Lecture 1.3 - Ethernet Library

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來自 University of California, Irvine 的課程

Arduino 接口

1479 個評分

University of California, Irvine

Arduino 接口

1479 個評分

課程 3（共 6 門，Specialization 物联网（IoT）程序设计入门）

Arduino senses the environment by receiving inputs from add-on devices such as sensors, and can control the world around it by adjusting lights, motors, and other actuators. In this class you will learn how and when to use the different types of sensors and how to connect them to the Arduino. Since the external world uses continuous or analog signals and the hardware is digital you will learn how these signals are converted back-and-forth and how this must be considered as you program your device. You'll also learn about the use of Arduino-specific shields and the shields software libraries to interface with the real world. Please note that this course does not include discussion forums.

從本節課中

Module 4

In this module we’ll examine Arduino shields and how they are used to extend the capabilities of an Arduino-based system. Shields are printed circuit boards that are stacked on top of the Arduino together with libraries that enable the new hardware to be used through an Arduino sketch. We’ll describe the benefits of shields and their basic construction as well as the use of libraries in interfacing with each shield. This module also describes how to connect your IoT device to the Internet using shields. We focus on an Ethernet shield in order to establish a wired network connection, and a WiFi shield to establish a wireless connection.

Lecture 1.1 - Arduino Shields6:03

Lecture 1.2 - Ethernet Shield7:51

Lecture 1.3 - Ethernet Library6:42

與講師見面

Ian Harris
Professor
Department of Computer Science

[MUSIC]

So now, we're gonna start talking about the Ethernet library.

The library that's used to control an Ethernet controller, an Ethernet shield.

So we'll start with #include.

So at the top, like with all these libraries,

you need to set a #include to include a bunch of header files, .h files.

There are a lot.

So for instance, Ethernet.h, EthernetClient.h, EthernetServer.h and

there are several more.

The easy way to do this in your sketch is just to go to the ITE,

go to Sketch > import library > Ethernet and

it'll just stick in all the #includes that you need for the Ethernet library.

So let's assume you do that, then there's several functions provided

to initialize the Ethernet interface to initialize the shield.

So these are the first things that you're gonna call when you wanna use the shield.

So Ethernet.begin, that's really the function, which is common name for

the first function in any library, but Ethernet.being, you call that function.

Now there are five possible arguments for Ethernet.begin.

Only the first one is required and the rest are optional.

We'll go through all them and we won't actually use all them.

Well, the first one we have to give the rest we probably won't, but

let's go through all of them.

So first argument that is required is the MAC address.

As the MAC address of your Ethernet shield, which you can make up, by the way.

And most fields you can just create which ever one you want to create and

you can use that.

[COUGH] Or if it has a sticker on there that indicates its MAC Address,

you can us that too.

IP Address, that's the IP address of your shield, optional.

DNS, so that's the IP Address of the domain name server that you wanna

connect to by default and you don't have to give that.

A gateway is the address of a router,

which knows how to route packets to other networks.

So if you're going outside of our local network, which we won't in this class.

But if you are, then you might want to specify a default gateway.

A default router to go to when it wants to send packets to an outside network,

it can go ask this gateway router for the path to say, how do I get this there?

Now if you don't give a gateway, it will find a gateway.

But sometimes a particular service will have it's own gateway to speed thing sup,

but you don't have to do that.

Also the subnet mask.

Subnet mask tells us which other IP addresses are in the local subnet.

So this is sort of a flexible idea, but

a subnet you can think of as your local network.

And generally, what happens with the IP addresses.

The prefix, so it's a 4-byte number, 4 numbers separated by decimals.

And the higher bytes, the most significant bytes are often held constant for

all the machines in a local subnet.

So in a local subnet maybe it's 128, 198,

200 are the first 3 numbers and then the last number is a 1 from my machine,

a 2 for the next machine, a 3 for the next machine and so on, but they share.

In that case, the local subnet shares the high 3 bytes.

So the subnet mask tells you which bits,

specifically are shared by the local subnet.

You don't have to specify this.

If you don't, it'll default to 255.255.255.0.

So what that means is remember 255 in binary is all 1s.

So all 1s, all 1s, all 1s and then all 0s.

So that's saying that the local subnet is just the last byte.

So, all the nodes in a local subnet share the same high 3 bytes.

That's what it says and that'll be the default if you don't set it, but

you don't have to.

Dynamic Host Connection Protocol.

So this is pretty important or DHCP.

So in order to communicate on the internet, you need an IP address.

Every node on the internet needs an IP address.

Now note that in our Ethernet type again, we did not have to specify and IP address.

So how is it gonna communicate on the Internet without an IP address?

Well, it has to get one.

It can get one dynamically.

So DHCP, Dynamic Host Connection Protocol is a protocol that a allows a node,

a new node on the network to request a new IP address.

Now this is generally enabled on routers by default.

So right here, right now my laptop is sitting over there and I open it up.

And when I opened it up, it connected to the local network at school and

it uses DHCP and the local network is setup for DHCP, that's turned on.

And my machine basically in order to communicate, it's first thing it does is

it asks through using this DHCP protocol, it asks for an IP address.

So, it gets it's IP address dynamically from the network and

then the network finds some available IP address and tells my machine.

So here's your IP address and

that's what we're assuming is being used that's very common.

So let's just assume that's whats being used and we don't have to specify

an IP address, we can just assume DHCP is being used to give us an IP address.

Now DHCP is invoked if you call Ethernet.begin ad it has no

IP address argument.

So if you just give it, if you call with a MAC address argument and nothing else,

no IP Address doesn't know anything, then it automatically invokes the DHCP.

Making a request, you don't have to deal with these details, but your machine will.

Your Ethernet shield will make a request to the local network to ask for

an IP address.

Well, after you call up again.

Most routers are configured for DHCP.

Now another way to do this, instead of allowing DHCP,

which gives dynamic IP addresses is that you can also use static IP addresses.

Static IP addresses are fixed IP addresses.

So that means every time you power your machine and connect to the network,

it gets exactly the same IP address.

Now a static IP address, that's more difficult to manage.

Basically, that IP address needs to be known across the world.

It has to be known to any machine that wants to connect to your machine,

it needs to know that static IP address.

It's useful for servers.

It's typical for servers, because like say,

www.uci.edu, there's a web server right there, which I can go to.

And in order for my machine, my laptop to connect to that server to look at

the school's web page, I need to be able to look up its IP address.

So it needs to be static, it needs to sit in the same place and

be in a table somewhere.

So when I request, it's address I can get it.

So servers often have static addresses, but dynamic addresses are very common for

non servers.

Thank You.

[MUSIC]

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