Arduino example for the FGM-3h magnetometer

Archive: The magnetometer is offline and we are no longer collecting data.

Getting started

This simple Arduino Mega example counts the pulses from the FGM-3h using an external counter and sends the result to the serial port. An interrupt counts timer overflows during the approximately five-second sample. First, the includes, constants and declarations:

#include "avr\io.h"
#define SAMPLE_DELAY 5  // number of seconds between samples
#define SAMPLE_DURATION 5  // number of seconds to sample for.
volatile int mult=0;
static uint32_t timer=0;
float GetData();

Setup

The setup function starts the serial port:

void setup()
 {
 Serial.begin(57600);
 delay(1000);
   Serial.println("Hello world");
 }

Loop

The loop takes a reading roughly every five seconds and sends it to the serial port:

 void loop()
 {
    if (millis() > timer) {
      timer = millis() + (SAMPLE_DELAY*1000);  //only run every 5 seconds or whatever
     float FGMreading;
      FGMreading = GetData();
      Serial.print("FGM Reading. Period =  ");
     Serial.print(FGMreading,5);
       Serial.println(" microseconds");
     }
 }

Timer overflow interrupt

The FGM square-wave output is connected to pin 47 on the Mega, which is connected to the counter for timer 5. The interrupt increments the overflow count:

ISR(TIMER5_COMPA_vect) {
   //Interrrupt service routine for clock counter - each time it reaches 65k, reset and increase mult
   //mult*65k is then added to the count at the end in GetData()
   if (TIFR5 & 1) {          			// if Timer/Counter 1 overflow flag
    		mult++;               // count number of Counter1 overflows
    	TIFR5 =(1<<TOV5);        			// clear Timer/Counter 1 overflow flag
   	}
  }

Counting and calculating the period

GetData resets the counter, counts pulses during the sample and combines the timer count with the number of overflows. The calculation then converts the frequency to a period in microseconds. The original page presented this as an excerpt without the GetData function declaration:

  unsigned long count;
 unsigned long Frequency;
  mult=0;
  TCCR5A=0; // reset timer/counter control register A
  TIMSK5 |=(1<<OCIE5A); //enable interrupt service routine
  //start counting
  bitSet(TCCR5B ,CS12); // Counter Clock source is external pin 47 of mega 2560
  bitSet(TCCR5B ,CS11); // Clock on rising edge
  delay(SAMPLE_DURATION*1000); //wait for sample duration and count the clicks
  //stop counting
  TCCR5B = 0;
  count = TCNT5;
  TCNT5 = 0;
  count+=0x10000 * mult; // the counter is 16 bit, so total count is count + mult*65k
  Frequency = count/SAMPLE_DURATION; //frequency in Hz
  unsigned long long oneSecond = 1000;
  oneSecond*=1000;
  oneSecond*=1000;
  oneSecond*=100;
  unsigned long long Period;
  Period = oneSecond / (unsigned long long) Frequency;
  float usPeriod;
  usPeriod = float (Period) / 100000;
  return usPeriod;
}

Possible improvements

• Use a more stable timing source, such as the pulse-per-second output from a GPS.
• Use an Ethernet shield to send data to a website.
• Send the pulse count itself and convert it to a period elsewhere.
• Enable the watchdog timer to reset the Arduino if it gets stuck.

Back to the magnetometer project