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# Reading the C++
Arduino · A new board
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## Today
**We are learning to** every sketch has two parts: setup() runs once when the board starts, then loop() runs again and again for ever. delay(ms) waits that many milliseconds, so the delays set the timing of a blink.
- I can say what setup() and loop() each do.
- I can work out a blink's timing from its delays.
- I can choose delays for a timing.
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## Why it matters
> MakeCode had 'on start' and 'forever'. An Arduino sketch has the same two parts, written as C++. Which part runs when, and how many times?
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💬 Discussion
## Predict
**The blink sketch runs for 10 seconds. How many times does setup() run?**
- Never: only loop() runs
- Twice
- Once
- Every time loop() runs
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✍️ Activity
## Try it
{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fengineering%2Farduino%2Fsetup-loop%2Fstep-2&code=widget%3A+arduino%0Aled%3A+13%0Aedit%3A+off%0Acode%3A+void+setup%28%29+%7B%0Acode%3A+pinMode%2813%2C+OUTPUT%29%3B%0Acode%3A+%7D%0Acode%3A%0Acode%3A+void+loop%28%29+%7B%0Acode%3A+digitalWrite%2813%2C+HIGH%29%3B%0Acode%3A+delay%28500%29%3B%0Acode%3A+digitalWrite%2813%2C+LOW%29%3B%0Acode%3A+delay%28500%29%3B%0Acode%3A+%7D%0A&fresh=1}}
[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/engineering/arduino/setup-loop/?full=step-2#step-2)
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✅ Check
## What happened?
* Once, when the board starts. Then loop() runs again and again: 10 times in 10 seconds here, as each loop takes 1 s.
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💬 Discussion
## Board time
Now fill in the times.
$$\text{setup(): pinMode(13, OUTPUT)}$$
1) $\text{loop(): digitalWrite(13, HIGH)}$ — LED on. Writing takes (almost) no time.
2) $\text{delay(500)}$ — Wait 500 ms with the LED on.
3) $\text{digitalWrite(13, LOW); delay(500)}$ — LED off for 500 ms.
4) $\text{end of loop()} \to \text{start of loop()}$ — 1000 ms per blink, for ever.
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✅ Check
## Board time
**Fill in the board time after each line of loop(), starting at 0 ms.**
| line | LED | time (ms) |
|---|---|---|
| digitalWrite(13, HIGH); | on | 0 |
| delay(500); | on | ? |
| digitalWrite(13, LOW); | off | ? |
| delay(500); | off | ? |
* Answer: 500, 500, 1000
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✅ Check
## Sam's blink
```cpp
void setup() {
pinMode(13, OUTPUT);
digitalWrite(13, HIGH);
delay(500);
digitalWrite(13, LOW);
delay(500);
}
void loop() {
}
```
**Sam put the blink lines in setup() and left loop() empty (below). What does the LED do?**
- Blinks for ever
- Stays on
- Never lights
- Blinks once, then stays off
* Answer: Blinks once, then stays off
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✍️ Activity
## Vary: change the timing
- **Twice as fast:**
- **A flash:**
[Open this step](https://studio.learningwaypoints.com/tutorials/engineering/arduino/setup-loop/?full=step-4#step-4)
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✍️ Activity
## Build it in Tinkercad
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✅ Check
## Count the blinks
**loop() is: HIGH, delay(300), LOW, delay(700). How many blinks in 10 seconds?**
* Answer: 10
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✅ Check
## Where it goes
**Where does pinMode(13, OUTPUT); belong?**
- In loop(): so it keeps working
- In setup(): it only needs doing once
- Before setup(), outside both
- It doesn't matter
* Answer: In setup(): it only needs doing once
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✍️ Activity
## Make: a lighthouse
[Open this step](https://studio.learningwaypoints.com/tutorials/engineering/arduino/setup-loop/?full=step-7#step-7)
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✅ Check
## Exit ticket
**What is the difference between setup() and loop()?**
* setup() runs once when the board starts; loop() runs again and again after it.