For all the Xilinx base design, define three global clock nets, which
are saved in the following three global variable: $sys_cpu_clk, $sys_dma_clk
and $sys_iodelay_clk.
These clock nets are connected to different clock sources depending of
the FPGA architecture used on the carrier. In general the following
frequencies are used:
- sys_cpu_clk - 100MHz
- sys_dma_clk - 200MHz or 250Mhz
- sys_iodelay_clk - 200MHz or 500Mhz
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