104 www.xilinx.com Ethernet AVB Endpoint User Guide
UG492 July 23, 2010
Chapter 11: Constraining the Core
PERIOD Constraints for Clock Nets

PLB_clk

The clock provided to PLB_clk must be constrained to the appropriate frequency. Note
the frequency range of the embedded processor to which this bus is connected. For
example, the maximum clock speed of the MicroBlaze™ processor is 100 MHz.
The following UCF syntax shows a 100 MHz period constraint being applied to the
PLB_clk signal:
NET "plb_clk" TNM_NET = "plb_clk";
TIMEGRP "plb_clock" = "plb_clk";
TIMESPEC "TS_plb_clock" = PERIOD "plb_clock" 10000 ps HIGH 50 %;

host_clk

The clock provided to host_clk must be constrained to the desired Management
Interface operating frequency of the Tri-Mode Ethernet MAC. If host_clk is connected to
the same clock source as any other Ethernet AVB Endpoint input clock (for example
PLB_clk or ref_clk), then this constraint is unnecessary and can be removed.
The maximum supported frequency of host_clk, as specified by the Tri-Mode Ethernet
MAC core, is 125 MHz.
The following UCF syntax shows a 125 MHz period constraint being applied to
host_clk:
NET "host_clk" TNM_NET = "host_clk";
TIMEGRP "host_clock" = "host_clk";
TIMESPEC "TS_host_clock" = PERIOD "host_clock" 8000 ps HIGH 50
%;

tx_clk

The interface clock of the Ethernet MACs transmitter must be constrained to the correct
maximum frequency. This is 125 MHz for 1-Gigabit Ethernet rates.
The following UCF syntax shows the necessary constraints being applied to tx_clk:
NET "tx_clk" TNM_NET = "tx_clk";
TIMEGRP "tx_clock" = "tx_clk";
TIMESPEC "TS_tx_clock" = PERIOD "tx_clock" 8000 ps HIGH 50 %;

rx_clk

The interface clock of the Ethernet MACs receiver must be constrained to the correct
maximum frequency. This is 125 MHz for 1-Gigabit Ethernet rates.
The following UCF syntax shows the necessary constraints being applied to rx_clk:
NET "rx_clk" TNM_NET = "rx_clk";
TIMEGRP "rx_clock" = "rx_clk";
TIMESPEC "TS_rx_clock" = PERIOD "rx_clock" 8000 ps HIGH 50 %;