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bypassed ALM register

ymiler
Employee
883 Views

Hi 

I would like to ask what is them meaning of "bypassed ALM register "

You can see below the schematic & data path :

 

ymiler_0-1651517915647.png

ymiler_1-1651517919805.png

According to this reports , Quartus use this FF as a combinatorial element like LUT 

Why ? the timing tool should calc setup & hold data path from FF to FF 

Why there is byapss on this FF ? 

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sstrell
Honored Contributor III
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Assuming this is a Hyperflex based device (Stratix 10 or Agilex), a bypassed ALM register means the hyper-retimer bypassed that ALM register and enabled a hyper-register in the fabric to improve performance.  The pink registers in the tech map viewer represent the hyper-registers.  Here's a good overview:

https://learning.intel.com/developer/learn/course/external/view/elearning/404/introduction-to-hyper-retiming

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ymiler
Employee
848 Views

OK

Thanks

 

 

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SyafieqS
Moderator
838 Views

Yishay,


Let me know if there is any other concern on this.

For hyperflex device, yes the Quartus will make use of hyper-register instead of alm reg to achieve better timing and faster frequency during hyper-retiming, thats the reason you alm reg is bypassed.


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ymiler
Employee
828 Views

Hi 

 

According to the intro of Hyper-Retiming  :

https://learning.intel.com/developer/learn/course/external/view/elearning/404/introduction-to-hyper-retiming

the tool have the option move the FF after the LUT :

ymiler_0-1651735692859.png

when something like this happens - we have another value ?

because before the retiming the FF dout is same value like the previos FF ,but after retiming the dout value is changed according to LUT output 

how do you keep the functionally equivalent ?

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sstrell
Honored Contributor III
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If you keep watching the training, you’ll see that the hyper-retimer will only make moves that maintain the functionality of the design. It follows a set of algorithmic rules to ensure this.
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SyafieqS
Moderator
788 Views

Yishay,


May I know if there is any update or other concern?


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ymiler
Employee
780 Views

Hi 

 

I still dont understand ,

According to your traing - the Hyper-Retiming tool keep the state 

Meaning - if the previous state was 3 FF between input -> output it will stay in the same num of FF - the Hyper-Retiming  can't add or remove FF in order to keep the functionlity 

ymiler_0-1652097755365.png

I dont understant it - lets see the top figure for example - when I probe by signal tap the value after the second FF I will get spesific value (after 3 LUT's) but after the retiming the value after second FF will get another value (only 2 LUT's instead of 3 LUT's )

So , when I try to debug my system I will get unexpect value .

please advice  

 

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ymiler
Employee
771 Views

Hi 

I wrote it again, more clarify 

 

I still don't understand,

According to your training - the Hyper-Retiming tool keep the functionality 

Meaning - if the previous state was 3 FF between input -> output it will be same num of FF after the Hyper-Retiming action it will be 3 FF between input -> output

The Hyper-Retiming can't add or remove FF because the tool should keep the functionality 

ymiler_1-1652106002221.png 

But still there is another implementation - this is effect the debugging 

For example, let’s see the figure above -

Left side (red arrow) - when I probe the value after the second FF (by signal tap) I will get specific value (after 3 LUT's)

But after the retiming - right side (green arrow) the value after second FF will be get another value (only 2 LUT's instead of 3 LUT's)

So, when I try to debug the FPGA I will get unexpected values. I have mismatch between my code and the implementation

this situation disturbs me to debug the FPGA  

please advice  

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sstrell
Honored Contributor III
762 Views

Again, the functionality is not changed when retiming.  If you had added Signal Tap to a design, it would actually prevent retiming moves like this to maintain the same functionality so you would see the correct signaling, but you would not have the most optimized design.  This is why when you finish debugging a design you want to remove all debugging hardware and recompile to optimize.

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ymiler
Employee
758 Views

Great 

 

Now everything is clear 

 

Thank you

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SyafieqS
Moderator
740 Views

I’m glad that your question has been addressed, I now transition this thread to community support. If you have a new question, feel free to open a new thread to get the support from Intel experts. Otherwise, the community users will continue to help you on this thread. Thank you.


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