Hamster High Speed Frequency Counter 950MHz timing error


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While trying to test Hamster's high speed frequency counter at 950MHz, a "WARNING:Par:468 - Your design did not meet timing" is generated.  Per instruction on http://hamsterworks.co.nz/mediawiki/index.php/High_Speed_Frequency_Counter,


I added 


NET test_signal_p LOC = "P51" | IOSTANDARD = LVDS_33 | PERIOD=1.051ns;
NET test_signal_n LOC = "P50" | IOSTANDARD = LVDS_33;
NET "prescaled" PERIOD=2.102;

to fast_freq_counter.ucf, and modified fast_freq_counter.vhd as followed
entity fast_freq_counter is
    Port ( clk32         : in  STD_LOGIC;
           test_signal_p : in  STD_LOGIC;
           test_signal_n : in  STD_LOGIC;
           prescaled : buffer  STD_LOGIC;
           tx            : out STD_LOGIC);
end fast_freq_counter;
      if rising_edge(test_signal) then
         prescaled <= not prescaled;
      end if;
   end process;
i_input_counter: input_counter PORT MAP(
      test_signal => prescaled,
      gray_count => gray_count

According to Design Summary, best case achievable for constraint "NET "test_signal" PERIOD = 1.051 ns HIGH 50%;" is 1.710ns.  All other constraints are met.   (Unmodified, the original 500MHz vhdl has best case achievable timing of 1.636ns.  So the modifications I made make implementation slower.) I seek guidance on solving Par:468 timing error.  Thank you for your time.

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Prescaled is running at 1/2 th the speed of the input signal, so the constraint should be about 2.102ns - you did add the extra timing constraint:


  NET "prescaled" PERIOD=2.102;


Which lets the tool know how fast it will be toggling.


I'm not home for three days or so, but can send you through my modified project then  if you want? 



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Thank you.  I did add the timing constraint " NET "prescaled" PERIOD=2.102;" to fast_freq_counter.ucf.  If I understand your reply correctly, prescaled should be the most stringent timing requirement in fast_freq_counter.ucf.  So I modified fast_freq_counter.ucf by deleting the timing constraint on test_signal_p.  My new fast_freq_counter.ucf is


NET clk32 LOC="P94"  | IOSTANDARD=LVTTL | PERIOD=31.25ns;      # CLK

NET test_signal_p  LOC = "P51" | IOSTANDARD = LVDS_33;
NET test_signal_n  LOC = "P50" | IOSTANDARD = LVDS_33;
NET "prescaled"  PERIOD=2.102;
This new constraint file was successfully implemented without generating any warning or error.   In Timing Constraints under Design Overview, it said the constraint "NET "prescale" PERIOD=2.102ns High 50%" was met.  I am going to see if I can get a function generator to test the counter.  Previously, I tested 500MHz version up to 112MHz with IOSTANDARD= both LVDS_33 and LVTTL.  I'll let you know in a day or two of the result.  Thanks again.
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Testing progress:

Did some testing with an oscillator built from a 74xx04 NOT-gate, the Hamster high speed frequency counter can count to 80MHz, the max

speed of the ring oscillator, without problem.  I was hoping the ring oscillator can go faster.  I am goingto  use the FPGA next to see if I can generate a higher frequency.  Thanks for your help.

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