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Altera_Forum's avatar
Altera_Forum
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13 years ago

Making a counter from 50MHz Clock?

Hi,

I know this might be a really silly question but I can't seem to get the theory down into HDL:

I want to make a 3579545Hz clock from a 50MHz clock input:

i know that

--- Quote Start ---

1/3579545 = 2.7936511484001458285899464876123e-7

Multiply that by 1000,000 and get 279.36511484001458285899464876123ns

--- Quote End ---

:D

but how i use this info to make a clock? that's another story.

I've tried just using a PLL, but it didnt like my desired output. I'm thinking it was in the wrong format but i don't know for sure.

In the megawizard, i told it that my input clock was 50MHz. the output could only be in MHz ps or ns. of course it did not like my desired output saying that tthe requested multiply/div factors were not achievable.

could someone please help?

P.S.

I tried reading the ALTPLL.pdf but it was very hard to do

14 Replies

  • Altera_Forum's avatar
    Altera_Forum
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    13 years ago

    why do you need a very specific frequency? The 50MHz clock probably isnt accurate enough in the first place (a few ppm error, so could actually be about 49.99MHz or 50.01) so creating a specific clock like yours wont be very exact

  • Altera_Forum's avatar
    Altera_Forum
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    13 years ago

    Have you thought of an accumulator like that in an NCO (i.e. DDS) that adds 3579545 modulo 50 million. The MSB becomes your required rate.

    It wouldn't be dead regular but it might be good for your device.
  • Altera_Forum's avatar
    Altera_Forum
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    13 years ago

    --- Quote Start ---

    Have you thought of an accumulator like that in an NCO (i.e. DDS) that adds 3579545 modulo 50 million. The MSB becomes your required rate.

    It wouldn't be dead regular but it might be good for your device.

    --- Quote End ---

    I've never even heard of such a thing. Do i google "Accumulator DDS"?
  • Altera_Forum's avatar
    Altera_Forum
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    13 years ago

    You have it in NCOs hidden there.

    just use a suitable width adder starting from zero adding 3579545 every system clock and carrying over result after (50 million -1) i.e. if it becomes > (50 million-1) then subtract 50 million from it and keep going nonstop.

    Then MSB is your rate.

    The generated clock is correct rate-wise but duty cycle varies.
  • Altera_Forum's avatar
    Altera_Forum
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    13 years ago

    --- Quote Start ---

    You have it in NCOs hidden there.

    just use a suitable width adder starting from zero adding 3579545 every system clock and carrying over result after (50 million -1) i.e. if it becomes > (50 million-1) then subtract 50 million from it and keep going nonstop.

    Then MSB is your rate.

    The generated clock is correct rate-wise but duty cycle varies.

    --- Quote End ---

    so then: 

    
if ((counter > 49,999,999) AND (counter < 3579545Hz))
    counter = counter - 50,000,000
endif;
if(counter == 3579545Hz)
    clk_enable <= "1"
else 
    clk_enable <= "0"
endif;

    is that what i need to do? (not actual VHDL syntax :) )
  • Altera_Forum's avatar
    Altera_Forum
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    13 years ago

    Wrong, I am afraid.

    choose counter of 26 bits (to cover 50 million) and I will add extra bit for overflow protection. I prefer to use variable here so that the count is corrected before it is too late. 

    
process(clk)
variable count : unsigned(26 downto 0) := (others => '0');
begin
if rising_edge(clk) then
    if count > 50000000-1 then
         count := count - 50000000;
    else
         count := count + 3579545;
    end if;
    my_clk <= std_logic(count(25));
end if;
end process;
  • Altera_Forum's avatar
    Altera_Forum
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    13 years ago

    so if bit 25 is set, then this is 3479545? interesting technique. never seen it before. :) I will give that a try thanks!!!

  • Altera_Forum's avatar
    Altera_Forum
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    13 years ago

    Refer to my correction of code above. Moreover you can factorise your values of 50MHz and new clock by dividing each by 5 then use modulo 10000000 instead saving few bits on adder.

    Notice this adder is a general case for any rate below clock rate and is equivalent to using explicit counters (which are adders anyway) for example to get half clock out of 50000000 then you will say add 25000000 modulo 50000000 then factorise it to the ratio 2:1 hence add 1 modulo 2 which is what we do by just having 1 bit toggle without the hassle of modulo but modulo serves a good generic concept.