ALTSHIFT_TABS automatically inserted into the design

Altera_Forum · ‎02-05-2016

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.

Altera_Forum · ‎02-05-2016

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:

■ Use the same clock and clock enable

■ Do not have any other secondary signals

■ 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 · ‎02-05-2016

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.