Our use of git

We do not want to develop the code but just use it.

In this case we decide, where in our file system tree we want to

install the source code of the library and then we download it with

git clone git://git.drogon.net/wiringPi

This will download the source code which we will have to compile

which is done with ./build. This is a shell script!

Have a look to see if you can understand what it does.

In order to keep the library up to date we go to the wiringPi

directory just created and we type:

git pull

This will update the source code to the latest version

Here is the installation manual of wiringPi.

A bit more on bash scripts

As explained before bash provides it own programming language with

• Assignments
• Conditional statements
• Loops

A bash script to setup the Pis

During the weekend I wanted to install the latest version of the

wiringPi library from git on all 15 Raspberry Pis in the lab.

I needed to

• Create a file system structure with
  /opt/ucc/micros/raspberry into which I put the wiringPi sources
• Change its owner to root:ucc such that members of the
  ucc group have access to the files
• Change the permissions to allow members of the
  ucc group to also write into these directories

A bash script to setup the Pis (2)

• Download the wiringPi source code into the /opt/ucc/micros/raspberry directory
• Modify a Makefile in the wiringPi subdirectory
• Compile the code and install using the build bash script supplied with wiringPi
• Compile all the examples
• Create the examples/solutions directory on /opt/ucc
  owned by uccstaff:uccstaff and only readable by this used
• Transfer the solutions of the exercise associated to this lecture
• Try out everything

... a lot of work

This is a lot of typing!

The solution: a shell script!

The shell script can only be executed by root because only the
root user can create directories in /opt

How to check if we are root?

A conditional statement in the script!

The rest of the script

Loops in bash

You can write

• for loops
• while loops
• until loops

Unfortunately I have not enough time to explain the full syntax of bash scripts.

You should have a look in the WEB for more information.

Often bash scripts are used for installations and you should be able

to understand what these scripts are doing before executing them.

Example of an endless loop in bash

You can also read command line arguments just like you do it in C:

It has many options. Please have a look at its man page!

One option allows to read and write GPIO pins:

gpio write $PIN $ON_OFF

If PIN is 0 and ON_OFF is 1 then the LED connected to GPIO pin 0 will go on.

A bit more on C programming

We have just seen the very basics of C programming and we need to learn

a bit more about the libraries we can use

Here we will look at

• String handling functions
• Some bit handling function
• Exiting a program gracefully

String handling functions

There is a series of functions to

• Compare strings strcmp and strncmp
• Copy strings strcpy and strncpy
• Locate a sub-string strstr

Treating command line arguments

Consider a wave generator where the user can choose the

type of waveform using command line arguments.

His options are:

• sine
• rectangular
• triangular
• sawtooth

First check it the number of arguments is correct

The test and assignment of the wave type

Bit handling functions

We need to learn a few bit handling functions before being able to prepare the data for the DAC:
| : bitwise or
& : bitwise and
data += 5 <=> data = data + 5
data |= 5 <=> data = data | 5
~data: invert all bits in data
data >> 4 all bits in data are shifted right by 4
data << 4 all bits in data are shifted left by 4

---++

Writing data to a DAC

Imagine a 12 bit digital to analogue converter with the following register outline:

The 12 bit data must be written 4 bits at a time to registers

which are selected by the RS bits.

The lowest significant digit goes to register 0
(both RS bits zero)

The medium significant digit to register 1
(RS=01)

And the highest significant digit to register 2
(RS=01)

To strobe the data into the register the strobe line must see a low to high transition

The DAC

We have a 12 bit DAC, which creates signal levels from 0V to Vcc

What is the max. number this DAC can take in decimal and in hex?

What is the signal resolution in ‰

This is Physics!

Preparing data for the DAC

Remember the bit layout of the DAC register?

We must write the

• lowest significant,
• medium significant and
• highest significant bits

We must “or” in the register select bits and read/write bits

(write is usually active low)

We must strobe in the data (a pulse on the strobe bit)

DAC preparation

Write a function which prepares a byte for the DAC

The function takes 3 arguments:

• The data nibble (4 bits of data)
• The register selection (0,1,2)
• The read/write bit

Solution: DAC preparation

Now the test program generating data for the DAC

Strobe

Now create a strobe function which takes DAC data (including the register bits)

and generates a strobe signal on the

STR line without touching the other bits

With a correctly prepared data and register byte this will

write the data to the correct register within the DAC

Solution: Strobe

Exiting a program gracefully

When we have a program like our blink program,

then the only way to exit the endless loop is a ^C

which will leave the LED in an unknown state.

Is there a way to exit the program gracefully and to

make sure the LED is always off once the program exits?

Yes, there is!

Killing the program

When we kill the program with ^C, a signal (SIGINT) is sent

to it which normally results in a brutal stop of the program.

It is however possible to capture the signal and

do some cleanup before the program finally exits.

Signal handler

Catching signals