How to set the sampling frequency in Arduino without using a delay I am reading an ECG signal using a single lead heart rate monitor. What I know is that when I use delay(15) it means that I am getting 66.67 readings in a second which corresponds to the sampling frequency. I want to set the sampling frequency to 250. This can be done by introducing delay(4). 

What I really need is to set the sampling frequency to 250 Hz without introducing a delay in the code because I am acquiring ECG data in real time. Also, I am sending the data to python for processing and the delay will affect the sampling frequency in the time domain analysis.

any detailed solution or code ??

PS. I am using Arduino UNO

Code:

#define SERIAL_BUFFER_SIZE 8192
#include &lt;SoftwareSerial.h&gt;
unsigned long Time;
void setup() {

  // initialize the serial communication:

  pinMode(10, INPUT); // Setup for leads off detection LO +
  pinMode(11, INPUT); // Setup for leads off detection LO -
  Serial.println("CLEARDATA"); //clears up 

  Serial.println("RESETTIMER"); //resets timer to 0

  Serial.begin(115200);

}

void loop() {
  if((digitalRead(10) == 1)||(digitalRead(11) == 1)){
   Serial.println('!');
  //  continue;
  }
  else{
   /*
     // send the value of analog input 0:
     int idata = analogRead(A0);
  //  float data = (idata*3.3)/1023;
      Serial.println(idata);
*/
  int sensorValue = analogRead(A0);
  float voltage = sensorValue * (3.3/1023.0);
  Serial.println(voltage);
  }
  //Wait for a bit
    delay(4);
}
Use the concept of blink without delay. 

Instead of using delay() to set the timeout you use a timestamp and check micros() against that to see whether your interval has passed:

const unsigned long READ_PERIOD = 4000;  // 4000 us: 250 Hz

void loop() {
    static unsigned long lastRead;
    if (micros() - lastRead &gt;= READ_PERIOD) {
        lastRead += READ_PERIOD;
        if (digitalRead(10) == 1 || digitalRead(11) == 1)
            Serial.println('!');
        else
            Serial.println(analogRead(A0) * (3.3/1024));
    }
}