Multi Tasking

Multi-Tasking: Delay

First, a Clarification

• We’ve learned about creating a button / latch toggle

  int currButtonVal = digitalRead(PIN_BUTTON);
  if (currButtonVal == LOW && prevButtonVal == HIGH) {
    // code to execute on button press goes here
  }
  prevButtonVal = currButtonVal;
  

• Latches allow us to react to single button presses
• Multi-tasking code is solving a different problem

Consider

What is your phone doing right now while you’re not using it?

Review: Basic LED Blink

void setup() {
  pinMode(D7, OUTPUT);
}

void loop() {
  digitalWrite(D7, HIGH);   
  delay(1000);  //what is happening during the 1 sec delay?       
       
  digitalWrite(D7, LOW);     
  delay(1000);                       
}

Questions

1. How do we get two LEDs to each blink on/off at 1 sec intervals?
2. How do we get one LEDs to blink on/off at 1 sec interval, and the second LED to blink at 2 sec intervals?
3. How do we get one LEDs to blink on/off at 1.442 sec interval, and the second LED to blink at 3.83 sec intervals?

Delay Problems

• delay() is blocking
  • This means the Photon 2 is essentially paused and can’t do anything else
• We need to be able to synchronize events and multi-task just like our computers and phones
• This means in the time between the LED turning off and turning on, the Photon 2 can do other things

Step 1: Let’s Use the Clock

• millis() is a function that returns the “relative current time”
  • Number of milliseconds that have elapsed since the Photon 2 last turned on / reset
• We can call millis() at different points in time to compare the passage of time
• When a specific time has elapsed, we can take an action

Note about millis() and long

• The “on” time is returned as an long

• long is like an int but has a larger capacity

• Normally long stores 32 bit numbers (positive and negative)

• range of long: -2,147,483,648 to 2,147,483,647

Note about millis() and unsigned long

• However, the “on” time will never be negative

• We can specify a variable as unsigned which means the lowest value is 0

• range of unsigned long: 0 to 4,294,967,295

• Syntax

  unsigned long currentTime = millis();
  

Step 2: Track the Current State

• State represents any variables or relevant data about our device at the current moment
• You decide what states are relevant for your program
  • E.g. AcceptingPayment, ReceivingData, DishesDry, AlarmOff
• For a single LED, the states are either ON (HIGH) or OFF (LOW)

Our New Plan

• if ledState == LOW AND interval is over
  • then ledState = HIGH
  • update time
  • turn on LED
• if ledState == HIGH AND interval is over
  • ledState = LOW
  • update time
  • turn off LED

Visual Representation

Code: Blink LED without Delay

unsigned long prevMillis = 0;	//last time we checked time
int ledState = LOW;					//initial state
int interval = 300;
void loop() {
  unsigned long curMillis = millis();     //current time
  //check if (now - previous) is more than our interval
  if (curMillis - prevMillis >= interval) {   
    prevMillis = curMillis;      //if YES, update previous
    if (ledState == LOW) {           //if LED ON, now it is OFF
      ledState = HIGH;                                
    } else {						//if LED ON, now it is OFF
      ledState = LOW;
    }
    digitalWrite(LED_PIN, ledState);
  }

Code: More Concise Blink LED without Delay

unsigned long prevMillis = 0;	//last time we checked time
int ledState = LOW;					//initial state
int interval = 300;
void loop() {
  unsigned long curMillis = millis();     //current time
  //check if (now - previous) is more than our interval
  if (curMillis - prevMillis >= interval) {   
    prevMillis = curMillis;      //if YES, update previous
    ledState = !ledState; //toggle true / false
    digitalWrite(LED_PIN, ledState);
  }
  

Lab

Lab Goals

• Blink LED1 every 300 ms
• Blink LED2 every 146 ms
• Create toggle button (latch) to turn ON BOARD on at rising edge and off again at the rising edge
• Track the number of times the button is pressed
• Publish the number of button presses to cloud every 10000 ms
• Extra Challenge
  • Change LED1 to be on for 300 ms and off for 700 ms
  • Change code so that when the toggle button is pressed, LED2 starts blinking every 146 milliseconds, and when the toggle button is pressed again, LED2 stops blinking

Stages to Build

1. Use delay to blink LED1 every 300 ms and then check for a button press (not a toggle); display Serial message if button pressed
2. Use millis() to fix blocking in #1 (we’ll do #1 and #2 together)
3. Use millis() to blink LED2 every 146 ms
4. Enable toggle button to turn ON BOARD LED on and off on the rising edge
5. Track number of button presses and use millis() to publish number of button presses every 10000 ms

Starting Code

const int PIN_BUTTON = D2;
const int PIN_LED1 = D3;
const int PIN_LED2 = D4;

void setup()
{
  pinMode(PIN_BUTTON, INPUT);
  pinMode(PIN_LED1, OUTPUT);
  pinMode(PIN_LED2, OUTPUT);
  Serial.begin(9600);
}

void loop()
{
  digitalWrite(PIN_LED1, HIGH);
  delay(300);
  digitalWrite(PIN_LED1, LOW);
  delay(300);
  digitalWrite(PIN_LED1, HIGH);
  delay(300);
  digitalWrite(PIN_LED1, LOW);
  delay(300);
}