Arduino 平台与C语言程序设计-week3-Arduino Programs-Lesson3

This series of articles are the study notes of "An Arduino platform and C Programming", by Prof. Harris, Department of Computer Science, University of California, Irvine. This article is the notes of week 3, Arduino Programs, lessen 3.

3. Lesson 3

3.1 Lecture3-1: Input andOutpu

We've been talking about pins and how they can be inputs and outputs, and be digital and analog. And so now, we're going to talk about how you do that from the code. So what code do you need to write, what function calls do you need to make  in order to use the pins as inputs and outputs, in order to use them as digital and analog.

3.1.1 Input/Output (1/O)

(1) These functions allow access to the pins

void pinMdoe(pin,mode);

(2) Sets a pin to act as either an input or an output

Some of the Pins can be inputs or outputs. Not all the pins, some of the pins can't be inputs, or can't be outputs. For instance, the analog pins. Analog pins, they can be inputs but they can't be outputs, but the standard digital pins, they can be input and output. So, before you use the pins, you have to select if the pin is going to act as an input or an output. And you should do this for all the pins that you're going to use. Before you try to read from it or write to it, you have to assign it to be an input or an output.
Now if you don't do this, something will happen. It will have a default value. So, by default, the pin will either be an input or output, and you should never rely on the default value.
Before you access the pins, you should call this function, this pinMode function, in order to assign the pin to either an input or output. It's a library function. It takes two arguments, pin and mode. The job of this is to set a pin to be either an input or an output.
Pin is the number of the pin

• 0 - 13 for the digital pins
• A0 - A5 for the analog pins

Mode is the I/O mode the pin is set to

• INPUT, OUTPUT or INPUT_PULLPUT
• INPUT_PULLPUT acts as input with reversed polarity

Basically what that does, is it makes the pin act like an input, but it reverses the polarity. So, when they drive zero volts on it, when they drive it low, you read it as a high. Or when they drive a high, you read it as a low. So it's reverse polarity input.

3.1.2 Digital Input

int digitalRead (Pin)

• Returns the state of an input pin 
• Returns either LOW (0 volts) or HIGH (5 volts)

Example:
int pinval;
pinval = digitalRead(3);

• pinval is set to the state o digital pin 3.

Let's say we're talking about the digital pins, and we've assigned this pin to be an input. We used pinMode, we made pin zero and input. Now, we wanna, in our code, actually read from the pin, read a value from the pin. So, for that, use this function called digitalRead. And it takes an argument, one argument, the pin. And it just returns the state of the pin, which is either low or high. Now that note that it returns an int, so if you look at the example, I say I create an int pinval. I say pinval = digitalRead(3). So what that does, is it reads the state of pin 3, assumes it's either 0 volts or 5 volts and returns low if it's 0 volts, high if it's 5 volts. Now notice that pinval is an int, so you can interpret that as a 0 and a 1. 0 meaning low, 1 meaning high.

3.1.3 Digital Output

void digitalWrite(pin, value);

• Assigns the state of an output pin
• Assigns either LOW (0 volts) or HIGH (5 volts)

Example: 
digitalWrite (3,HIGH);

• Digital pin 3 is set HIGH (5 volts)

And then if you want to write to a pin, you call digitalWrite. That takes two arguments, the pin, and the value that you want to write to it. And the value has to be either low or high, which would be zero or one. Zero volts, five volts. So, it assigns state of the pin. So if you say digitalWrite like I do here, digitalWrite (3, HIGH) it's going to set pin 3 to high, which would be 5 volts.

3.1.4 Analog Input

int analogRead (Pin)

• Returns the state of an analog input pin 
• Returns integer from 0 to 1023
• 0 for 0 volts, 1023 for 5 volts

Example:
int pinval;
pinval = analogRead (A3);

• Pin must be an analog pin.

Now, for the analog pins, remember the analog, we can't drive analog output data. We can't take a pin and make it an analog output, but we can read analog input data of the analog pins in any rate. Now, what actually happens is that the analog value, remember our processor, it is a digital processor. So, it really needs to see digital values. But we want to be able to read analog values, so the microprocessor inside it, has an analog to digital converter. Which, when you call this function analogRead, it reads the analog value off of the pin and then converts it to digital using the analog to digital converter and then our code gets the result. So, what happens is, analogRead returns the state of the analog input pin, but it returns an integer from 0 to 1023. Now 0 is the bottom of the range, so if you've got 0 volts on this pin, it's going to return you a 0. And the top of the range is 5 volts. So if it sees 5 volts then it's going to return you a 1023.

3.2 Lecture3-2: Blink Example

in this set of slides, we'll talk about the blink example, a piece of code, a very simple piece of code that everybody needs to run on their Arduino to sanity check it, to make sure everything's working.

3.2.1 Delay

void delay(msec);

• Pauses the program for milliseconds (ms)
• Useful for human interaction

Example:
digitalWrite (3,HIGH);
delay (1000);
digitalWrite (3,LOW);

• Pin 3 is HIGH for 1 second

And this user interface has to be slowed down, right? It has to be at the speed that a human can perceive it, right? So, you can't just output some data and expect the human to read it in one clock cycle and then change it. If you want the human to see something, to hear something, it has to be present for a certain amount of time, or else the human can't even perceive it.

3.2.2 Blink Example

• Blink is the generic simple example for embedded systems: Like “hello, world”
• File > Examples > 01. Basics > Blink
• Causes an LED to blink
• LED is built-in, no wiring required

(1) the LED connect to pin 13

So what happens in the Arduino, this Blink example, it blinks and one of the LEDs built into the Arduino. And so there's an LED built into the Arduino uno that is connect to pin 13 and actually is right next to pin 13.

(2) Blink Sketch

// the setup function runs once when you press reset or power the board
void setup() {
  // initialize digital pin 13 as an output.
  pinMode(13, OUTPUT);
}

// the loop function runs over and over again forever
void loop() {
  digitalWrite(13, HIGH);   // turn the LED on (HIGH is the voltage level)
  delay(1000);              // wait for a second
  digitalWrite(13, LOW);    // turn the LED off by making the voltage LOW
  delay(1000);              // wait for a second
}

3.3 Video Demonstration: Arduino Blink Example