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TECHNICAL ARTICLE can go on during the delay function, so in effect, it brings most other activity to a halt. For alternative approaches to controlling timing, see the millis() function [8] used on line 49 in the sketch below. Arduino veterans usually avoid the use of delay() for timing of events longer than 10’s of milliseconds unless the Arduino sketch is very simple. However, certain things do go on while the delay() function is controlling the Atmega chip because the delay function does not disable interrupts. Serial communication that appears at the RX pin is recorded, PWM (analogWrite) values and pin states are maintained, and interrupts will work as they should. Author’s Comment From what we have seen above, the delay() implementation behaves like a busy loop with interrupts enabled. Variables A variable is a place for storing a piece of data. It has a name, a type, and a value. For example, the line 26 from the sketch above declares a variable with the name ledPin, the type int, and an initial value of 13. It’s being used to indicate which Arduino pin the LED is connected to and since it never changes, it’s declared as const, which could give the linker a hint to place it in a section other then precious RAM. Every time the name ledPin appears in the code, its value will be retrieved. In this case, the person writing the program could have chosen not to bother creating the ledPin variable and could have simply written 13 everywhere they needed to specify a pin number instead. The advantage of using a variable is that it’s easier to move the LED to a different pin—you only need to edit the one line that assigns the initial value to the variable. Often, however, the value of a variable will change while the sketch runs (lines 29, 34, 49). Due to the lack of insightful comments, it’s not quite obvious to me why the variable in line 30 is not defined as const as well since it does not change. Functions A function (otherwise known as a procedure or subroutine) is a named piece of code that can be used from elsewhere in a sketch. The setup() function [9] as seen in Listing 2: delay-example.ino int ledPin = 13; // LED connected to digital pin 13 1 2 3 4 5 6 7 8 9 10 11 12 13 14 void setup() { pinMode(ledPin, OUTPUT); // sets the digital pin as output } void loop() { digitalWrite(ledPin, HIGH); // sets the LED on delay(500); // waits 500 msec digitalWrite(ledPin, LOW); // sets the LED off delay(500); // waits 500 msec } Figure 3 Listing 3: BlinkWithoutDelay.ino continued 24 // constants won’t change. Used here to 25 // set pin numbers: 26 27 const int ledPin = 13; // the number of the LED pin 28 // Variables will change: 29 int ledState = LOW; // ledState used to set the LED 30 31 long previousMillis = 0; // will store last time LED was updated 32 // the following variable is a long because the time, measured in milliseconds, 33 // will quickly become a bigger number than can be stored in an int. 34 35 long interval = 1000; // interval at which to blink (milliseconds) 36 void setup() { 37 // set the digital pin as output; 38 pinMode(ledPin, OUTPUT) 39 } Figure 4 Listing 4: BlinkWithoutDelay.ino continued 41 void loop() 42 { 43 44 // here is where you’d put code that needs to be running all the time. 45 // check to see if it’s time to blink the LED; that is, if the 46 // difference between the current time and last time you blinked 47 // the LED is bigger than the interval at which you want to 48 // blink the LED. 49 50 unsigned long currentMillis = millis(); 51 if(currentMillis – previousMillis > interval) { 52 // save the last time you blinked the LED 53 54 previousMillis = currentMillis; 55 // if the LED is off turn it on and vice-versa: 56 if (ledState = LOW) 57 ledState = HIGH; 58 else 59 60 ledState = LOW; 61 // set the LED with the ledState of the variable: 62 digitalWrite(ledPin, ledState); 63 } 64 } Figure 5 line 36 is called once, when the sketch starts. It’s a good place to configure pin modes and to initialize libraries. The pinMode() function [10] line 38 configures a digital pin as either an input or an output. To use it, you pass the number of the pin to configure and the constant INPUT or OUTPUT. When configured as an input, a pin can detect the state of a sensor like a “Push” button. As an output, it can drive an actuator like an LED, which, in our case, utilizes the digitalWrite() function [11] line 62 to EEWeb | Electrical Engineering Community Visit www.eeweb.com 18 TECHNICAL ARTICLE
TECHNICAL ARTICLE change state (Figure 5). The loop() function [12] line 41 is called over and over and is the heart of most sketches. Both setup() and loop() need to be included in your sketch, even if you don’t need them for anything. Author’s Comment From what we have seen so far, it looks like an Arduino sketch behaves like a cyclic executive with interrupts. Stay tuned for more hands-on stuff in the next part of this series of articles where we’ll investigate Processing! References [1] ”Das Blinkenlights” http://en.wikipedia.org/wiki/ Blinkenlights [2] ”www.blinkenlights.nl” http://www.blinkenlights.nl [3] ”First Sketch” http://arduino.cc/en/Tutorial/Sketch [4] ”Processing” http://processing.org [5] ”delay()” http://arduino.cc/en/Reference/Delay BeStar® ACOUSTICS & SENSORS PRODUCTS Speakers Buzzers Piezo Elements Back-up Alarms Horns Sirens/Bells Beacons Microphones Sensors INDUSTRIES Automotive Durables Medical Industrial Mobile Fire / Safety Security Consumer Leisure [6] ”vTaskDelay()” http://www.freertos.org/a00127.html [7] ”usleep()” http://pubs.opengroup.org/onlinepubs/009695399/functions/usleep.html [8] ”millis()” http://arduino.cc/en/Reference/millis [9] ”setup()” http://arduino.cc/hu/Reference/Setup [10] ”pinMode()” http://arduino.cc/en/Reference/pin- Mode [11] ”digitalWrite()” http://arduino.cc/en/Reference/digitalWrite [12] ”loop()” http://arduino.cc/en/Reference/loop About the Author Robert Berger is a highly respected and experienced embedded real-time expert and CEO of Reliable Embedded Systems, a leading embedded training consultancy. Robert consults and trains people all over the globe on a mission to help them create better embedded software. He specializes in training and consulting for embedded systems, from small real-time systems to multi-core embedded Linux. ■ Teamwork • Technology • Invention • Listen • Hear Preferred acoustic component supplier to OEMs worldwide bestartech.com | sales@bestartech.com | 520.439.9204 EEWeb | Electrical Engineering Community Visit www.eeweb.com 19 QS9000 • TS/ISO16949 • ISO14001 • ISO13485 • ISO9001 TECHNICAL ARTICLE
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