Vitis HLS change of datatype makes variable unused

Question

I'm trying to implement a signal processing algorithm in Vitis HLS. For this, I read in a few variables via AXI Stream and AXI Lite.

Simulation and synthesis did already work with the complete project.

Now I wanted to make the center frequency 'fc' adjustable via AXI Lite. I noticed, that at some point all my AXI Lite variables are marked as unused in the synthesis report (resource utilization does also drop - so they probably are really not used by the synthesis tool).

I was able to track the issue down to a single change: changing the datatype of 'fc' from float to 'ap_fixed<32,27>'.

However, I don't understand in the slightest how this would result in the observed error and couldn't find any information in that direction.

Below is a code snipped of the only function where 'fc' is actually used. All other functions only pass the read value from the top function to CalculateElement().

void CalculateElement(in2_t phi, float fc,  in3_t xpos, in1_t &w_real, in1_t &w_imag){
    constexpr double pi = 3.14159265359;
    constexpr double c_0 = 299792458; // in m/s
    constexpr double mm_to_m = 1000; // adjust xpos from mm to m
    float temp = 2*pi/c_0*mm_to_m;
    in3_t k = temp*fc;

    // -1 in complex exponent and -1 from propagation direction (incoming wave) 
    // cancel each other out
    w_real = hls::cos(k*hls::cos(phi)*xpos)/4;
    w_imag = hls::sin(k*hls::cos(phi)*xpos)/4;
}

I would be really thankful for any help.

Mt_266 · Accepted Answer · 2024-11-10 19:59:50Z

1

I found the problem. The fixed point precision was too low. This resulted in temp*fc always evaluating to zero. The compiler then optimized everything away as multiplication with zero is zero again - and sin(0) or cos(0) can also be calculated at compile time, therefore no CORDIC-Hardware was inferred. After playing a bit with the fixed point ranges everything works as it should.

answered Nov 10, 2024 at 19:59

Mt_266

414 bronze badges

Sign up to request clarification or add additional context in comments.

Collectives™ on Stack Overflow

Vitis HLS change of datatype makes variable unused

1 Answer 1

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

Comments

Your Answer

Sign up or log in

Post as a guest

Related