Hello,
I am running into an issue with my data logging. I am using an UNO to log data onto an SD card (no problems there). I use the millis() function instead of the delay function to make sure I am recording data onto my SD card every 1 second. My recorded data shows that is does in fact record every second.
However, the issue is 86400 seconds does not equal 24 hours. I have kept track of when I start the program and when I end it. So, for example I have uploaded my program at 7:00 am on Aug 10th, and ended the program at 9:00 am on Aug 12th. That results in exactly 50 hours of running time. At 1 second intervals that should be 50 x 60 x 60=180,000 seconds. When I imported my data to Excel I expected to have 180,000 lines of data. To my surprise that was not the case. I ran this experiment multiple times, but always ended up with less time (or Excel rows) than I expected. For instance in one of my experiments i ran it for 4 days. I ended with with 3.26 days worth of data (or Excel lines). Another time I ran the experiment for 3 days and ended with 1.7 days worth of data (or Excel lines).
I divided the actual time by the data recorded time (4 / 3.26) and got 1.23 & 1.76 respectively. Hence, I thought that my program cycle time was actually taking 1.23 seconds or 1.76 seconds for every cycle instead of the intended 1 second.
I ran the below code to measure the actual cycle time. I have read previously that opening a file, writing, and closing will take some time (up to 100 ms perhaps). However, my code measured less than 20 milliseconds for the entire process. Also my understanding of using millis() to delay instead of the delay() function ensures that a total of 1 second (referring to this particular example) has passed. For example, since opening, writing, and closing a file takes about 20 ms therefor and additional 980 ms must pass before entering the IF loop again. Where as, the delay() function will take a total of 1020ms.
NOTE: After running the below code you will see that on the serial monitor it will show less than 20ms for the opening, writing, and closing of the SD file. However, when checking the actual TEXT file on the SD card you will see about 980ms. I believe the reason is FileCloseTime is read after opening, writing, and closing the file, hence it is calculating the duration lapsed since this function was called last. 980ms is the difference between the 1000ms of the "IF" loop and the 20ms it takes to open, write, and close the SD file. Unless im wrong, this is further proof that it should be writing 86400 lines of data in a span of 24 hours.
Does anyone have any insight on where this delay is occurring and how to measure it?
Below is the code I used:
//FOR PROGRAM CYCLE TIME
const unsigned long eventInterval = 1000; //milli seconds
unsigned long FileStartTime = 0; //milli seconds
unsigned long FileCloseTime = 0; // milli seconds
unsigned long previousTime = 0;
unsigned long currentTime = 0;
//FUNCIONS
void writefile();
///for file program
#include <SD.h>
#include <SPI.h>
#include <Wire.h>
File myFile;
int pinCS = 10; // Pin 10 on Arduino Uno
void setup()
{
Serial.begin(115200); // open serial over USB
pinMode(pinCS, OUTPUT);
if (SD.begin())
{
Serial.println("SD card is ready to use.");
} else
{
Serial.println("SD card initialization failed");
return;
}
}
void loop() {
currentTime=millis();
if (currentTime - previousTime >= eventInterval) {
writefile();
Serial.print("Duration since last IF loop entrance <");Serial.print(currentTime-previousTime);Serial.println(">ms");
previousTime = currentTime;
}
}
void writefile(){
FileStartTime=millis();
myFile = SD.open("test.txt", FILE_WRITE);
if (myFile) {
myFile.print("Duration since last IF loop entrance ");
myFile.print(currentTime-previousTime); //milli seconds time
myFile.print("\t");
myFile.print("Duration of file opening, writing, closing ");
myFile.println(FileStartTime-FileCloseTime); //milli seconds time
myFile.close(); // close the file
}
// if the file didn't open, print an error:
else {
Serial.println("error opening test.txt");
}
FileCloseTime=millis();
Serial.print("write file function (total time) <");Serial.print(FileCloseTime-FileStartTime);Serial.println(">ms");
}
