernet 1000BASE-X PCS/PMA or SGMII v9.1
UG155 March 24, 2008
Chapter 12: Constraining the Core R
the HDL source code for the example design and with the information contained in
Chapter 7, “1000BASE-X with RocketIO Transceivers.”
Clock Period Constraints
The clock provided to userclk must be constrained for a clock frequency of 62.5 MHz.
The clock provided to userclk2 must be constrained for a clock frequency of 125 MHz.
The following UCF syntax shows the necessary constraints being applied to the example
design.
############################################################
# PCS/PMA Clock period Constraints: please do not relax #
############################################################
NET "brefclk_ibufg" TNM_NET = "brefclk";
TIMESPEC "ts_brefclk" = PERIOD "brefclk" 16 ns HIGH 50 %;
NET "rocketio/rxrecclk" TNM_NET = "rxrecclk";
TIMESPEC "ts_rxrecclk" = PERIOD "rxrecclk" 16 ns;
NET "clk0" TNM_NET = "clk0";
TIMESPEC "ts_clk0" = PERIOD "clk0" "ts_brefclk";
NET "clk2x180" TNM_NET = "clk_tx";
TIMESPEC "ts_tx_clk" = PERIOD "clk_tx" "ts_brefclk"/2 PHASE + 4 nS HIGH
50 %;
Setting MGT Attributes
MGT attributes can be set by either of these methods:
•
Directly from HDL source code during MGT instantiation (see the HDL source code
for the example design)
•
From the UCF
Attributes set from a UCF take priority. The UCF for the example design defines some
user-modifiable attributes as illustrated in the following example. All attributes used in the
example design UCF are based on the GT_ETHERNET_1 defaults.
############################################################
# Rocket I/O constraints: #
# please refer to Rocket I/O documentation #
############################################################
INST "rocketio/mgt" TX_CRC_USE = FALSE;
INST "rocketio/mgt" RX_CRC_USE = FALSE;
INST "rocketio/mgt" REF_CLK_V_SEL = 1;
INST "rocketio/mgt" TERMINATION_IMP = 50;
INST "rocketio/mgt" TX_DIFF_CTRL = 500;
INST "rocketio/mgt" TX_PREEMPHASIS = 0;
MGT Transceiver Placement Constraints
The following UCF syntax illustrates the MGT transceiver placement for the example
design. Special attention must be made to the placement of the SERDES alignment flip-flop
as described in the RocketIO Transceiver User Guide (Chapter 2, SERDES Alignment, Ports, and
Attributes, ENPCOMMAALIGN, ENMCOMMAALIGN). This is the single flip-flop
illustrated in
Figure 7-1 .