Why I have a delay when using an external clock with a NI-DAQ (USB 6211) data acquisition board?

Hi, I have this code of MATLAB for obtaining the number of counts from an Avalanche Photodiode detector with the NI-DAQ model USB 6211. I am adding an external clock to obtain the counts but when running the code it is showing but it starts with a delay and when changing the signal it has a delay also to show the changes. I would appreciate your help.

dataFocus = [];
h = animatedline();
acq_time=0.01;
N = 1;
start(dq,"Continuous")
while true
    tic
    dataIn= read(dq,seconds(acq_time));
    timing=toc() 
    n0 = max(dataIn.Dev1_ctr0)-min(dataIn.Dev1_ctr0);
    dataFocus = n0/acq_time;
    addpoints(h,N,dataFocus);
    drawnow
    N=N+1;
    
end

Answer 1

[Anurag Ojha]

Hello Adriana,

The delay you are experiencing in your code is due to the use of the “tic” and “toc” functions. These functions measure the elapsed time between their calls, but they are not accurate for measuring small time intervals like “acq_time”. Instead, you can use the “datetime” function to get the current time and calculate the elapsed time between iterations.

Here's an updated version of your code that uses “datetime” for timing:

dataFocus = []; 
h = animatedline(); 
acq_time = 0.01; 
N = 1; 
start(dq, "Continuous") 
prevTime = datetime('now'); 

while true 
    dataIn = read(dq, seconds(acq_time)); 
    currTime = datetime('now'); 
    timing = seconds(currTime - prevTime); 
    prevTime = currTime; 
    n0 = max(dataIn.Dev1_ctr0) - min(dataIn.Dev1_ctr0); 
    dataFocus = n0 / timing; 
    addpoints(h, N, dataFocus); 
    drawnow 
    N = N + 1; 
end 

Refer to the following MATLAB documentation for more information:

• https://www.mathworks.com/help/matlab/ref/datetime.html#mw_e5a21fa8-24fe-4412-ae91-e89f7242f731

I hope this resolves your issue.