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Tri-state bus data bus delayed

SDavi9
Beginner
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I have a design where my data bus is tri-stated and everything seems to work correctly till the tri-state buffer. For some reason the output of the tri-state buffer is delayed by a clock from the input ? Could there be some Quartus setting that might be doing this ? 

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ak6dn
Valued Contributor III
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You don't say what version of Quartus or even what device you are using. Or if your tr-state bus is on chip or off chip.

That being said, physically any FPGA I am aware of does not have a true on-chip tri-state bus per se, but it is implemented with logic and multiplexors.

So logically your Verilog/VHDL may be written as a tri-state bus, but during synthesis it is turned into logic, specifically an N:1 multiplexor for the driver function.

Depending on how wide your N:1 mux is (ie, number of discrete source drivers) and how fast your clock rate is, it might just be the logic delay is longer than a clock cycle.

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SDavi9
Beginner
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We are using Quartus 15.1.  FPGA: Cyclone V GX (5CGXFC4F7M11I7) and the 32 bit tri-state bus is implemented on chip and is created by a simple standard VHDL code that is instantiated in my top level design. The output of the tri-state buffer feeds directly into the bi-directional data bus. The bi-directional bus is also feed back into the design where it is clocked\latched along with the control signals to catch the read data for the read cycle process. Note my clock is 100Mhz !

 

Your answer that the tri-state bus in synthesis is turned into N:1 multiplexor is interesting. Besides shifting my data earlier by a clock - is there anything else I can do to fix this ?

 

Also this current design is based upon an earlier design from a different chip (Cyclone V 5CEFA7U19I7N) where this issue was not seen ????

 

I looked at the RTL of the design and a very clear straight forward tri-state buffer was created as expected. Is there another level for synthesis that changes this tri-state buffer into the N:1 multiplexor that you mentioned ? Is there anyway to view this ?

 

Thank you once again for all your help

 

Shmuel Davis

 

 

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FvM
Valued Contributor III
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Hello,

I presume, Technology Map Viewer shows the gate level implementation.

By the way, now you are talking about clock, previously it sounds like the "delay" occors in pure combinational circuit. It's probably better to show an exact implementation.

Regards

Frank

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SDavi9
Beginner
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Thanks for your reply,

 

So below is the VHDL code at the upper level 

 

Signal usbDmuxOUT, usbDreadSMout_withbufferedZEROES :std_logic_vector(31 downto 0);

.....

usbDreadSMout is my "read cycle" output data from an internal state machine defined as :-

 

usbDreadSMout :out std_logic_vector(7 downto 0);

......

usbDreadCYCLEchanSELen is my open enable for usbDreadSMout from the same internal state machine 

......

usbDWRSMout is my "write cycle" output data from an internal state machine defined as :-

usbDWRSMout :out std_logic_vector(31 downto 0);

......

usbDen is my open enable for usbDWRSMout  from the same internal state machine 

 

My Bi-directional bus USBD  [ usbDWRSMout :out std_logic_vector(31 downto 0); ] is implemented as follows :-

 

usbDreadSMout_withbufferedZEROES(7 downto 0) <= usbDreadSMout; -- DEBUG USB429
usbDreadSMout_withbufferedZEROES(31 downto <= (others => '0'); -- DEBUG USB429

usbDmuxOUT <= usbDreadSMout_withbufferedZEROES when usbDreadCYCLEchanSELen = '1' else usbDWRSMout; -- DEBUG USB429

USBD <= usbDmuxOUT when usbDen = '1' or usbDreadCYCLEchanSELen = '1' else (others => 'Z');

 

Below I have attached the RTL implementation of the above

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SDavi9
Beginner
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Below I have added a shot of the Technology Map Viewer and in fact there does appear to be a latch on the usbDmuxOUT signals for some reason ??? 

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SDavi9
Beginner
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And in fact when I clicked on one of these Latches and asked for its location in the RTL view I in fact got the following 

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FvM
Valued Contributor III
2,862 Views

Registers may be shifted forth and back along the data path during synthesis, but without changing the logic function or latency.
After seeing the RTL and technology map snippets, I wonder where registering has been originally demanded in the code, e.g. which assignments are clocked.

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SDavi9
Beginner
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If this "shifting" of registers back and forth along the data path during synthesis happens - I would understand if this was done with a known expected latch in the actual design. But as can be seen in the RTL the latching of the data occurs in the Write and Read state machine blocks ! After the data leaves these SM blocks - there are no other registers in the design !

 

Why does the Quartus Synthesizer add this additional latch to implement the tri state buffer AND is there any way to prevent the synthesizer from doing this ?

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ak6dn
Valued Contributor III
2,806 Views

Are you referencing actual clocked registers? Or latches?

Quartus can move registers around and can replicate and/or merge registers as it sees fit.

If you write some clocked logic equations where you inadvertently don't always assign a value to the output Quartus may infer a latch and insert it.

So be very specific in your terminology of register vs latch when describing your logic.

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SDavi9
Beginner
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Sorry I was referring to the addition of a clocked register.

 

The problem can be seen clearly in the RTL representation attached : USB429 - usbD birectional output implementation - RTL.jpg

Where the Tri State buffer is connected directly to the output pin usbD[31..0]

 

Then in the Technology Map View shown in : USB429 - usbD birectional output implementation - Technology Map Viewer 1.jpg

 

and then expanded in : USB429 - usbD birectional output implementation - Technology Map Viewer 2.jpg

 

That a clocked register for some reason has been placed between my Mux and the Tri state buffer for some reason.

??????

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FvM
Valued Contributor III
2,828 Views
Don't believe that Quartus adds registers. That's why I'm asking for more detailed code.
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ShengN_Intel
Employee
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SDavi9
Beginner
2,777 Views

Dear Sheng,

Thank you for your reply.

I looked at those posts and the only thing that I saw that seemed relevant to my design was "when CASE or IF statements do not cover all possible input conditions...". In my SM for both the read and write cycles I use a CASE statement and I added the code :-

 

when others =>
    usbD <= (others => '0'); -- DEBUG USB429
end case;

 

However this didn't help, the clocked register shown in USB429 - usbD birectional output implementation - Technology Map Viewer 2.jpg above still exits and on the scope the output data is still delayed by 1 clock ?

 

Any other ideas ?

 

Best regards

 

shmuel 

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sstrell
Honored Contributor III
2,758 Views

Can you repost your code in a little bit easier to read format?

Clocked registers don't get magically added to a combinatorial design.  There must be other code (an IP?) that has a clocked process that might be causing this.

 

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SDavi9
Beginner
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So I have attached a file that represents all the parts of my code that do the writes to the usbD bidirectional data bus.

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ShengN_Intel
Employee
2,734 Views

Hi shmuel,

 

That clocked register is due to rising-edge triggered of clocked process. For example implementing a Rising Edge D Flip Flop check the code and technology map viewer images below:

Screenshot 2023-02-09 141838.pngScreenshot 2023-02-09 141652.png

If commented out if(rising_edge(Clk)) then, the clocked register is no more visible check images below:

Screenshot 2023-02-09 142656.pngScreenshot 2023-02-09 142725.png

In technology map viewer, at clocked register, you can Locate Node -> Locate in Design File

 

Thanks,

Best regards,

Sheng

p/s: If any answer from the community or Intel Support are helpful, please feel free to give best answer.

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ShengN_Intel
Employee
2,615 Views

Hi shmuel,

 

I had compiled the code provided with both Quartus II 15.1 and Quartus Standard 22.1 and got the results below instead (image):

Screenshot 2023-02-13 103222.png

But one thing can be confirmed that clocked register is because of the clock rising-edge trigger condition in process block. For example, by commenting out the usbCLK'event (elsif usbCLK = '1' /*and usbCLK'event*/ then) in FT601niosWRITEsm. I'm getting the technology map viewer result changes below (image):

Before: Screenshot 2023-02-13 103406.png             After: Screenshot 2023-02-13 104231.png

 

Thanks,

Best Regards,

Sheng

 

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SDavi9
Beginner
2,571 Views

Dear Sheng,

 

Thank you so much for all your help. 

In the first image that you showed above, as you indicated, there is no register between Mux and the IO_OBUF as there is in my design shown in the image below 

SDavi9_0-1676370438384.png

 

"But one thing can be confirmed that clocked register is because of the clock rising-edge trigger condition in process block. For example, by commenting out the usbCLK'event (elsif usbCLK = '1' /*and usbCLK'event*/ then) in FT601niosWRITEsm. I'm getting the technology map viewer result changes below (image)"

I see that from the Technology Map Viewer images that you show that the registers of the state machine have disappeared. However isn't this expected when you remove the clock from a clocked process that builds a statement machine - all the registers will be removed ? Also, the registers that disappeared are within the state machine block - it is not the register that I see that is implemented in the figure above associated with my tri state buffer ?

I expanded my Technology Map View of the write state machine as below and got the following :-

 

SDavi9_1-1676370915427.png

I am pretty sure that all the registers there will also disappear if I comment out the Clock in the clock process ?!

 

Bests regards

 

Shmuel 

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ShengN_Intel
Employee
2,554 Views

Hi,

 

I just compiled the original code you provided and didn't make any change to the code (check previously attached test.qar). Then I got the first image result where there is no register between Mux and the IO_OBUF. (May be you have use usbDmuxOUT in any clock process and didn't provide that part of code I'm not sure)

 

By commenting out the usbCLK'event (elsif usbCLK = '1' /*and usbCLK'event*/ then) in FT601niosWRITEsm is just to show you that's possible to make the clocked register disappear.

 

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ShengN_Intel
Employee
2,475 Views

Hi Shmuel,


Let me know if you have any further concern or consideration?


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