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writing and reading bytes from SD card

Created Tue, 01 Jan 2013 06:28:25 +0000 by spencoid


Tue, 01 Jan 2013 06:28:25 +0000

i can't get the following (also attached as a text file) simple test to work. i am trying to convert a long integer into bytes, write them to the SD card and read them back and convert back to a long integer. the conversion seems to work because as long as i just convert and do not write to the card it works. i am not stuck on converting to bytes and back but thought this was the only way to write a 32 bit integer to the SD card. i am open to any other method that achieves the same thing.

#include <SD.h>    // was suggested that these need to be declared first
#include <SPI.h>  // this was recommended, not sure why

File SDfile;
unsigned char   bytes[sizeof(long)];

void setup() {
  pinMode(9, INPUT);
  pinMode(4, OUTPUT);  
  digitalWrite(4, HIGH);      // chip select on SD card
  digitalWrite(9, HIGH);      // turn on pullup resistors
  pinMode(8, INPUT);
  digitalWrite(8, HIGH);      // turn on pullup resistors
  pinMode(35, INPUT);
  digitalWrite(35, HIGH);     // turn on pullup resistors
  if (!SD.begin(4)) {
    Serial.println("initialization failed!");
  Serial.println("initialization success!");
  long in_value = 20000;
  long out_value = 0;
  unsigned char read_byte;
  SDfile.close();  //  
  SDfile ="test.txt", FILE_WRITE);
      if (SDfile) {
        Serial.print("opening SD for writing ");
        Serial.println("SD card failure"); 
    for (int i = 0; i < sizeof(long); i++){
        bytes[i] = in_value & 0xFF;
        in_value >>= 8;
//    SDfile.write(bytes, sizeof(long)); // tried using this instead of writing individual bytes in above for loop
    SDfile ="test.txt");
//    for (int i = 0; i < sizeof(long); i++){  // this does not work if the same bytes are written to and then read from the SD card
//      read_byte =;             // must have something to do with bit or byte order that i have not been able to figure out
//      out_value |= (read_byte << (i * 8));
//    }
    for (int i = 0; i < sizeof(long); i++){ // this works fine so it shows that the conversion from integer to bytes and back is correct
      out_value |= (bytes[i] << (i * 8));
    Serial.print("got value "); 

void loop() {    



Tue, 01 Jan 2013 08:20:03 +0000

i think i figured this out. i think it was just that i was not removing the old test file. i am used to perl in which files are overwritten unless opened for "append" so i didn't realize that i was not writing a new file by opening for writing.