How to cut clock speed in half?

One way would be to use a PLL to produce a 25 MHz clock from the 50 MHz clock. Check the megafunctions.

Another good way would be to use a clock enable.

reg clk_enable;

always @ (posedge clk) clk_enable <= ! clk_enable;

^^ enable is "1" just for every other clock

Then you wrap arround every piece of code you need to run at 25 MHz like this

always @ (posedge clk)

begin

if (clk_enable) begin

// Your code goes here

end

end