What is the best way to program such a FIFO buffer, written in an interrupt and read in the main to save to the external file.

I am now using the following code as example. So I use a ticker to interrupt the system and fill a buffer with timestamp values. In the main-function I check if there is data in the buffer and if so I write it to the external file.

However now I still get different timestamp intervals. It seems sometimes the main is writing the 512bytes to the external file and then the ticker is no longer called. When the 512bytes have been written, the ticker was suppose to be called e.g. 20x times and so the interrupt service routine is executed 20x immediately after each other.

How to solve this problem or how to rework the code? I have already tried to increase the priority of ticker, but without result.

void TimerISR(void){
    afBuffer[BufferIndexIn++] = timer.read();  
//    fprintf(Datalogging,"%f\n",afBuffer[BufferIndexIn]);
    if(BufferIndexIn == BUFFERSIZE)
        BufferIndexIn = 0; 
    led3 = !led3;
        
}
 
int main(void) {
    led4 = 1;
    
    Datalogging = fopen("/local/TEST.csv", "w");
    
    BufferIndexIn = 0;
    BufferIndexOut = 0;
    
    timer.reset();
    timer.start();
    NVIC_SetPriority(TIMER3_IRQn, 1); // set mbed tickers to higher priority than other things
    TimerIRQ.attach_us(&TimerISR,500);
    
    bool bMeasurementRunning = TRUE;    
    while(bMeasurementRunning) {
        if (BufferIndexOut != BufferIndexIn)
        {
            fprintf(Datalogging,"%f\n",afBuffer[BufferIndexOut]);
            BufferIndexOut++;
            if(BufferIndexOut == BUFFERSIZE)
                BufferIndexOut = 0;
            led1 = !led1;
        }
        if (afBuffer[BufferIndexOut]>1)
        {
            TimerIRQ.detach();
            led2 = !led2;
            bMeasurementRunning = FALSE;   
        }
    }
    fclose(Datalogging);
}

Sorry, I just noticed that you're using LocalFileSystem, for some reason I was assuming it was an SD card. The localFileSystem is implemented almost as a debug breakpoint, the main CPU core is halted while the interface chip does the flash access. No matter what you do no interrupts will trigger during file system access.

Hello Andy A,

Thanks for your answer. I have now changed the setup to store the data on an external SD-card and I am using the FIFO buffer. However with below code I still get very few times a delay of 100ms. Not sure where this is coming from now??

void TimerISR(void){
    afBuffer[BufferIndexIn++] = timer.read();  
    if(BufferIndexIn == BUFFERSIZE)
        BufferIndexIn = 0; 
    led3 = !led3;   
}
 
void SaveData(){
    fprintf(Datalogging,"%f\n",afBuffer[BufferIndexOut]);
    BufferIndexOut++;
    if(BufferIndexOut == BUFFERSIZE)
        BufferIndexOut = 0;
    led1 = !led1;
}
 
int main(void) {
    led4 = 1;
    led2 = 1;
    
    //Datalogging = fopen("/local/TEST.csv", "w");
    Datalogging = fopen("/sd/TEST.csv", "w");
    
    BufferIndexIn = 0;
    BufferIndexOut = 0;
    
    timer.reset();
    timer.start();
    TimerIRQ.attach_us(&TimerISR,100);
    NVIC_SetPriority(TIMER3_IRQn, 0); // set mbed tickers to higher priority than other things
    
    bool bMeasurementRunning = TRUE;    
    while(bMeasurementRunning) {
        if (BufferIndexOut != BufferIndexIn)
        {
            SaveData();
        }
        if (afBuffer[BufferIndexOut]>1)
        {
            TimerIRQ.detach();
            led2 = 0;
            bMeasurementRunning = FALSE;   
        }
    }
    fclose(Datalogging);
}

No idea. That looks to be exactly the same structure as I've used before.

Any chance of a buffer overflow? Is the dropout exactly 100ms or 102.4ms each time and your buffer 1000 or 1024 items deep? 1000 items at 100us = 100ms. Maybe add

if (BufferIndexIn  == BufferIndexOut)
  led2 = 1;

to the end of TimerISR() to turn an LED on if there is an overflow just to be sure.

It seems there is indeed an overflow occuring. The reason is that the FIFO buffer is filled faster than it can be emptied. Because the SD writing still happens with 512bytes and stil takes several ms. Is there anyway to speed the SD-card write up. I have already increased the SPI frequency from 1MHz to the mbed max of 12.5MHz, but the write time is not decreased by 10x. So there seems to be a lot of overhead?

The bus speed will decrease the time to transfer the data but the bulk of the write time is the internal flash time of the SD card. About the only way to decrease that is to use a faster SD card.