And talkin’ the talk…

Now that I think about it, that last bit of bloggy catharsis will probably be read to the end by maybe three people on the whole internet…if I include my mom. I’m sure the real coders felt a yawn coming on at the second line, and aside from the two other newbies out there at the “Just reading anything I can find.” stage, everyone else is probably thinking “Ok! Enough already… just gimme the code!”

So here you go. My first stable datalogger build* for a Tinyduino stack consisting of: a processor board, a BMA250 accelerometer sheild,  the microSD adapter, and a protoboard, jumpered to a chronodot RTC. The whole thing runs ‘unregulated’ off of 3 AA batteries, so I keep an eye on the bandgap voltage to (hopefully) stop data writes to the SD card when the battery voltage gets too low. But this part has not been tested yet, so don’t trust your life, or data for your thesis, to that kludge. Build yourself a real voltage divider!

*This scrapbook of code was cobbled together from the work of other people who actually know what they are doing, and my only real contribution was the hours I spent wading through forum threads collecting examples, before bolting this Frankenstein together.  So I take no credit for any of it, and provide no guarantees whatsoever that it wont brick your system the first time you run it. I also hope that anyone I’ve missed in the credits, forgives the accidental oversight, as some details get lost in the process of all that cutting and pasting.

// Date, Time and Alarm functions using a DS3231 chronodot RTC connected via I2C // and Wire library from
// based largely on Jean-Claude Wippler from JeeLab’s
// Clear alarm interupt code from
// Get temp from,22301.0.html [which does not use the RTCLIB!]
// BMA250_I2C_Sketch.pde – from
// which links to
// combined with internal voltage reading trick from:
// floats to string conversion:

//note: angle brackets which should be around library names are missing here due to formatting weirdness in WordPress

#include SD.h
#include Wire.h
#include SPI.h // not used here, but needed to prevent a RTClib compile error
#include avr/sleep.h
#include RTClib.h

#ifndef cbi //defs for stopping the ADC during sleep mode
#define cbi(sfr, bit) (_SFR_BYTE(sfr) &= ~_BV(bit))
#ifndef sbi
#define sbi(sfr, bit) (_SFR_BYTE(sfr) |= _BV(bit))

#define DS3231_I2C_ADDRESS 104 //for the RTC temp reading function

#define BMA250 0x18
#define BW 0x08 //7.81Hz bandwith
#define GSEL 0x03 // set range 0x03 – 2g, 0x05 – 4, 0x08 – 8g, 0x0C – 16g
//#define DELAY 10000 is this line an orphan?

byte Alarmhour = 1;
byte Alarmminute = 1;
byte dummyRegister;
volatile int state = LOW;
volatile boolean clockInterrupt = false;
int SampleInterval = 1;
// power-down time in minutes before interupt triggers next sample

byte tMSB, tLSB; //for the RTC temp reading function
float temp3231;

const int chipSelect = 10; //sd card chip select

uint8_t dataArray[16];
//variables for accellerometer reading
int8_t BMAtemp;
float BMAtempfloat;
int x,y,z;
String BMAdata; //for passing back data from bma read function
char TimeStampbuffer[ ]= “0000/00/00,00:00:00, “;

int ledpin = 13; //led indicator pin

void setup () {

digitalWrite(INTERRUPT_PIN, HIGH);//pull up the interrupt pin
pinMode(13, OUTPUT); // initialize the LED pin as an output.
digitalWrite(13, HIGH); // turn the LED on to warn against SD card removal

//RTC.adjust(DateTime(__DATE__, __TIME__));
//the above line set the time with code compile time – you only run this line ONCE!
clearClockTrigger(); //stops RTC from holding the interrupt low if system reset
// time for next alarm
DateTime now =;
Alarmhour = now.hour();
Alarmminute = now.minute()+ SampleInterval ;
if (Alarmminute > 59) { //error catch – if Alarmminute=60 the interrupt never triggers due to rollover
Alarmminute = 0; Alarmhour = Alarmhour+1; if (Alarmhour > 23) {Alarmhour =0;}

initializeBMA(); //initialize the accelerometer

//get the SD card ready
pinMode(chipSelect, OUTPUT); //make sure that the default chip select pin is set to output, even if you don’t use it
Serial.print(“Initializing SD card…”);
if (!SD.begin(chipSelect)) { // see if the card is present and can be initialized:
Serial.println(“Card failed, or not present”); // don’t do anything more:
Serial.println(“card initialized.”);
File dataFile =“datalog.txt”, FILE_WRITE); //PRINT THE DATA FILE HEADER
if (dataFile) { // if the file is available, write to it:
dataFile.println(“YYYY/MM/DD, HH:MM:SS, Vcc(mV), X = , Y = , Z = , BMATemp (C) , RTC temp (C)”);
else { //if the file isn’t open, pop up an error:
Serial.println(“Error opening datalog.txt file!”);

void loop () {

if (clockInterrupt) {

//read in our data
BMAdata = String(“”); //clear out the datastring
read3AxisAcceleration(); //loads up the dataString
DateTime now =; // Read the time and date from the RTC
sprintf(TimeStampbuffer, “%04d/%02d/%02d,%02d:%02d:%02d,”, now.year(), now.month(),, now.hour(), now.minute(), now.second());
Serial.println(“Timestamp Y/M/D, HH:MM:SS, Vcc = , X = , Y = , Z = , BMATemp (C) , RTC temp (C)”);
Serial.print(TimeStampbuffer); Serial.print(readVcc()); Serial.print(“,”);
Serial.print(BMAdata); Serial.print(“,”); Serial.println(get3231Temp());

//write data to the SD card
// note that only one file can be open at a time,so you have to close this one before opening another.
File dataFile =“datalog.txt”, FILE_WRITE);

if (dataFile) { // if the file is available, write to it:
dataFile.print(TimeStampbuffer);dataFile.print(readVcc()); dataFile.print(“,”);
dataFile.print(BMAdata); dataFile.print(“,”); dataFile.println(get3231Temp());
else { //if the file isn’t open, pop up an error:
Serial.println(“Error opening datalog.txt file”);

// setNextAlarmTime();
Alarmhour = now.hour(); Alarmminute = now.minute()+SampleInterval;
if (Alarmminute > 59) { //error catch – if alarmminute=60 the interrupt never triggers due to rollover!
Alarmminute =0; Alarmhour = Alarmhour+1; if (Alarmhour > 23) {Alarmhour =0;}
RTC.setAlarm1Simple(Alarmhour, Alarmminute);

Serial.print(“Alarm Enabled at: “);
Serial.print(now.hour(), DEC); Serial.print(‘:’); Serial.println(now.minute(), DEC);
Serial.print(“Going to Sleep for “); Serial.print(SampleInterval);Serial.println(” minutes.”);
//a delay long enough to boot out the serial coms before sleeping.


//Serial.println(“Alarm 1 has been Triggered!”);

void sleepNow() {
digitalWrite(13, LOW);
cbi(ADCSRA,ADEN); // Switch ADC OFF
attachInterrupt(0,clockTrigger, LOW);
sbi(ADCSRA,ADEN); // Switch ADC converter ON
pinMode(13, OUTPUT); digitalWrite(13, HIGH);
// turn the LED on to warn against SD card removal
// But I have some conflict between the SD chip select line, and the led – so the darned thing never lit!

void clockTrigger() {
clockInterrupt = true; //do something quick, flip a flag, and handle in loop();

void clearClockTrigger()
Wire.beginTransmission(0x68); //Tell devices on the bus we are talking to the DS3231
Wire.write(0x0F); //Tell the device which address we want to read or write
Wire.endTransmission(); //Before you can write to and clear the alarm flag you have to read the flag first!
Wire.requestFrom(0x68,1); // Read one byte; // In this example we are not interest in actually using the bye
Wire.beginTransmission(0x68); //Tell devices on the bus we are talking to the DS3231
Wire.write(0x0F); //Tell the device which address we want to read or write
Wire.write(0b00000000); //Write the byte. The last 0 bit resets Alarm 1
clockInterrupt=false; //Finally clear the flag we used to indicate the trigger occurred

// could also use RTC.getTemperature() from the library here as in:
//RTC.convertTemperature(); //convert current temperature into registers
//Serial.print(RTC.getTemperature()); //read registers and display the temperature

float get3231Temp()
//temp registers (11h-12h) get updated automatically every 64s
Wire.requestFrom(DS3231_I2C_ADDRESS, 2);

if(Wire.available()) {
tMSB =; //2’s complement int portion
tLSB =; //fraction portion

temp3231 = ((((short)tMSB << 8 | (short)tLSB) >> 6) / 4.0); // Allows for readings below freezing – Thanks to Coding Badly
//temp3231 = (temp3231 * 1.8 + 32.0); // Convert Celcius to Fahrenheit
return temp3231;

else {
temp3231 = 255.0; //Use a value of 255 to error flag that we did not get temp data from the ds3231

return temp3231;

byte read3AxisAcceleration()
for(int i = 0; i < 7;i++)
dataArray[i] =;

BMAtemp = dataArray[6];
x = dataArray[1] << 8; x |= dataArray[0]; x >>= 6;
y = dataArray[3] << 8; y |= dataArray[2]; y >>= 6;
z = dataArray[5] << 8; z |= dataArray[4]; z >>= 6;

BMAdata += String(x);
BMAdata += “,”;
BMAdata += String(y);
BMAdata += “,”;
BMAdata += String(z);
BMAdata += “,”;
BMAtempfloat = (BMAtemp*0.5)+24.0;
// add digits of BMAtempfloat value to datastring
BMAdata += ((int)BMAtempfloat);
BMAdata += “.”;
int temp = (BMAtempfloat – (int)BMAtempfloat) * 100;
BMAdata += (abs(temp));
//the following 2 lines also to convert float to string
//dtostrf(floatVariable2convert, minStringWidthIncDecimalPoint, numVarsAfterDecimal, charBuffer);
//for example: dtostrf(BMAtempfloat, 5, 2, dtostrfbuffer); dataString += dtostrfbuffer;

byte initializeBMA()
Wire.write(0x0F); //set g
Wire.write(0x10); //set bandwith

long readVcc() { //trick to read the Vin using internal 1.1 v as a refrence
long result;
// Read 1.1V reference against AVcc
ADMUX = _BV(REFS0) | _BV(MUX3) | _BV(MUX2) | _BV(MUX1);
delay(2); // Wait for Vref to settle
ADCSRA |= _BV(ADSC); // Convert
while (bit_is_set(ADCSRA,ADSC));
result = ADCL;
result |= ADCH<<8;
result = 1126400L / result; // Back-calculate AVcc in mV
return result;

2 thoughts on “And talkin’ the talk…

    1. edmallon Post author

      I usually comment out any print statements before compiling for battery powered runs, along with any led indicator blinking, as that’s just wasted power. But thanks for the great link there Mike, as I clearly have a long way to go to make that code into something more elegant.

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