Received waveform frequency is way too low than it was sent

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You have many variables in your setup:

1) nios runs at known speed but your tone frequency depends on instructions (nios is no good as NCO) so your frequency at generation is unknown

2) buffering adds further time uncertainty

3) plotter needs to be scaled correctly but I don't think you can

so with your given info it is not doable.

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Thank you kaz for your reply.

Regarding point 1 and point 2, instead of getting Vref from fpga block, now I have the Vref data in C code (as below, just for testing), so I am no longer doing any buffering, still I obtain the same result where the period of a complete cycle of sinewave is 0.16 minutes. So, if I understand correctly, now the data is available in C code straightforward, so shouldn't I expect the data can be plotted faster?

Regarding point 3, I don't understand how to scale the plotter.

float Vref = {0.00,0.06,0.13,0.19,0.25,0.31,0.37,0.43,0.48,0.54,0.59,0.64,0.68,0.73,0.77,0.81,0.84,0.88,0.90,0.93,0.95,0.97,0.98,0.99,1.00,1.00,1.00,0.99,0.98,0.97,0.95,0.93,0.90,0.88,0.84,0.81,0.77,0.73,0.68,0.64,0.59,0.54,0.48,0.43,0.37,0.31,0.25,0.19,0.13,0.06,0.00,-0.06,-0.13,-0.19,-0.25,    -0.31,    -0.37,    -0.43,    -0.48,    -0.54,    -0.59,    -0.64,    -0.68,    -0.73,    -0.77,    -0.81,    -0.84,    -0.88,    -0.90,    -0.93,    -0.95,    -0.97,    -0.98,    -0.99,    -1.00,    -1.00,    -1.00,    -0.99,    -0.98,    -0.97,    -0.95,    -0.93,    -0.90,    -0.88,    -0.84,    -0.81,    -0.77,    -0.73,    -0.68,    -0.64,    -0.59,    -0.54,    -0.48,    -0.43,    -0.37,    -0.31,    -0.25,    -0.19,    -0.13,    -0.06};

Please let me know if I could provide more information, I actually want to know which aspect could be the root of the problem could it be with C code or Matlab code or the uart programming...

I also posted the Matlab code here:

max_packets = 20000; %maximum number of packets to keep in memory (all are saved to log file)
update_plot = 1; %how often we should update the plot
plot_length = 12; %maximum length of plot
figure('Position', ); % new figure
ax1 = subplot(3,1,1); % top subplot
ax2 = subplot(3,1,2); % bottom subplot
ax3 = subplot(3,1,3);
drawnow;
s = serial(port);
cleanupObj = onCleanup(@()cleanupFunc(s)); %clean-up function
set(s,'BaudRate',115200,'timeout',2); %timeout is in seconds 
fopen(s);
vref= NaN(12,max_packets);
j = 0;
tic; %start timer
while 1; %do forever...
          
    %get data line by line 
    line = fscanf(s);
     = disect_line(line);
    if c == 1 %start recording from set 1 
             
        
        vref(1,j) = vo; 
        
        
        for i = 2:12 %get set 2 to 12
            line = fscanf(s);
             = disect_line(line);
            if c == i
                              
                vref(i,j) = vo;
               
            else
                fprintf('\nError\n');
            end
        end
        
       
       dlmwrite(logfile,,'delimiter',',','-append'); %save data from all 12 sets to csv file
       
       plot(ax1, start:j, vref(1,start:j).');
            
            for k = 1:1
                text(j, vref(k, j), num2str(k), 'Parent', ax1);
            end
            xlim(ax1, );
            
            ylabel(ax1, 'Vref');
           
            title(ax1, );   
        
        
end
end
function   = disect_line(line)
c = str2double(line(1:2)); %set number
vo = str2double(line(4:7));
end
function cleanupFunc(s)
fprintf('\nClosed serial port.\n')
fclose(s);
delete(s);
clear s;
end