adalogger.ino

#include <SPI.h>
#include <SD.h>

// Set the pins used
#define cardSelect 4

File logfile;

// blink out an error code
void error(uint8_t errno) {
  while(1) {
    uint8_t i;
    for (i=0; i<errno; i++) {
      digitalWrite(13, HIGH);
      delay(100);
      digitalWrite(13, LOW);
      delay(100);
    }
    for (i=errno; i<10; i++) {
      delay(200);
    }
  }
}

// This line is not needed if you have Adafruit SAMD board package 1.6.2+
//   #define Serial SerialUSB

void setup() {
  // connect at 115200 so we can read the GPS fast enough and echo without dropping chars
  // also spit it out
  Serial.begin(115200);
  Serial.println("\r\nAnalog logger test");
  pinMode(13, OUTPUT);


  // see if the card is present and can be initialized:
  if (!SD.begin(cardSelect)) {
    Serial.println("Card init. failed!");
    error(2);
  }
  char filename[15];
  strcpy(filename, "/ANALOG00.TXT");
  for (uint8_t i = 0; i < 100; i++) {
    filename[7] = '0' + i/10;
    filename[8] = '0' + i%10;
    // create if does not exist, do not open existing, write, sync after write
    if (! SD.exists(filename)) {
      break;
    }
  }

  logfile = SD.open(filename, FILE_WRITE);
  if( ! logfile ) {
    Serial.print("Couldnt create "); 
    Serial.println(filename);
    error(3);
  }
  Serial.print("Writing to "); 
  Serial.println(filename);

  pinMode(13, OUTPUT);
  pinMode(8, OUTPUT);
  Serial.println("Ready!");
}

uint8_t i=0;
void loop() {
  digitalWrite(8, HIGH);
  logfile.print("A0 = "); logfile.println(analogRead(0));
  Serial.print("A0 = "); Serial.println(analogRead(0));
  digitalWrite(8, LOW);
  
  delay(100);
}

The sketch still writes incremental filenames, but doesn't write any data to the files.
(I'm using an RTC, would love to be able to write a valid timestamp as well...)
When I add logfile.flush() at the end of my loop, all is well.
I suppose this is the same as putting the Open and Close into the loop, yes?

As per documentation of SD (https://www.arduino.cc/en/Reference/SDCardNotes):

Opening/Closing files
When you use file.write(), it doesn't write to the card until you flush() or close(). Whenever you open a file, be sure to close it to save your data.

The documentation of the "Adafruit Feather M0 Adalogger" needs to be updated, as "https://learn.adafruit.com/adafruit-feather-m0-adalogger/using-the-sd-card" is saying that it writes on the SD-Card once the buffer is full and with this code it will not.

If anyone has been waiting 10 years for this to behave as described in the Adafruit Learn article, just update the loop:

void loop() {
i = i + 1;
digitalWrite(8, HIGH);
logfile.print("A0 = "); logfile.println(analogRead(0));
Serial.print("A0 = "); Serial.println(analogRead(0));
digitalWrite(8, LOW);

if (i >= 50) {
logfile.flush();
i = 0;
}

delay(100);
}