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Raspberry Pi II

Network

Connecting to a network

  • wired connection using the built-in RJ45 connector
  • Wi-Fi connection using a USB Wi-Fi adapter
    • start Wifi Config from the desktop

Using a Web Browse

  • Epiphany browse is builti-in

SSH

  • Should change the private host ID keys
    • Otherwise , they have the same default values.

One you install raspbian on your Pi, you get these default keys ,which are the same as everybody else's default keys.

sudo rm /etc/ssh/ssh_host_* && sudo dpkg-reconfigure openssh-server

Internet Protocal Family

  • IP (Internet protocal)
    • Host naming scheme , define IP address
    • Host-to-host connection, unreliable
  • UDP (Unreliable Datagram Protocal)
    • Process-to-process communication
    • Process naming
    • Also unreliable
  • TCP (Transmission Control Protocal)
    • Process-to-process communication and naming
    • Process naming
    • Reliable communication

Domain Names

IP addresses are not easy for humans to memorize, so we use domain names. eg. cc.com

Each domain name must be resolved to an IP address to send packets.

So ,for that we use what's called domain naming system, DNS.

  • DNS is a hierarchical naming system used to determine IP addresses from domain names
  • Gigantic, distributed tables are accessed by performing a DNS Lookup

When you visit cnn.com, your web browser sends a message to a DNS server and says and look, what is the address of cnn.com? And if DNS server knows that address, it sends the IP address back to you. If it doesn't , then it goes and asks another DNS server higher up. The ip address will store in a local cache to save time.

nslookup

nslookup baidu.com
Server:        192.168.1.1
Address:    192.168.1.1#53

Non-authoritative answer:
Name:    baidu.com
Address: 180.149.132.47
Name:    baidu.com
Address: 220.181.57.217
Name:    baidu.com
Address: 111.13.101.208
Name:    baidu.com
Address: 123.125.114.144
  • #53 is the port

Internect Connections

  1. A socket is an endpoint of a connection
  2. client and server both have sockets
  3. A port is a 16-bit integer that identifies a process

Sockets on the Client

Creating a generic network client:

  1. Create a socket
  2. Connect socket to server
  3. Send some data ( a request )
  4. Receive some data ( a response )
  5. Close the socket

Creating a Socket

import socket
mysock = socket.socket( socket.AF_INET , socket.SOCK_STREAM )
  • AF_INET declares Address Family Internet
  • SOCK_STREAM idicates TCP (connection-based)

Connect Socket to Server

>>> host = socket.gethostbyname( 'www.baidu.com' ) #不解析也可以
>>> PORT = 80
>>> mysock.connect( host, PORT )  # mysock.connect( (host, PORT) ) for 2.7

Sending Data on a Socket

message = "GET / HTTP/1.1\r\n\r\n"
mysock.sendall(message)
  • sendall() send the data , and tries until it succeeds.

Receiving Data on a Socket

data = mysock.recv(1024)
  • recv() returns the data on the socket
    • Blocking wait, by default , you can change that
  • python will resize the data buffer for you1

Closing a Socket

mysock.close()  # free up port

Exceptions

  • Errors may happen when working with sockets
    • Socket cannot be opened
    • Data cannot be sent

Socket Exceptions

  • socket.error
    • All socket errors
  • gaierror
    • getaddressinfo() - no host found

Sockets on the Server

  1. Create a socket
  2. Bind the socket to an IP address and port
  3. Listen for a connection
  4. Accept the connection
  5. Receive the request
  6. Send the response

Creating and Binding a Socket

mysock = socket.socket( socket.AF_INET , socket.SOCK_STREAM )
mysock.bind( "",1234 ) # mysock.bind( ("",1234) ) 2.7
  • bind() : the 1st argument it the host that you want to sssociate it with
    • "" like a wildcard , it allows it to receive from any host

Listening and Accepting a Connect

mysock.listen(5)
conn , addr = mysock.accept()
  • listen() starts listening for a connect()
    • argument is called backlog , it's the number of requests allowed to wait for service.
  • accept() accepts a connection request
    • return a connection (for sending/receiving) and an address (IP, port)

Sending and Receiving

data = conn.recv(1000)
conn.sendall(data)

conn.close()
mysock.close()

A Live Server

while True:
    conn, addr = mysock.accept()
    data = conn.recv(1000)
    if not data:
        break
    conn.sendall(data)
    
conn.close()
mysock.close()

Camera

Enabling the Camera

  • Use raspi-config to enable the CSI interface
    • sudo raspi-config
  • Select Enable Camera , done

python-picamera Library

  • install the library

    sudo apt-get update
sudo apt-get install python3-picamera

  • Using the library

    import picamera
camera = picamera.PiCamera()

Camera Functions

  • Take a picture
    • camera.capture("pict.jpg")
  • Changing camera settings
    • There are many
    • camera.hflip = True
    • camera.vflip = True
    • camera.brightness = 50
    • camera.sharpness = 0
  • View video on RPi screen
    • camera.start_preview()
    • camera.stop_preview()

  • Video Recording

    camera.start_recording( "vid.h264" )
time.sleep(10)
camera.stop_recording()

Capturing Image

Capture an image, send it on a connection.

mysocket = socket.socket()
mysocket.connect( ( 'aserver' , 8000 ) )
conn = mysocket.makefile( 'wb' )
camera.capture( conn, 'jpeg' )
  • Need a file-like object to capture to

Timelapse Photography

  • camera.capture_continuous()
    • takes photos over time
  • Takes one argument , a file name
  • {counter} and {timestamp} can be substituted
  • ex. "picture{counter}.jpg" produces picture1.jpg, picture2.jpg, etc.
camera = picamera.PiCamera()
for filename in
    camera.capture_continuous('img{counter}.jpg'):
    time.sleep(300) # 不知道什么用意
  • Iterate through all images, infinite loop
  • 5 minute delya between images