Plotting of a real-time signal acquired via USB @10 Hz; is Matlab fast enough?

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Alessandro Russo 2016-5-18

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https://ww2.mathworks.cn/matlabcentral/answers/284816-plotting-of-a-real-time-signal-acquired-via-usb-10-hz-is-matlab-fast-enough

评论： Ramon Martinez Cancino 2023-11-28

Hi everyone! I recently opened a topic about this, i resume briefly the situation:

i have a bluetooth development board that receives values from a sensor, every 100 ms (that is to say 10 Hz, temporized by a timer counter of the source of the signal); as soon as it receives a value, it sends it to the PC via usb. In Matlab, i developed a GUI with a plot in it that sets the parameters for the function "serial", that is for example to stay opened for 10 second and receive a buffer of 8 bits (which is the dimension of the single transmitted datum). After this, in a while loop i plot the value and just update the time and samples arrays.

The problem is that this small cycle (code below) seems to be too slow, so i sometimes lose values! I used "tic" and "tac" to see how long does it take to complete a cycle, and sometimes it takes 500 ms, so i lose 4 values! Also removing the plot an just using "fread" as instruction in the cycle, the situation doesn't improve, it seems that the "slowing" instruction is indeed fread (by removing it, the cycle takes around 10-20 ms to complete). How can i solve this and get correctly all the values? Please note that while running this program anything else on the PC is opened, so it must be at full performance.

 As said i have two versione of the code, one with the while and one with a timer counter that fires every 100 ms, but the problem is the same. Here is the while version, just the GUI function that starts the acquiring and plotting!
% --- Executes on button press in startvis.
function startvis_Callback(hObject, eventdata, handles)
% hObject    handle to startvis (see GCBO)
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    structure with handles and user data (see GUIDATA)
hgui=getappdata(0,'hgui');
setappdata(hgui,'startv',1);
axes(handles.figvis)
priorPorts = instrfind;
delete(priorPorts);
handles.s=serial ('COM3');
set(handles.s,'BaudRate', 38400, 'DataBits', 8, 'Parity', 'none','StopBits', 1, 'FlowControl', 'none');
set(handles.s, 'InputBufferSize', 4);
set(handles.s, 'Timeout',100);
set(handles.s, 'Terminator', 'CR/LF');
fopen(handles.s);
array=[];
T=0;
i=0;
guidata(hObject, handles)
while (getappdata(hgui,'startv')==1)
    tic
    i=i+1;
    a=fread(handles.s);
    array=[array a(1)*3/255];
    if i>1
      T=[T T(end)+0.1];
    end
 if i>100
    i=0;
     array=a(1)*3/255;
end
     plot(array),grid on,xlabel('Sample'),ylabel('Voltage [V]')
     axis([0,100,0,3.5])    
     pause(0.00000000001);
     toc
end
fclose(h);