2018
June Wednesday 20

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Arduino Time Intervals; Timming

2017 OCTOBER | by Gene Casanova

Senior Systems Engineer


Using Time Intervals

Processes progress by clockm ticks within any computer system, including an Arduino. Using the clock ticks, is the proficent method for timming process executions.

The logical method for using timming is by first saving a timestamp at a critcal moment in a process, and then later in the process, subtract a second point in time with the timestamp to get the interval time; the time passed since the timestamp.


Using Timming In A Process

When precision timming is not an issue in a program, use the "millis()" function for obtaining the number of milliseconds passed/elapsed since the Arduino microcomputer IC chip was powered.

Example:

uint32_t ts1 = millis();
// Some process code goes here.
uint32_t ts2 = millis();

// Output the elasped time in milliseconds:
Serial.println( ts2-ts1 );}	//	Basic math here.

When timming precision is crtical, use the "micros()" function; it returns the number of microseconds since the Arduino was powered, with a precision of 4 or 8 μsec, depending on the clock speed of the microcomputer.

Example:

uint32_t ts1 = micros();

// a process goes here.

uint32_t ts2 = micros();

// Output the elasped time in microseconds:
Serial.println( ts2-ts1 );

BEWARE, the examples above will work most of the time, but not always. The registers used to count milli- or microseconds, since the Arduino boot time, have a limited capacity of 32 bits. This means, when they reach their highest possible value (0xffffffff, or equally 4294967295) they will overflow and reset to zero.

If the overflow occurs anywhere outside the two calls to millis() or micros(), the code above will work fine, because ts2 will be greater than ts1 and the result of the subtraction will be correct.

If the overflow occurs between the first and the second call to millis() or micros(), then there is a problem. ts2 will not be greater than ts1, and simply subtracting ts2 - ts1 will yield a wrong result.

The overflow of the counters occurs once every 49.7 days for the milliseconds counter.

The microseconds counter overflows much more often: every 71.5 minutes!

It may not be always possible to tolerate a wrong calculation, even if it occurs seldom.

Calculate the time interval between those two timestamps as the sum of two sub-intervals: dt1 which is the interval between t1 and the overflow point (timestamp = 0), and dt2, which is the interval between 0 and t1. Then the total interval dt would be:

uint32_t dt= dt1 + dt2;

Calculate dt2 = t1 − 0; is t1:

uint32_t dt2 = t1;

Calculating dt1 is a bit more complicated, and requires to invert all the bits of t2, and then add +1 to it.

Add +1 to get the actual interval between t2 and 0:

uint32_t dt1 = 1 + ~t2;

Everything together:

uint32_t dt1 = 1 + ~t2;
uint32_t dt2 = t1;
uint32_t dt = dt1 + dt2;

Simplified to:

uint32_t dt = 1 + t1 + ~t2;

Next, discriminate the two cases:

uint32_t dt = t1 > t2 ? 1 + t1 + ~t2 : t2 - t1;

The statement above checks if t1 > t2 or not, and then performs the appropriate calculation.


Example Process

/*
 * Example Interval Timer Process.
 * Example state machine.
*/
const int ledPin =  LED_BUILTIN;	// the number of the LED pin.

int ledState = LOW;             // ledState used to set the LED

unsigned long previousMillis = 0;  // Used to save last time LED process updated

// A Constant Value:
const long interval = 1000;           // the Interval to do something void setup() { // set the digital pin as output: pinMode(ledPin, OUTPUT); } void loop() { // LOGIC: // Check if time passed to meet interval; IF the difference // between the current time and last time blinked the LED is bigger than // the interval to blink the LED. unsigned long currentMillis = millis(); if (currentMillis - previousMillis >= interval) { // save last time blinked the LED previousMillis = currentMillis; // If LED Off, Turn It On and vice-versa: if (ledState == LOW) { ledState = HIGH; } else { ledState = LOW; } // Set LED with the ledState of the variable: digitalWrite(ledPin, ledState); } }

 


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Use The Technology Wisely & Keep It Simple

- Cheers!

Gene Casanova


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