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Problem with sampling of the outputs of a CARRY4 delay line chain

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msdarvishi

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Hello friends,

I would like to share my current issue with you and get your advise about it. I am working on a CARRY4 delay line based design in order to sample its output and in each clock rising edge, I would like to compare the sampled output with the old latched output by CARRY4 delay line chain.
I configured a delay line with CARRY4 primitives with 256 states that captures a digital signal called trigger coming from an LFSR (Linear Feedback Shift Register). At each rising clock transition of tdc_clock that is 400 MHz, I sample the captured signal by a DFlipFlop and compare it with its previous captured signal. As you see in the attached timing diagram, I provide the compared_output signal by subtracting the latched_output (captured by delay line) and thesampled_outout to detect any difference. A flag called ODC_result_sig will be raised if the comparison result is not 0. But I do not know why my compared result is not always 0 because I did not inject any extra delay to the digital signal and ODC_result_sig is raised unnecessarily. Could you please verify my timing diagram and give me your idea to figure out the problem?

I also send you my architecture for sampling and comparison as well as the definitions of sampling circuit and its initialization and the comparison method in the following:


------------------ Sampling circuit with DFF -----------------

Code VHDL - [expand]
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Inst_DFF: DFF PORT MAP(
D => latched_output_sig,
Q => sampled_output_sig,
QN => open,
CLK => clock_400MHz_AND,
RESET =>  sync_reset
);


-----------------------------------------------------------------



------------- writing a process for compared_output calculation -------

Code VHDL - [expand]
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process (clock_400MHz_AND, latched_output_sig, sampled_output_sig)
begin
  if(RISING_EDGE(clock_400MHz_AND)) then
   compared_output <= sampled_output_sig - latched_output_sig ;
   compared_output_1bit <= sampled_output_sig(255 downto 254) - latched_output_sig(255 downto 254);
   compared_output_first <= sampled_output_sig(15 downto 0) - latched_output_sig(15 downto 0);
  end if;
end process;

-----------------------------------------------------------------------


I thank in advance for your always kind consideration and assist.
 

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ads-ee

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I'm not at all sure how you expect any of this to work.

The delays from the AX to the DMUX carry output is 0.43 ns in the -3 speed grade of a Virtex 7. Given that you have 256 bits and 4-bits per CARRY4 means you have 64 CARRY4s, therefore 0.43*64 = 27.52 ns of cell delays alone. That isn't even close to being clockable at 400 MHz.

Besides that I'm not sure what your problem is with the simulation, so you delayed the latched_output with a register and called it sampled_output, since it's delayed unless the latched_output never changes the sampled_output will differ from the latched_output every time latched_output is updated with a new value. So the simulation waveform looks correct to me.
 

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How is the trigger signal "captured" without a clock?
 

msdarvishi

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How is the trigger signal "captured" without a clock?


The trigger signal is coming fro an LFSR circuit and it's like you see in the timing diagram...

- - - Updated - - -

I'm not at all sure how you expect any of this to work.

The delays from the AX to the DMUX carry output is 0.43 ns in the -3 speed grade of a Virtex 7. Given that you have 256 bits and 4-bits per CARRY4 means you have 64 CARRY4s, therefore 0.43*64 = 27.52 ns of cell delays alone. That isn't even close to being clockable at 400 MHz.

Besides that I'm not sure what your problem is with the simulation, so you delayed the latched_output with a register and called it sampled_output, since it's delayed unless the latched_output never changes the sampled_output will differ from the latched_output every time latched_output is updated with a new value. So the simulation waveform looks correct to me.


Thanks @ads-ee for your reply.
I do not car about those delays that you mentioned. I want to compare the latched_output signal with the sampled_output signal at each rising clock transition and detect any undesired delay occured in the trigger signal. I mean, if I inject a delay into the trigger signal, since its latched_output value is changed with the new clock cycle, it will be different as of sampled_output signal in the previous clock cycle. So, in this case, the compared_output must go to '1' state since the values of latched_output and sampled_output are different. But I see the compared_output signal varies between '0' and '1' states while no delay is injected into the latched_output signal !! It seems weird. I would cordially appreciate to have any suggestion by you.

Thanks,
 

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I do not car about those delays that you mentioned.
You should care it won't work in hardware. The primitive you are using is a carry chain, the AX-DMUX output delay defines the delay from end to end of the 256 bit chain. By the time you capture in sampled_output using the 400 MHz clock the chain has only propagated the carry around 5 CARRY4 primitives.

I still don't get why you think there is anything wrong with the simulation. compared_output changes on the clock edge every time sampled_output and latched_output are different. You can see it stops doing that when latched_output doesn't change for 4 clock cycles around 495 ns to 517 ns.

I'm thinking your problem with the waveform has more to do with not understanding 0 delay simulations and thinking the change in value should be captured at the leading clock edge instead of the trailing edge of the value change.

Code:
[FONT=Courier New]   __    __    __    __    __    __    __    __
__|  |__|  |__|  |__|  |__|  |__|  |__|  |__|  |
__ _____ _____ _________________ _____ _____ ___
__X__a__X__b__X__c______________X__d__X__e__X___
__ _____ _____ _____ _________________ _____ ___
__X_____X__a__X__b__X__c______________X__d__X___
__ _____ _____ _____ _________________ _____ ___
__X_____X_a=?_X_b=a_X_c=b_X_c=c_______X_d=c_X___[/FONT]

Remember you are using this for compared_output:

Code VHDL - [expand]
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if(RISING_EDGE(clock_400MHz_AND)) then
   compared_output <= sampled_output_sig - latched_output_sig ;

 

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