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AR# 42713

Spartan-6 GTP: Clock Forwarding Hints


The clock forwarding feature allows sharing ofone REFCLK signal to both GTP in the dual or to both East and Westduals. However, there are some caveats to keep in mind while forwarding the clock from East to West and vice-versa.


Simulation and Hardware

Usually, the Wizard is the safest way to implement the correct clock tree. The CORE Generator softwareadds some logic that is needed for the simulation only; you can find it in file s6_gtpwizard_v1_10.vhd:

simulation : if WRAPPER_SIMULATION = 1 generate
TILE0_PLLLKDET0_OUT <= tile0_plllkdet0_i2;
if (TILE0_GTPRESET0_IN = '1') then
count00 <= "00000";
elsif(TILE0_CLK00_IN'event and TILE0_CLK00_IN ='1') then
if((count00 = "10100") or (tile0_plllkdet0_i = '0')) then
count00 <= "00000";
count00 <= count00 + "00001";
end if;
end if;
end process;
end generate simulation;

In some cases,you mightprefer to keep this piece of code in the hardwareimplementation, but it is not really needed. If you decide to change the clock forwarding tree, and, if, for some reason you want to implement this piece of logic, please do not forget to replace TILE0_CLK00_IN with the new toggling clock source. If you forget this, the GTP will never get out of reset.

Power Saving

When the reference clock is forwarded from East to West, or vice-versa, and the forwarding tile is not used, you can power down part of the unused GTP. Only the signal REFCLKPWRDNBx must be set to '1', otherwise, the forwarding circuit will not be powered.
AR# 42713
Date 12/15/2012
Status Active
Type General Article
  • Spartan-6 LXT