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I am using a raspberry pi to grab data through a SPI port. Attached to the SPI port is a PIC, programmed so that it samples on an A/D at 12.8kHz, and stores the samples in a 256 long buffer. (to cope with linux process pause and runs).

I have a linux program that reads the SPI and writes to a file. a couple of times per second I am getting missing data, I believe because linux is taking too long to return to my C program.

I understand that Linux (Raspbian) is not real-time and a process/program can be interrupted at any time, and that is why I am using a buffer in the PIC, to ride through these interruptions.

The problem is that sometimes the delay where the program waits is too large, and so I am missing some of the data. This happens maybe once every few seconds:

I would like suggestions for reducing the time, or at least setting a maximum time between process run time slices.

I have tried running with a nice level of -15 and -20.

Is there a way to reduce time slices?

Is there a memory disk on Raspbian that I can try writing my file to, incase it is the writing of the file that is causing it?

Here is some output from top:

top - 14:01:23 up  2:25,  3 users,  load average: 1.91, 2.09, 2.18
Tasks:  76 total,   1 running,  75 sleeping,   0 stopped,   0 zombie
%Cpu(s):  7.6 us, 28.9 sy,  0.0 ni, 63.2 id,  0.0 wa,  0.0 hi,  0.3 si,  0.0 st
KiB Mem:    185732 total,   105708 used,    80024 free,    21112 buffers
KiB Swap:   102396 total,        0 used,   102396 free,    43916 cached

  PID USER      PR  NI  VIRT  RES  SHR S  %CPU %MEM    TIME+  COMMAND                                                  
  353 root      20   0     0    0    0 D  15.3  0.0  17:01.87 spi0                                                     
13296 root       5 -15  2652 1468 1344 D  12.1  0.8   0:09.21 a.out                                                    
13386 pi        20   0  4708 2500 2064 R   1.6  1.3   0:00.40 top                                                      
 4707 pi        20   0  4176 2572 1976 S   1.0  1.4   2:48.66 watch                                                    
11547 root      20   0     0    0    0 S   0.6  0.0   0:03.59 kworker/u2:1                                             
   16 root      20   0     0    0    0 S   0.3  0.0   0:05.60 kworker/0:1                                              
 2524 pi        20   0  9288 3296 2708 S   0.3  1.8   0:00.62 sshd                                                     
12448 root      20   0     0    0    0 S   0.3  0.0   0:02.98 kworker/u2:4                                             
    1 root      20   0  2168 1368 1260 S   0.0  0.7   0:02.22 init                                                     
    2 root      20   0     0    0    0 S   0.0  0.0   0:00.00 kthreadd                                                 
    3 root      20   0     0    0    0 S   0.0  0.0   0:06.88 ksoftirqd/0                                              
    5 root       0 -20     0    0    0 S   0.0  0.0   0:00.00  

I can see from top that the a.out and spi0 process are using the most CPU time, but they are not maxing it out. Here you can see my process has a nice of -15.

Maybe one of the processes that is running is interfering? Here are my running processes, a.out is my SPI reading program.

pi@raspberrypi ~/frd/src/raspi/xcodefrd $ ps -ae
  PID TTY          TIME CMD
    1 ?        00:00:02 init
    2 ?        00:00:00 kthreadd
    3 ?        00:00:07 ksoftirqd/0
    5 ?        00:00:00 kworker/0:0H
    7 ?        00:00:00 khelper
    8 ?        00:00:00 kdevtmpfs
    9 ?        00:00:00 netns
   10 ?        00:00:00 perf
   11 ?        00:00:00 khungtaskd
   12 ?        00:00:00 writeback
   13 ?        00:00:00 crypto
   14 ?        00:00:00 bioset
   15 ?        00:00:00 kblockd
   16 ?        00:00:06 kworker/0:1
   17 ?        00:00:00 rpciod
   18 ?        00:00:00 kswapd0
   19 ?        00:00:00 fsnotify_mark
   20 ?        00:00:00 nfsiod
   26 ?        00:00:00 kthrotld
   27 ?        00:00:00 VCHIQ-0
   28 ?        00:00:00 VCHIQr-0
   29 ?        00:00:00 VCHIQs-0
   30 ?        00:00:00 iscsi_eh
   31 ?        00:00:00 dwc_otg
   32 ?        00:00:00 DWC Notificatio
   34 ?        00:00:00 kworker/0:2
   35 ?        00:00:01 mmcqd/0
   36 ?        00:00:00 VCHIQka-0
   37 ?        00:00:00 SMIO
   38 ?        00:00:00 deferwq
   40 ?        00:00:00 jbd2/mmcblk0p6-
   41 ?        00:00:00 ext4-rsv-conver
  156 ?        00:00:00 udevd
  286 ?        00:00:00 udevd
  293 ?        00:00:00 udevd
  321 ?        00:00:00 cfg80211
  353 ?        00:18:14 spi0
  411 ?        00:00:00 kworker/0:1H
 1792 ?        00:00:06 ifplugd
 1795 ?        00:00:01 ifplugd
 1797 ?        00:00:01 ifplugd
 1800 ?        00:00:00 wpa_supplicant
 1877 ?        00:00:00 dhclient
 2246 ?        00:00:00 rsyslogd
 2248 ?        00:00:00 thd
 2296 ?        00:00:00 cron
 2326 ?        00:00:00 dbus-daemon
 2393 ?        00:00:01 ntpd
 2422 ?        00:00:00 sshd
 2513 tty1     00:00:00 getty
 2514 tty2     00:00:00 getty
 2515 tty3     00:00:00 getty
 2516 tty4     00:00:00 getty
 2517 tty5     00:00:00 getty
 2518 tty6     00:00:00 getty
 2519 ?        00:00:00 getty
 2520 ?        00:00:00 sshd
 2524 ?        00:00:04 sshd
 2525 pts/0    00:00:02 bash
 2545 ?        00:00:00 sshd
 2549 ?        00:00:39 sshd
 2550 ?        00:00:13 sftp-server
 2551 ?        00:00:00 sshd
 2555 ?        00:00:00 sshd
 2556 pts/1    00:00:01 bash
 2583 ?        00:00:00 sshd
 2587 ?        00:00:03 sshd
 2588 pts/2    00:00:01 bash
 4707 pts/2    00:02:56 watch
 8843 ?        00:00:06 kworker/u2:0
11547 ?        00:00:04 kworker/u2:1
12448 ?        00:00:03 kworker/u2:4
13186 ?        00:00:03 kworker/u2:2
13295 pts/0    00:00:00 sudo
13296 pts/0    00:01:09 a.out
13690 ?        00:00:01 kworker/u2:3
14083 pts/0    00:00:00 ps

This is what my data stream looks like, you can spot the bit where the SPI reads were delayed:

The data stream, you can see missing data for about half a cycle around sample 3850

You may of course want a look at my code. I have tried different buffer sizes.

#include <stdio.h>
#include <stdlib.h>
#include <wiringPi.h>
#include <sys/timeb.h>
#include <time.h>
#include <sys/timeb.h>

#include "frdSPI.h"
#include "frdBuffer.h"

#define TRIGGERVAL 50

#define BUFF_SIZE 32

int main(void){

    unsigned char buffer[BUFF_SIZE];
    int i, sample, channel = 0 , triggerdifference, oldsample, qtytowriteout = 0;
    unsigned long int samplenumber=0;
    unsigned long int differencenumber=0;

    FILE *fp;
    char filename[100];

    struct timeb recordtime;


    // Following is test code to see how to make filename and use it to write a file.
    ftime( &recordtime );
    sprintf( filename, "/frd/data/%ld%03ld.startup", recordtime.time, recordtime.millitm);

    fp = fopen( filename ,"w" );
    fprintf( fp, "test and more %lu", samplenumber);
    fclose(fp);



    if ( wiringPiSPISetup (channel, 500000) < 0){
        fprintf (stderr, "SPI Setup failed! Check module is loaded and run app as root.\n");
        exit(-1);
    }

    while(1){
        wiringPiSPIDataRW ( channel, &buffer, BUFF_SIZE );

        for(i=0; i<BUFF_SIZE; i++){
            sample=parseSPIDataStream(buffer[i]);
            if( sample > -1 ){
                samplenumber++;
                oldsample = circularbufferreadwrite( sample );

                if( calculateDifferenceValue(sample, &triggerdifference))
                {
                    differencenumber++;
                    if( triggerdifference > TRIGGERVAL || triggerdifference < (-1 * TRIGGERVAL) ){
                        //printf("%d \n", triggerdifference);
                        if( qtytowriteout == 0 )
                        {
                            ftime( &recordtime );
                            sprintf( filename, "/tmp/%ld%03ld.csv", recordtime.time, recordtime.millitm);
                            fp = fopen(filename,"w");
                            samplenumber=0;
                        }
                        if( qtytowriteout < 4000 )
                            qtytowriteout = qtytowriteout + 12800;
                    }
                }

                if( qtytowriteout && (fp!=(FILE *)NULL))
                {
                    //printf("%lu,%d\n", samplenumber, oldsample);
                    fprintf(fp, "%lu,%d\n", samplenumber, oldsample);
                    qtytowriteout--;
                    if( qtytowriteout == 0 ){
                        fclose(fp);
                        printf("\b");
                    }
                }
            }
        }
    }
}

Thank you for any help and suggestions in advance.

EDIT I just ran dd if=/dev/zero of=/dev/null as well, to take up some CPU time, and this has made the problems significantly worse, so I think I am on the right track, but I could do with some pointers as to how to fix it. Here is a graph of the data when using more CPU time:

enter image description here

  • Make sure your spi device is able to cope with 500khz: it needs an internal clock of at least 4 times that. You can try a different faster driver pigpio. – meuh Jul 28 '17 at 17:18
  • The pic spi can handle it. Pic uses 20MHz clock for a 5MHz internal clock. Will have a look at pigpio, thanks. – J Rodgers Jul 28 '17 at 19:38

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