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Altera_Forum's avatar
Altera_Forum
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10 years ago

ALTSHIFT_TABS automatically inserted into the design

I had the following in my design: 

r_reg1 <= { r_reg1, inputpin };

Then I had to introduce a condition to select which register input data will be shifted into, I did it this way: 

if(r_condition)
        r_reg1 <= { r_reg1, inputpin };
        else    r_reg2 <= { r_reg2, inputpin };

And my project stops compiling because Quartus tries to use another M9K block for Altshift_Tabs megafunction, and all the RAM blocks are already in use.

Why Quartus is doing it? Why it does not do what it was instructed to do - just use two registers and shift input data in respective one on the condition?

Please advise. Thank you.

2 Replies

  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    10 years ago

    I found the solution.

    Assignments -> Settings -> Analysis & Synthesis settings -> Auto shift register replacement=OFF.

    --- Quote Start ---

    To infer shift registers, synthesis tools detect a group of shift registers of the samelength and convert them to an ALTSHIFT_TAPS megafunction. To be detected, all theshift registers must have the following characteristics:

    &#9632; Use the same clock and clock enable

    &#9632; Do not have any other secondary signals

    &#9632; Have equally spaced taps that are at least three registers apart

    --- Quote End ---

    --- Quote Start ---

    If theAuto Shift Register Recognition setting is set to Auto, Quartus II integratedsynthesis uses the Optimization Technique setting, logic and RAM utilizationinformation about the design, and timing information from Timing-Driven Synthesisto determine which shift registers are implemented in RAM blocks for logic.

    --- Quote End ---

    From the Altera's guide "6. Recommended HDL Coding Styles".
  • Altera_Forum's avatar
    Altera_Forum
    Icon for Honored Contributor rankHonored Contributor
    10 years ago

    If you're happy to leave it global, thats fine - but you can set that setting per module - either in the assignments editor or modifying the .qsf file by hand.