Using files written on SD Card

Using Arduino Programming Questions
#1

Hi all! I am building a coin counting system utilizing a 16x2 LCD shield and SeedStudio SD Card shield on the Mega 2560. The SD is used for both data logging and retention. I am writing values onto the SD card into a .txt file. Is it possible to use these files to begin the count at the last written number? For clarification, if the count is at 100 and the power goes out, I need the count to start at 100 again after power is lost.

Included is a shortened version of the code.

Thanks for any help!

#include <SPI.h>
#include <SD.h>
int dataString = "";

#include <LiquidCrystal.h>
LiquidCrystal lcd(8, 9, 4, 5, 6, 7);
#include "Timer.h"
Timer t;
const int input_12 = 41;
int last_in_12_state = 0;

const int chipSelect = 4;

int count = 0;
int coin = 0;
int hourly_count = 0;
int sensorPin = A0;
int Update = 0;


pinMode (input_12, INPUT);
  
  pinMode (chipSelect, OUTPUT); pinMode (53, OUTPUT);
  
  // Open serial communications and wait for port to open:
  Serial.begin(9600);
  while (!Serial) {}
  Serial.print("Initializing SD card...");
  // see if the card is present and can be initialized:
  if (!SD.begin(chipSelect)) {
    Serial.println("Card failed, or not present");
    return;}
  Serial.println("card initialized.");
// set up inputs as inputs:
 
 // set up the LCD's number of columns and rows:
  lcd.begin(16, 2);
  // Print a message to the LCD.
  lcd.clear();
  t.every(10*60*1000*6,UpdateResults); //update every 60 min
  // t.every(10000,UpdateResults); //update every 10 Sec

  lcd.setCursor(0,0);
  lcd.print("ACI");
  lcd.setCursor(0,1); 
  lcd.print("NOTE TIME");
  delay(5000);
  lcd.clear();
}

void loop() {
 dataString =   int(count);  
 dataString =   int(hourly_count);

in_state_12 = digitalRead(input_12);
  if (in_state_12 != last_in_12_state){
   if (in_state_12 == LOW){    
     count ++;
  }}
 last_in_12_state = in_state_12;

coin = (count/4);

 t.update();

 
   
  File dataFile = SD.open("datalog.txt", FILE_WRITE);

  // if the file is available, write to it:
  if (dataFile) {
    dataFile.print("Hour:");dataFile.print(hourly_count);dataFile.print(";");
    dataFile.print("Count:");dataFile.println(count); 
    dataFile.close();
  }
  // if the file isn't open, pop up an error:
  else {
    Serial.println("error opening datalog.txt");
  }
}
#2

Of course it's possible. You can read the file.

#3

aarg:
Of course it's possible. You can read the file.

Hi aarg. Would you be able to expand? How can you do this?

#4

The read() function of the SD library would seem like a good place to start.

#5

use SD.read() to read each line while counting - there you have your latest index if each line represents one dataset.

Then close the file and open it again with SD.open(filepath, FILE_WRITE) - this should append new data to the file.

#6

beejayf:
use SD.read() to read each line while counting - there you have your latest index if each line represents one dataset.

Then close the file and open it again with SD.open(filepath, FILE_WRITE) - this should append new data to the file.

Thanks beejayf. But how do you individually select the latest index?

#7

But how do you individually select the latest index?

As suggested, each time you read a record add one to a counter. Once you have read each record in the file the counter will tell you how many there were. If you need the data in the last record then it will be in whatever variable or variables you saved it to when reading the file to count the number of records.

#8