Read data continuously from accelerometer through serial COM

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Francesco Lucarelli 2021-6-10

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https://ww2.mathworks.cn/matlabcentral/answers/853055-read-data-continuously-from-accelerometer-through-serial-com

编辑： Walter Roberson 2021-6-29

Hi all,

i would like to read continuos data from an accelerometer.Before i was using hterm but it does not support long recording.

I was wondering if i can do that with matlab

baud 8200000 (13byes=1+4+2+2+2+2)

Can anyone help me with this?

Thanks a lot in advance!

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Francesco Lucarelli 2021-6-29

编辑：Francesco Lucarelli 2021-6-29

在 MATLAB Online 中打开

Hi Walter,

Thanks again for your feedback. Much appreciated.

Question: do you need to do anything with the data as it is being read? Or do you just need to record the data for later analysis?Answer: The analysis is done after the acquisition, so i just need to record the data for later analysis

I do have an SSD on the host system.

Question:What is your target samples per second ?

Answer: the sampling frequency is at 26khz

That' why i was thinking about matlab, to store data directly and then analysing them. I wrote a small script like the one below:

%%
device=serialport("COM3",8200000);
n=1;
while n>0    
    read(device,13,"uint16")
    n=n+1;
end
%%

Could you give me an example of how i can improve it?

Thanks a lot for your help and your time

Kind Regards

Walter Roberson 2021-6-29

编辑：Walter Roberson 2021-6-29

在 MATLAB Online 中打开

%read a nominal second's worth of data at a time
%in this version, store it all at first -- it should be about 600 megabytes
blocksize = 13;         %bytes
test_duration = 30*60;  %seconds
target_sps = 26000;
bytes_per_second = target_sps * blocksize;
buffer = zeros(bytes_per_second, test_duration, 'uint8');
device = serialport("COM3",8200000);
full_cols = 0; partial = 0;
start_time = tic;
for K = 1 : test_duration
   try
     thisbuffer = read(device, bytes_per_second, 'uint8');
     blen = length(thisbuffer);
     buffer(1:blen,K) = thisbuffer;
     if blen == bytes_per_second
         full_cols = K;
         partial = 0;
     else
       full_cols = K - 1;
       partial = blen;
       break;
     end
   catch ME
       full_cols = K - 1;
       partial = 0;
       break
   end
end
read_time = toc(start_time);
clear device
bytes_received = full_cols * bytes_per_second + partial;
bytes_per_second = bytes_received / read_time;
samples_per_second = bytes_received / blocksize / read_time;
fprintf('Actual throughput: %.3f bytes/second, %.3f samples/second\n', bytes_per_second, samples_per_second);
[fid, msg] = fopen('testlog.bin', 'w');
if fid < 0
   fprintf('Could not open output file because: "%s"\n', msg);
   fprintf('Data is still in buffer(1:%d)\n', bytes_received);
else
    fwrite(fid, buffer(1:bytes_received), '*uint8');
    fclose(fid);
end

Start with a shorter test.

If the logic works and the throughput is high enough, clone the function and rearrange slightly to fwrite() thisbuffer to the file as you go, and see whether the throughput drops more than you are willing to live with.

The more data you can record in memory before writing to disk, the more efficient writing will be -- but the more risk that everything will get lost if something goes wrong. For example, this code has no STOP logic in it, and if you control-C then it will not write to disk (but buffer will still exist). It would be more robust to write results to disk as you go, but you need to accumulate enough to be worth writing.

Generally speaking, operating systems need to be fed at least 4096 bytes to write efficiently, preferably 16K at least, 64 K is better.

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