Lecture7:AccessingTheRealWorld < Embedded

Accessing the Real World

Lecture 7

Uli Raich

UCC semester 2017/2018

Slide 1 of 12

Access libraries

As we have already seen, the Raspberry Pi flat cable connector

and the cobbler + bread board, give access to external hardware though

gpio
I2C bus
SPI
serial port interface

Dedicated interfaces to camera and display

Slide 2 of 12

Software access

Of course you can access the external hardware through

the BCM-2837 interfaces and their registers directly, however,

this is not for the faint-hearted (read the 200 page manual first!)

The easier way to access these devices are ready made

libraries giving you a simpler API for access

To my knowledge there are 2 such libraries around

(at least these are the most popular ones):

Slide 3 of 12

How to download and install

Both libraries can be downloaded as git source archives,

which allows you to have the very latest version and

to keep your version up to date.

git is a revision control system allowing many developers

to work on the same project. You check out the current version,

work on it and you can upload to the git server

the modifications you made.

You can also create a new code branch where you implement

new functionality which may be specific to what you want

to use the code for, or it may be a try to implement new options

which may later be discarded or, in case everybody is interested

in this new functionality it may be merged back into the main branch.

Slide 4 of 12

Getting the LED blink program to work

The wiringPi library has included a few example programs to show its use.

The most simple one is a program making a LED blink.

WiringPi has its own numbering system for the GPIO pins:

Slide 5 of 12

Creating blink.c

As you would expect, wiringPi has its own include file

which you must use in order to access the library:

#include <wiringPi.h>

On our systems we have installed this include file is in /usr/include

on the Pi file system while the library itself is in /usr/lib

As these are the standard positions for include files and libraries on a

Linux system all we have to do in the Makefile is to add

-lwiringPi to the LDLIBS macro.

Slide 6 of 12

The C code of blink.c

Slide 7 of 12

The Makefile to build blink

Slide 8 of 12

Ohm's law

If you consider that the LED has no resistance and the Raspberry Pi

drives the GPIO pins with 3.3V and you connect as shown in the circuit diagram,

then what is the current flowing through the LED?

Slide 9 of 12

!WiringPi functions

The library has functions to

Setup and initialize the library
This will open the needed device drivers and give access to them
Core functions: Functions to
- set gpio pins to input or output and
- to read and write from /to them
- define pull-ups/pull-downs
Some Pi specific functions liking getting the version no
Timing functions (e.g. delays)

Slide 10 of 12

!WiringPi functions (2)

Priority, interrupts, threads
I2C bus functions
Serial line functions
SPI functions
Miscellaneous function

Slide 11 of 12

The gpio command

In addition to the library functions a new command:
gpio
is provided. This can be used to set gpio pins from bash
It has however many more options. Look them up in its man page.

Slide 12 of 12

-- Uli Raich - 2017-09-27

Comments

I	Attachment	History	Action	Size	Date	Who
png	blink.png	r1	manage	48.9 K	2017-09-27 - 14:26	UnknownUser
png	blinkMakefile.png	r1	manage	6.6 K	2017-09-27 - 16:36	UnknownUser
png	gpioNumbering.png	r1	manage	157.9 K	2017-09-27 - 14:26	UnknownUser
png	ledschematics.png	r1	manage	7.2 K	2017-09-27 - 16:48	UnknownUser
png	piRemoteDesktop.png	r1	manage	378.8 K	2017-09-27 - 14:53	UnknownUser
png	reminna.png	r1	manage	3.9 K	2017-09-27 - 14:53	UnknownUser
png	ssh-X.png	r1	manage	293.5 K	2017-09-27 - 15:07	UnknownUser

This topic: Embedded_Systems > WebHome > LectureSlides > Lecture7:AccessingTheRealWorld
Topic revision: r4 - 2017-10-16 - uli