How to control clock frequency?

You have to add some VHDL code that uses a counter to divide the clock.

For example you create a counter (in VHDL) with the clock as inputclock that counts to a million (or more). And once the counter reaches a million you change the state from the LED (so on or off).

A PLL is too complex for this purpose and I doubt that is what you want/need to do.

So something like this (this is AHDL, with probably some syntax errors):

SUBDESIGN counter

(

clock : INPUT;

LED_OUT : OUTPUT;

)

VARIABLE

count[20..0] : DFF; //create a counter that can count to 2^21-1 =1048576

LED : DFF; //help flipflop

BEGIN

count[].clk = clock; //link clocks

LED.clk = clock;

count[].d = count[].q+1; //start counting

If Counter[21].q==1 Then

LED = not LED; //change state

Counter = 0; //reset counter

end if

LED_OUT = LED; //Link help flipflop to output

END;