Arduino busy during pulse counting

Forum 2005-2010 (read only) Hardware Interfacing
1

This is a follow up from another topic, yet is a specific issue.

I'm running this code at the moment

const int frqPin = 5; //Input Pin
const int oneSecond = 1000; //Meassured time
int frqPinState = LOW;
int frq = 0;
int prevFrqPinState = LOW;
unsigned long timer = 0;
void setup()
{
  Serial.begin(9600);
  pinMode(frqPin, INPUT); //Declared Pin 5 as Input
}

void loop()
{
  frqPinState = digitalRead(frqPin); //Reads State of Input Pin
  if (frqPinState == LOW) //Creates a previos state
  {
    prevFrqPinState = LOW;
  }
  if (frqPinState == HIGH && prevFrqPinState == LOW) //Pulses Counter
  {
    prevFrqPinState = frqPinState;
    frq++;
  }
  if (millis() - timer > oneSecond) //Time Counter
  {
    timer = millis(); //Resets Time
    Serial.println(frq); //Prints Frequency
    frq = 0; //Resets Frequency
  }
}

I'm feeding a 5.6K TTL Pulse to (T1)PD5 on AtMega368U
The purpose of this is to count how many pulses I get every second, then I want to feed this count to a serial port.

My issue lies on the fact that the count alters by a specific amount constantly, as if the Arduino is not reading some pulses every so often.

This is what I get now...

5665
5665
5665
5659
5665
5659
5666
5659
5665
5659
5665
5659
5665
5660
5665
5666
5659
5665
5659
5665
5659
5665
5660
5665
5660

If I understand correctly, the moment the Arduino uses the serial pins it might not be reading my pulses...any ideas?

2

Your input pulses are too fast for that method. What is happening is that you are missing pulses by two mechinisims:-

  1. In the arduino going off to service regular interrupts, like up dating the millis() counter.
  2. Each step takes a finite time to execute, therefore timing each high and low is at least plus or minus one one count. That gives a possible spread of 4 counts per pulse.

At that speed you need to use interrupts and an hardware counter / timer.

3

digitalRead takes more than 100 microseconds to run once. You can use direct port manipulation to speed it up.

4

I have tried a new approach, following RIDDICK's example using PulseIn:

http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1272558914/30

Here he uses a average of a specific amount of pulses over time in microseconds. I have come up with this code:

const uint8_t pin = 5; // digital pin #5
const uint32_t oneSecond = 1000;
uint32_t timer = 0;
void setup() {
  Serial.begin(9600);
  pinMode(pin,INPUT);
  digitalWrite(pin,HIGH); // pullup
}
void loop() {
  uint32_t sts = micros(); // start time stamp
  const uint32_t c = 3000; // wait for 1000 pulses
  for (uint32_t i=c; i>0; i--)
    pulseIn(pin,HIGH);
  uint32_t ets = micros(); // end time stamp
  if (millis() - timer > oneSecond)
  {
  Serial.println((c*1e6/(ets-sts))); // output Hz
  timer = millis();
  }
}

And now my outputs look like this:

5680.66
5680.66
5680.66
5680.66
5680.66
5680.66
5680.66
5680.66
5680.61
5680.61
5680.61
5680.61
5680.61
5680.61
5680.66
5680.66
5680.66
5680.66
5680.66
5680.66
5680.66
5680.66
5680.66
5680.66
5680.66
5680.66
5680.66
5680.66
5680.66
5680.66
5680.66
5680.66
5680.66
5680.66
5680.66

This is very good!
Thank you for the tips!

Now, I need to learn how to use a Cmos/Static Ram to store a calibration value. I have a older one here, a HM6264LP-70 I like to use, I will open a new Thread to get some help doing such a thing.

5

You can use the built in EPROM in the Arduino for that.

6

You mean, I can access the Static Ram for my own use? How do I go about this?

7

Nevermind I got it :slight_smile: