My code below reads data from an ADC and 3 digital pins via a Mega and prints to an SD Shield (Deek Robot). I want to be able to read the data every 50ms (20ms ideally). I can't get below 250ms.

I've read countless forum posts regarding the same issue and understand that I need to create 2x 512 byte buffer, store data to one while the other is being written to the SD card. A forum post mentioned using the Adafruit GPS library for an example of double-buffering but I can't seem to find it. Another suggests changing the internal buffer from 64bytes to 256bytes in hardwareserial.cpp but I can't even navigate to it on my w***ows laptop.

I've run dataString.Length and the max length is 38 for the 8 columns of data, plus a long (for recording ms) another byte for the "\t" and 3 bytes of headroom = 46bytes in all, round it up to 50bytes/sample at a 50ms sampling rate is 1000bytes/second. Darn it, I forgot to add the separating-commas between the columns = plus another 7 bytes; call it 55 in total = 1100bytes/second. I know it can be done but I can't do it; not at my current knowledge/skill level.

Thank you in advance for your help.

//include relevant libraries 
#include <EnableInterrupt.h>
#include <SPI.h>
#include <SD.h>

//globally define pins
#define BUSY 3    //purple*
#define RD 4      //yellow* RD+CS tied together
#define RESET 5   //grey* 
#define CONVST 6  //green* CONVSTA+CONVSTB soldered together on the board
#define RANGE 7   //blue*   *not permanent - double check!!!*

#define DB0 22
#define DB1 23
#define DB2 24
#define DB3 25
#define DB4 26
#define DB5 27
#define DB6 28
#define DB7 29
#define DB8 30
#define DB9 31
#define DB10 32
#define DB11 33
#define DB12 34
#define DB13 35
#define DB14 36
#define DB15 37

//byte statusLed       = 13;
byte sensorPin1       = 38;
byte sensorPin2       = 40;
byte sensorPin3       = 42;

int sensorValue[8];
int rawData[16];
//change the size  of these arrays to match the amount of channels being read by the ADC 
int adcChannel[5];
int adcData[5];
int channelCount=5;

volatile byte pulseCount1;
volatile byte pulseCount2;
volatile byte pulseCount3;

int pulses[3];

// cs pin for sd-shield *NB 53 for Mega
const int chipSelect = 53;
// SCK 52
// MISO 50
// MOSI 11

unsigned long oldTime;

void setup() {             //set up the hardware

  //sd card.....  
  while (!Serial) {
    ; // wait for serial port to connect. Needed for native USB port only

  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");
    // don't do anything more:
    while (1);
  Serial.println("card initialized.");

  enableInterrupt(BUSY, bitBang, FALLING);

  oldTime            = 0;

  pinMode(DB0, INPUT_PULLUP);
  pinMode(DB1, INPUT_PULLUP);
  pinMode(DB2, INPUT_PULLUP);
  pinMode(DB3, INPUT_PULLUP);
  pinMode(DB4, INPUT_PULLUP);
  pinMode(DB5, INPUT_PULLUP);
  pinMode(DB6, INPUT_PULLUP);
  pinMode(DB7, INPUT_PULLUP);
  pinMode(DB8, INPUT_PULLUP);
  pinMode(DB9, INPUT_PULLUP);
  pinMode(DB10, INPUT_PULLUP);
  pinMode(DB11, INPUT_PULLUP);
  pinMode(DB12, INPUT_PULLUP);
  pinMode(DB13, INPUT_PULLUP);
  pinMode(DB14, INPUT_PULLUP);
  pinMode(DB15, INPUT_PULLUP);

  pinMode(RESET, OUTPUT);
  pinMode(CONVST, OUTPUT);
  pinMode(RD, OUTPUT);
  pinMode(RANGE, OUTPUT);
  pinMode(BUSY, INPUT);

  //reset ADC to begin conversion
  digitalWrite(RESET, HIGH);
  digitalWrite(RESET, LOW);

  digitalWrite(CONVST, LOW);
  digitalWrite(RD, HIGH);
  digitalWrite(RANGE, LOW);
  digitalWrite(BUSY, LOW);


  // Set up the status LED line as an output
//  pinMode(statusLed, OUTPUT);
//  digitalWrite(statusLed, HIGH);  // We have an active-low LED attached

  pinMode(sensorPin1, INPUT);
  digitalWrite(sensorPin1, HIGH);

  pinMode(sensorPin2, INPUT_PULLUP);
  digitalWrite(sensorPin2, HIGH);

  pinMode(sensorPin3, INPUT_PULLUP);
  digitalWrite(sensorPin3, HIGH);

  pulseCount1        = 0;
  pulseCount2        = 0;
  pulseCount3        = 0;

  oldTime            = 0;

  // the hall effect sensors are configured to trigger on a FALLING state change
  // (transition from HIGH state to LOW state)
  enableInterrupt(sensorPin1, pulseCounter1, FALLING);
  enableInterrupt(sensorPin2, pulseCounter2, FALLING);
  enableInterrupt(sensorPin3, pulseCounter3, FALLING);


void loop() {

  //change this value to preffered sampling rate
  if ((millis() - oldTime) == 50) {

    oldTime = millis();


    //tell ADC to start read (converting analogue input to digital output)
    digitalWrite(CONVST, LOW);
    digitalWrite(CONVST, HIGH);
    //when read-conversion is complete ADC sends the BUSY pin low triggering the bitBang ISR

    //print ADC data from array within the bitBang ISR to a string
    String adcString = "";
    for(int thisChannel=0; thisChannel<channelCount; thisChannel++){
      adcString += String(adcData[thisChannel]);
      adcString += ",";

    //disable interrupt to access current pulse count
    //grab current pulse count and store in a variable
    pulses[0] = pulseCount1;
    pulses[1] = pulseCount2;
    pulses[2] = pulseCount3;
    //reset pulse count
    pulseCount1 = 0;
    pulseCount2 = 0;
    pulseCount3 = 0;
    //enable interrupt and start incrementing pulse count again
    enableInterrupt(sensorPin1, pulseCounter1, FALLING);
    enableInterrupt(sensorPin2, pulseCounter2, FALLING);
    enableInterrupt(sensorPin3, pulseCounter3, FALLING);

    String pulseString = "";
    for(int i=0; i<3; i++){
      pulseString += String(pulses[i]);
      if (i<2) {
        pulseString += ",";

   String dataString = String(adcString + pulseString);
  // open the file. note that only one file can be open at a time,
  // so you have to close this one before opening another.
  File dataFile = SD.open("dataLog.txt", FILE_WRITE);

  // if the file is available, write to it:
  if (dataFile) {
    // print to the serial port too:
  // if the file isn't open, pop up an error:
  else {
    Serial.println("error opening dataLog.txt");


//start again

//grabbing ADC data ISR (for n channel)
void bitBang ()  {
  //a for loop to bitbang the values fom the analogue (ADC) channels (maximum 8) sequentially storing them in a variable array (equal to the channels of the ADC)
  for(int thisChannel=0; thisChannel<channelCount; thisChannel++){
  //send ADC read pin low to bitbang the first channel  
  digitalWrite(RD, LOW);
  //read the state of the 16 pins and store in a variable
  rawData[0] = digitalRead(DB15);
  rawData[1] = digitalRead(DB14);
  rawData[2] = digitalRead(DB13);
  rawData[3] = digitalRead(DB12);
  rawData[4] = digitalRead(DB11);
  rawData[5] = digitalRead(DB10);
  rawData[6] = digitalRead(DB9);
  rawData[7] = digitalRead(DB8);
  rawData[8] = digitalRead(DB7);
  rawData[9] = digitalRead(DB6);
  rawData[10] = digitalRead(DB5);
  rawData[11] = digitalRead(DB4);
  rawData[12] = digitalRead(DB3);
  rawData[13] = digitalRead(DB2);
  rawData[14] = digitalRead(DB1);
  rawData[15] = digitalRead(DB0);
  //convert into 16bit 2s compliment and store in a variable array 
  adcData[thisChannel] = rawData[0] | (rawData[1] << 1) | (rawData[2] << 2) | (rawData[3] << 3) | (rawData[4] << 4) | (rawData[5] << 5) | (rawData[6] << 6) | (rawData[7] << 7) |  (rawData[8] << 8) | (rawData[9] << 9) | (rawData[10] << 10) | (rawData[11] << 11) | (rawData[12] << 12) | (rawData[13] << 13) | (rawData[14] << 14) | (rawData[15] << 15);
  //send ADC pin high to say we have read the first channel 
  digitalWrite(RD, HIGH);
  //repeat for n channels

//counting pulses ISRs
void pulseCounter1(){
  // Increment the pulse counter
void pulseCounter2(){
  // Increment the pulse counter
void pulseCounter3(){
  // Increment the pulse counter
  • 1
    if ((millis() - oldTime) == 50) { .... what happens if the program briefly hangs up and misses the 50ms difference?
    – jsotola
    Commented Jul 11, 2019 at 22:21
  • Did you think about jsotola's comment? If not, I suggest you think about it very very carefully. The advice in the answer below is good advice, but it might just be the comment that saves you - and exposes a new problem when you realise the answer....
    – GMc
    Commented Jul 11, 2019 at 22:30
  • I'm running tests on it now. Do you know what? As little as 10 days ago that part of code said if ((millis() - oldTime >= 50) { and it has said that for months. I don't know when or how the double equal crept in. I'll let you know the outcome soon. Thanks
    – Microk
    Commented Jul 12, 2019 at 11:03
  • Yeah, that seems to have fixed the issue. Absolutely kicking myself for somehow adding in the bug without realising it, oh well, humanum est errare as the old saying goes. Thank you for everyone for your help, especially to the person that voted my question up thus awarding me enough reputation to comment everywhere - a very special day for me. What is the etiquette here? I can't accept a comment as an answer so should I accept the @Michel Keijzer one? thanks again
    – Microk
    Commented Jul 12, 2019 at 11:36

2 Answers 2


Some tips:

  • Do not open the open and close the file in every loop sequence (I think you can use the flush command to save/update the file.
  • Do not save strings, but save the raw data and pulse string. This will take 16 * 2 (raw data + 3 bytes for the pulse data = 35 bytes per 16 samples, meaning 35 bytes/samples * 20 samples/s = 700 bytes (I think your calculation of 1100 bytes is incorrect).
  • Try to avoid string, and even more string concatenation. I don't know if this will be the bottleneck, but in any case String operations are expensive and for concatenation there is a chance (or is always the case) that memory needs to be allocated dynamically. Use a string buffer instead.
  • 1
    Thanks for your suggestions. I have read about strings being problematic, and the C string class should be used instead. I will implement the first 2 bulletpoints today but will have to do a bit of research before taking on the third. i will kyp.
    – Microk
    Commented Jul 12, 2019 at 11:08
  • File I/O operations are normally quite expensive, so it's good to keep them to a minimum. The third point might be a bit more work, but except for very simple sketches, try using C string (char* instead). Tip: to test with the C strings, do not use an Arduino, but do it on a PC, that's much more quicker, than if the code works or you know how c strings work, move it to the Arduino IDE. Commented Jul 12, 2019 at 11:12
  • OK, so I use command prompt on my laptop?
    – Microk
    Commented Jul 12, 2019 at 11:17
  • No, I suggest you install either Visual Studio 2019 or Eclipse C(++) ... both are free. You create your non Arduino specific code on the PC with either those two IDEs, and when it works you copy it to the Arduino IDE. The advantage is that you can use debugging and you don't have to upload the program/sketch every time. Commented Jul 12, 2019 at 11:22
  • That's great advice, I'm definitely gonna take it :) Thanks again.
    – Microk
    Commented Jul 12, 2019 at 11:38

So it turns out the actual problem was this line of code if ((millis() - oldTime) == 50) { it needed to be changed to if ((millis() - oldTime >= 50) {

I really appreciate all the help everyone gave me to get to the bottom of it and especially thankful to @Michel for handing some solid advice to someone who isn't in the know. Hopefully I will be able to repay in kind to another learner one day. :D

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