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2 years, 8 months ago.

how to convert binary data to float value?

Hello all, For my data logger project I'm logging data to a SD card. Here I'm storing the data in binary format. I'm logging accelerometer and temp&humidity sensors. I could read data byte by byte. Hence to print ADXL data (two bytes), I combine each two byte data. Hence four bytes has to be combined to get temparature data. Unfortunately for temparature I failed to print actual data. There is some conversion problem. I'm looking some help regarding how to convert data to floats.

Code to read and print binary to uint16_t ( accelerometer data): working

 while ((nr = fgetc(logFile)) != EOF){
             pc.printf(" \r\n %d ",nr);
              acc_con[i] = nr;
              if (i == 1){ 
              acc = (acc_con[1]<<8) | acc_con[0];
              pc.printf(" \r\n %i ",acc);
              i = 0;} 
              else i++; 
             }

Code to read and print binary to float ( temparature data): Not working

while ((nr = fgetc(logFile)) != EOF){
             pc.printf(" \r\n %d ",nr);
               humicon[i] = nr;
               if (i == 3){
                hum = (humicon[0] << 24) | (humicon[1] << 16) | (humicon[2] << 8) | humicon[3];
                pc.printf(" \r\n %f ",hum);
                i = 0;}
                else i++;
                }

The byte by byte raw data from variable nr is: 24, 58, 46, 66, and the output from variable hum is: 406466112.000000

How did you write humicon in the first place? And what is the format you read it from your device like?

posted by Erik Olieman 13 Dec 2016

I don't know much about formats. I get the correct output with below code.

int main() {
    
    int nr;
    FILE *fp = fopen("/sd/PCE000.bin", "rb");
    float read[1];
    fread(read,sizeof(float),1,fp);
    pc.printf("\r\n %f",read[0]);
      fclose(fp); 
  }

I would like to know the formula for the conversion. Thank you.

posted by Mohan gandhi Vinnakota 14 Dec 2016

It is working. It is little endien byte order.

while ((nr = fgetc(logFile)) !=EOF){
               humicon[i] = nr;
               if (i == 3){
                float x;
               const unsigned char bytes[] = {humicon[0],humicon[1],humicon[2],humicon[3]};
               memcpy(&x, bytes, sizeof x);
               pc.printf("%f\n", x);
                i = 0;}
                else i++;
                }
posted by Mohan gandhi Vinnakota 14 Dec 2016
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