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TCP/IP commnunications while GPIO interrupt

Former Member

Hi,

 

I am developing an audio recording system using Raspberry Pi B+.

 

To guarantee the sampling rate, an external GPIO interrupt with 16 kHz  was implemented.

Then, for each interrupt, A/D conversion having 16-bit resolution was followed to get a sample.

Finally, for every 100ms, the buffered 1600 samples were transmitted to the server PC by applying TCP/IP-based communications.

 

However, I found that several samples were missed whenever the buffered samples were transmitted by TCP/IP.

I doubt that the GPIO interrupt for A/D conversion was missed whenever the TCP/IP socket communications are executed.

 

The core program codes written in C are as follows.

+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

        struct sockaddr_in server_addr;

 

        //External GPIO interrupt setting      

        wiringPiSetup();

        pinMode(5, INPUT);

        wiringPiISR(5, INT_EDGE_FALLING, &ISR);

 

         //Setting for TCP/IP socket communications

        Socket = socket(AF_INET, SOCK_DGRAM, 0);
        bzero((char *)&server_addr, sizeof(server_addr));

        server_addr.sin_family = AF_INET;
        server_addr.sin_addr.s_addr = inet_addr("192.168.0.3");
        server_addr.sin_port = htons(8101);

        connect(Socket, (struct sockaddr *)&server_addr, sizeof(server_addr);

 

        while(1)

        {

              //Buffered samples transmitting

              if(buffFullFlag == TRUE) write(Socket, buf0, BUF_LEN*sizeof(unsigned short));    

         }

 

+++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

The attached file shows an example of the waveform of transmitted data.

 

Could you tell me the reason and solution about my problem ??

Attachments:
imageGPIO interrupt error.pdf
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  • 0

    Hi Sam,

     

    You'll see this issue because while you may be using an ISR in the WiringPi library, your code is still running in user space (as I understand) and thus has to still battle with other processes. I'm not familiar with WiringPi however so I could be wrong. You could run as root user to see if this makes a difference. Linux user apps are not suited for such responsiveness where jitter is a concern - you will need to either do this in a custom driver, or use an external microcontroller perhaps.

  • 0 in reply to shabaz

    You are also not "double buffering". So, when your 1600 sampes are done, the main process starts to transmit the buffer to the server, while the ISR is starting to refill the buffer with new samples...

     

    When you have interrupts at 16kHz, the interrupt comes every 60 microseconds. This is quite fast. A microcontroller can, with a little care probably achieve this performance. But if you want to do this on a normal Linux system you have to make sure that no system device driver disables interrupts for a period like this. From your graph it could be that the interrupts are disabled for a period like 500 microseconds, meaning you are losing around ten samples.

     

    I looked up how the GPIO interrupts work. The interrupt fires, and then tries to schedule your function to run in userspace. If the system decides that doing "something else" is preferable above running your program then that will happen. It will run your function within miliseconds, but sometimes something else will have to be done first. In your case, you will definitvely miss interrupts if you schedule them to happen every 60 microseconds.

     

    To solve this you could write a kernel driver that handles the ISR. Or you could use a DMA buffer to store the samples (is the 16bit ADC part of the BCM2835?)

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  • 0 in reply to shabaz

    You are also not "double buffering". So, when your 1600 sampes are done, the main process starts to transmit the buffer to the server, while the ISR is starting to refill the buffer with new samples...

     

    When you have interrupts at 16kHz, the interrupt comes every 60 microseconds. This is quite fast. A microcontroller can, with a little care probably achieve this performance. But if you want to do this on a normal Linux system you have to make sure that no system device driver disables interrupts for a period like this. From your graph it could be that the interrupts are disabled for a period like 500 microseconds, meaning you are losing around ten samples.

     

    I looked up how the GPIO interrupts work. The interrupt fires, and then tries to schedule your function to run in userspace. If the system decides that doing "something else" is preferable above running your program then that will happen. It will run your function within miliseconds, but sometimes something else will have to be done first. In your case, you will definitvely miss interrupts if you schedule them to happen every 60 microseconds.

     

    To solve this you could write a kernel driver that handles the ISR. Or you could use a DMA buffer to store the samples (is the 16bit ADC part of the BCM2835?)

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