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## November 2003, Issue 96       Published by Linux Journal

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##### Copyright © 1996-2003 Specialized Systems Consultants, Inc.

 ...making Linux just a little more fun! Ecol By Javier Malonda

The Ecol comic strip is written for escomposlinux.org (ECOL), the web site tha t supports, es.comp.os.linux, the Spanish USENET newsgroup for Linux. The strips are drawn in Spanish and then translated to English by the author.

These images are scaled down to minimize horizontal scrolling. To see a panel in all its clarity, click on it.

All Ecol cartoons are at tira.escomposlinux.org (Spanish), comic.escomposlinux.org (English) and http://tira.puntbarra.com/ (Catalan). The Catalan version is translated by the people who run the site; only a few episodes are currently available.

These cartoons are copyright Javier Malonda. They may be copied, linked or distributed by any means. However, you may not distribute modifications. If you link to a cartoon, please notify Javier, who would appreciate hearing from you.

Copyright © 2003, Javier Malonda. Copying license http://www.linuxgazette.com/copying.html
Published in Issue 96 of Linux Gazette, November 2003

 ...making Linux just a little more fun! Linux Radio Station By Phil Hughes

In late September I wrote an article about radio station automation for the Linux Journal web site. You can find the article The comments I received indicates that there is interest. What I would like to do here is to see if there are some people who would actually be interested in working on such a project. I have establesed a palce on the LG Projects Wiki to further develop this work.

The comments the article received indicated that many of the pieces were in place. Most I knew about and this didn't surprise me. However, I want to build a solution rather than present a shopping list. That solution has to include various pieces of software, all playing together along with support. The pieces I see are:

• Audio conversion tools
• Audio editor
• Streamer
• Station automation
• Logging
• Transmitter
The first three items are pretty obvious and there are lots of choices out there. Logging is equally obvious. The other two items warrant further discussion once I get the basic concepts out.

## Customer Base

You can look at this potential customer base in three ways:

• Where you can get some money from
• What sort of fun code you can write
• What good political ends you can reach

The market is huge and very diverse. As a result of that diversity there are lots of choices of where you would like to fit into this project. For example, if you have a political ax to grind, helping your favorite political or religious cause get a radio voice or build a more efficient radio voice is likely your place. If, however, you just want to be in it for the money, there are tens of thousands of radio stations that could use your help and would pay for it.

Just looking at FM broadcast stations, there are 100 available frequencies on the band. In the US, these frequencies are allocated based on the signal strength of other stations on the same and adjacent frequencies. I don't have the numbers handy but that easily means thousands of stations in the US alone. Toss in AM broadcast and shortwave stations and you have the potential customer base for a product. If that isn't enough, Internet-only stations could use the same system.

Hopefully, if you are still reading, this is starting to make sense. I willdescribe the overall project idea and then talk about the specifics from above.

## An Overall Look

Let me present how a traditional radio station works. By traditional, I mean how stations have worked for many years. That will make it easier to see where automation makes sense.

A typical station is composed of one or more studios and a transmitter facility. A studio is nothing more than a soundproof room with some audio equipment. The transmitter may be in the same building or at a remote location connected by a dedicated wire or radio link. While there is some room for talking about the transmitter link, I want to concentrate on the studio end.

Generally, one studio will be live. That means whatever is happening in that studio will be sent directly to the transmitter. The alternative to a live studio would be the station either re-broadcasting some external feed or something that they have pre-recorded. In any case, whenever the transmitter is on the air, there must be some source of program material. Additional studios are available to build the pre-recorded program material.

Each studio will likely contain one or more live microphones, multiple sources for pre-recorded material (CD players, turntable and tape decks) and something to record a program (tape recorder or mini-disk are the most common). Also included is a audio mixer board and monitor system so multiple sources can be mixed and edited.

Small stations will typically have one studio with a person that queues and starts pre-recorded sources and makes live announcements that can include news and weather. For a typical music station, the majority of this person's time is spent waiting for the current song to end so they can queue the next one possibly inserting some commentary between songs. They may also be logging what they play so the station can pay the necessary royalties.

## Station Automation

This is the most obvious piece of the system--replacing the live tedium of queuing CD tracks with an automated system. The most basic step would be to just save all the tracks on a disk in a computer system and allow the person to pre-select what they would like played during a specified time block. This is relatively trivial to do. We can, however, do a lot more than this.

Most stations develop a play list that the live DJs need to follow. This list includes songs that can be played along with guidelines for how often they can be played. Armed with that list, a program could easily make all the necessary decisions to offer what appears to be a random selection of music within the necessary constraints.

Next comes the announcements. They can be divided into:

• Commercials
• Information that will be supplied live or almost live
• Information that could be automated

Automation of commercials is not much different than the music play lists. The primary difference is that there will likely be specific times when a commercial must be broadcast. Nothing magic here--just another type of event to put into a scheduling program.

The live or almost live program material is that which must be put together by a person. For example, a news broadcast. This could either be done live by a person in the studio (or remotely over an Internet connection) or it could be pre-recorded. If the news was pre-recorded as a set of items then later news broadcasts could re-use the appropriate portions of a previous broadcast. In any case, there is still nothing particularly difficult about this--it is just another event to schedule.

I separated out information that could be automated because it is an additional project. That is, it does not have to be part of this original package. Two things come to mind here:

• Time announcements
• Weather

Both of these announcements are really nothing more than building a human voice announcement out of some digital data. Both have been done--it just becomes a matter of integration.

Some stations allow call-in requests. This means a human responds to a request, checks to see if the song is available and hasn't been played more than is considered correct by the play list and then schedules it. This seems like a perfect place to use a web page. Listeners could place the requests and the software would appropriately adjust what was to be played. If the requested material was not available it could advise you of that fact or even order heavily requested material.

## Transmitter

While a traditional transmitter is far away from the scope of this project, a transmitter option deserves mention. In my searching for FM broadcast transmitters for a project in Nicaragua I ran across a company that offers an FM broadcast transmitter on a PCI card. They have a new card on the way and I have offered to write the Linux driver for it.

If you saw this project as just fun or something that would work for your college dorm, a card such as this might be just the ticket. You could offer a web interface to select program material and broadcast to nearby FM receivers. Being a radio guy as well as a computer guy, this interests me a lot.

## Overall Scope

That covers all the pieces from the geek end. But, as I said in the beginning, I want to present a solution rather than a shopping list. That 100mw station in your basement that your family can listen to is not going to be a good commercial customer but tens of thousands of commercial stations certainly could be.

Integrating all the software necessary to offer a solution is the first part. Installation and support comes next. A radio station that has full-time employees running the station and advertisers paying hundreds or thousands of dollars for a commercial will quickly see the benefits of a system such as this. The biggest hurdle will be showing them that the solution will be supported. That is, that their station will not be off the air because they made this choice.

For a small station, this might mean knowing that they could call someone and have them come in and fix a problem. For a larger station it could mean training of on-site personnel. There are other levels of support including spare systems, shared servers and so forth. In other words, an assortment of different markets where the same software is offered but the support potential varies.

## What next?

That's up to you. I am excited about the project. Unfortunately, I have a "day job" which does not give me the necessary time to put all this together and even if it was together, I don't want to go into the software support business.

My hope is that the right people are out there that want to do the pieces. That is my real reason for writing this article. I am happy to offer input, help set direction, offer a mailing list of discussion forum and even publicize the product. If you are interested in participating, write at phil@ssc.com and let me know your interests. Or go out to the LG Projects Wiki and chime in. Maybe Linux-controlled radio will be here before you know it.

Phil Hughes is the publisher of Linux Journal, and thereby Linux Gazette. He dreams of permanently tele-commuting from his home on the Pacific coast of the Olympic Peninsula. As an employer, he is "Vicious, Evil, Mean, & Nasty, but kind of mellow" as a boss should be.

Copyright © 2003, Phil Hughes. Copying license http://www.linuxgazette.com/copying.html
Published in Issue 96 of Linux Gazette, November 2003

 ...making Linux just a little more fun! Designing a Scientific Poster with Xfig and TEX By C.E.C. Artime, J.A. Baro

Designing a scientific poster

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# Designing a scientific poster with Xfig and TEX

A traditionally geeky user base has made the free software panorama plenty of scientifical tools. A key feature of scientific work is public defense of reseach methods and results in scientific meetings, and this is usually accomplished by means of oral presentations or posters. Both require the authors to develop some visual support: slide shows for oral presentations, with plenty of tools available (OpenOffice, LATEX,...), and a poster proper for the poster sessions. These take the form of an A0-sized paper sheet, and a visual exposition of the materials involved is crucial.

A general tool for manipulating images, as the GIMP, can be very useful when most of the poster is to be filled with photos and analogic images. Notwithstanding, scientific graphics are best handled with a vectorial graphics tool. This article will present a brief review of tools for incorporating graphics in scientific posters.

## Designing: Xfig

The GIMP is a bitmap- or raster-oriented tool, and as such, aimed at dealing with individual pixels. This approach is suitable for photos and artistic material. Technical drawings take a rather different approach: they are better described by their geometric elements, rather than by their constitutive pixels. This "vectorial" approach has the additional advantage that it produces resolution-independent graphics, thus allowing arbitrary zooming and enlargement.

Several free vectorial programs are currently available: Sodipodi, Karbon14, Sketch... Some integrate nicely with the Gnome and KDE desktop environments. Even the GIMP, since version 1.2, includes a plug-in enabling quite elaborate vectorial design, Gfig; it lies under the menu (Filters->Render). However, the classic among X Window applications is Xfig.

Xfig is a thoroughly proven program that makes use of the X Athena widget toolkit. Thus, it is a very mature product and, consequently, its stability is beyond doubt. This is a highly appreciated feature as work on posters frequently goes on until the very dead line.

A further consequence of being a mature project is that Xfig has been enriched by contributions of a wide community of users. This has produced a comprehensive set of galleries for various fields (electronics, UML, music, maps, etc.).

It is a reliable program as it stores its information in a readily accessible text format, from which information retrieval is quite straightforward. This kind of format does not follow the modern trend, that is, it is not a SGML-based system. Rather, it consists of the minimal amount of information for describing the graphic, stored as ASCII characters. A thorough description is available, e.g., at The FIG file format or, in a Debian system, in the file /usr/share/doc/xfig/FORMAT3.2.gz. Its regularity makes it suitable to be processed with the classical Unix/GNU text filters: sed, awk and such. So chances are that you prefer ASCII to the XML format. On the other hand, important applications as Gnuplot and GNU plotutils can produce FIG files; a comprehensive list is available in the file FIGAPPS, within the Xfig distribution.

Another nice feature of Xfig lies with its ability to export using LATEX friendly formats. This is true for both text and graphics. A “special flag” is assigned to each piece of text, signaling whether it is to be exported 'raw' or 'as TEX'. Graphics are coded according to the LATEX package options chosen by the user. More on this later, so keep reading or look up the LATEX.AND.XFIG and PDFLATEX.AND.XFIG files in the Xfig distribution

In order to be fair, it is time now to expose some not-so-desirable Xfig features. It did not age badly, it is rather that Xfig drags several drawbacks inherited from old techniques.

Users of modern programs will certainly get dissapointed by its single level of undo. So it pays to be a little cautious when designing.

As mentioned above, Xfig makes use of Athena widgets, and suffers from their limilations. For example, it is not easy to handle accented characters in dialog windows (as when using the Edit tool), a very frustrating feature for most European users. Note that it allows the use of the Meta key as a Compose key, thus permitting accented characters when using the Text tool on the canvas.

On the other hand, Xfig handles a lot of export formats. In fact, some of them are especially useful for LATEX users. However, its support for modern graphic formats, like XML-based SGV (Scalable Vector Graphics), is still experimental.

## Post-processing

### LATEX

LATEX is a set of macros built on the top of the TEX typesetting system. It is widely used to publish scientific papers, reports and books. Despite many users are unaware, slides are easily composed too. However, designing a full poster with LATEX alone is a daunting task, as many objects must be arranged in a 2D space with possibly complex interconnections among them. So the composition can be visually arranged with Xfig, and lately refined with a TEX editor.

To facilitate the integration of the visual work with the edition of TEX source code, Xfig users are empowered with a broad range of options. First, they have the choice of exporting a file as either a LATEX chunk or as a complete LATEX file. Second, a given graphical element is possibly translated to several TEX expressions, depending of which macros are available or are preferred by the user; this point is discussed in the next paragraph.

### fig2dev

The Xfig web site also hosts transfig, which includes a program to export from Fig format to several “devices”, fig2dev. This functionality is available from the GUI as well, under the Export option of the File menu. As we intend to finally automatize the full task as much as possible, emphasis is put on the stand-alone program.

Transformation of Fig documents into TEX elements can be accomplished in several ways. Each way differs in the number of graphical elements it can deal with (for example: rotated text, filled rectangles), at the cost of requiring more sophisticated LATEX packages: epic, eepic, eepicemu, pictex. The best results are obtained with combined PostScript (or PDF) and TeX: this assures that all elements seen in the Xfig screen will be present in the printable output. Two warnings: specially flagged text will be shown as raw TEX in Xfig, but nicely typeset in the final document; and some previewers, as Xdvi (distributed with teTEX), are not able to represent some kinds of elements, as rotated text, so better use dvips and a PostScript viewer.

### dvips

Processing a TEX file produces by default DVI format files, standing for DeVice Independent. This format is ready to be transformed into the set of displaying instructions. The most widespread format for final documents is PostScript, featuring a complete programming language understood by almost every high-quality printer.

The program dvips converts a DVI file into a PostScript, and is distributed with the main TEX packages, like tetex. It allows several interesting options, giving access to the powerful features of PostScript. One of them is the ability of setting the paper size, be it in a measure unit, as cm, or by a standard name, as A1. This feature is accessed by the -t and -T options.

### poster

The output of dvips is fine if one has access to an A0-size capable printer. This is often not the case. An acceptable alternative would be printing 16 A4 pages, or 8 A3 pages. The poster package tiles a one-page PostScript into small pieces, that can be pasted together to form the whole image. Guide lines indicating where to apply the guillotine cutter are printed as well. Poster is available at the KDE web site, and as a Debian package.

### psresize

Should we give away small copies of our poster, psresize is the tool of choice to build them. It is included in the psutils Debian package.

### bash

The entire process must be repeated if any change is made in the Xfig drawing. We suggest that you consider seriously the little extra work of gluing all the steps mentioned through a shell script. Here we present one developed for the most widespread shell, GNU bash. Calling the following script from within the directory containing the Fig file, the whole proccess is performed. Note that it appends a LATEX preamble and postamble, required by several LATEX features used by the author, and several other as required by Xfig exports.

(We just realised the inclusion of a simple script meant to perform the conversion from the Fig format to an EPS, TeXfig2eps, in the Xfig distribution. As our discussion shows, it may not be enough in many cases.)

#!/bin/bash
################################################
# Processes the FIG source file for the poster #
################################################

######################################################
#                                                    #
# Usage:                                             #
#         1. Design a poster with Xfig.              #
#         2. Save it as a normal FIG file, with      #
#            the name, e.g., myfile.fig'.     	     #
#         3. Substitute BASE=myfile' for the line   #
#            BASE=poster' in this script.	     #
#         4. Run this script within the directory    #
#            where myfile.fig' lies.	             #
#         5. The output files will be:		     #
#            myfile-a0.ps' - one poster-sized page  #
#            myfile-a4-1.ps' - a4-sized mini-poster #
#            myfile-a4-16.ps' - sixteen tile pages  #
#						     #
######################################################

# basename

BASE=poster

# convert the FIG into LaTeX + PostScript

echo '
\documentclass{article}
\usepackage{amsmath}          % symbols and equations
\usepackage{amsfonts}         % needed for blackboard characters
\usepackage[spanish]{babel}   % titles in our language
\usepackage[latin9]{inputenc} % accented characters
\usepackage[a0paper,margin=0cm,nohead,nofoot]{geometry} % margins
\usepackage{graphicx}         % including figures
\usepackage{color}            % import from Xfig
\usepackage{epic}             % complements the following
\usepackage{eepic}            % Xfig filling
\usepackage{rotating}         % text in figures
\usepackage{type1cm}          % arbitrarily sized fonts
\begin{document}
' > $BASE.tex fig2dev -L pstex$BASE.fig > $BASE.pstex echo -n ' \begin{picture}(0,0) \includegraphics{'$BASE'.pstex}
\end{picture}
' >> $BASE.tex fig2dev -L pstex_t$BASE.fig >> $BASE.tex echo '\end{document}' >>$BASE.tex

# typeset the source LaTeX

latex $BASE # transform into a PostScript of size almost DIN A0 dvips -o$BASE-a0.ps -T 84cm,118cm $BASE # build the small A4 version of the poster psresize -Pa0 -pa4$BASE-a0.ps $BASE-a4-1.ps # build the 16 A4 pages composing the poster dvips -o$BASE.eps -T 84cm,118cm -E $BASE poster -mA4 -pA0$BASE.eps > $BASE-a4-16.ps  Some of the commands used in the script need further explanation. The following paragraphs try to clarify these issues. The \usepackage{color} is suggested in /usr/share/doc/xfig/README.Debian in Debian systems. Latest versions of Xfig's pstex use the \RGB command for setting text's color. Other packages were included as needed, often with a trial and error procedure. The example does not make use of all them, but it can be illustrative as a comprehensive list of packages that we have ever needed for our own posters. Please note that psresize operates on .ps files, and poster on .eps files. Thus dvips is executed twice, without and with the -E option. ## Example In order to exemplify the use of this script, we first create the graphic layout with Xfig. By using the following command $ xfig -paper a0 -portrait

the canvas adopts the size and orientation of the poster we need. We design a sample poster shown here in a screenshot capture and here in Fig format. We save it with the name poster.fig. The basename poster is assigned to the BASE variable in the first lines of the script. After running the script within the same directory where poster.fig lies, the following output files are of interest:
• poster-a0.ps: contains the A0-sized postcript, ready to be printed in an A0-capable printer or plotter.
• poster-a4-1.ps: the poster scaled down to A4 size, to be printed and distributed among the poster session attendees.
• poster-a4-16.ps: a postscript file containing sixteen A4 pages that, put together, form the A0-sized poster.

## Very big fonts: a hack is needed

The standard tetex distribution uses by default bitmapped fonts, which requires discrete magsteps. This leads to warnings like

LaTeX Font Warning: Font shape OT1/cmr/m/n' in size <30> not available
(Font)              size <24.88> substituted on input line 5.

LaTeX Font Warning: Size substitutions with differences
(Font)              up to 5.12pt have occurred.

As stated in the tetex documentation, \usepackage{type1cm} allows the use of Computer Modern fonts at arbitrary type sizes. It must overwrite font definitions, so it may require to be included after other packages.

Note that transfig uses a fixed set of font sizes, handled through the macro

#define TEXFONTSIZE(S)        (texfontsizes[((S) <= MAXFONTSIZE) ? round(S)\
: (MAXFONTSIZE-1))])

that indirectly limits the maximum size of fonts to be of 41 points. For the example being shown, we had to edit transfig's sources (file texfonts.h) in order to change that macro to
#define TEXFONTSIZE(S)   (S)

The Debian way:
$su Password: # apt-get remove transfig Reading Package Lists... Done Building Dependency Tree... Done The following packages will be REMOVED: transfig 0 packages upgraded, 0 newly installed, 1 to remove and 3 not upgraded. Need to get 0B of archives. After unpacking 799kB will be freed. Do you want to continue? [Y/n] (Reading database ... 67797 files and directories currently installed.) Removing transfig ... # exit$ apt-get source transfig
Reading Package Lists... Done
Building Dependency Tree... Done
Need to get 340kB of source archives.
Get:1 ftp://ftp.es.debian.org testing/main transfig 1:3.2.4-rel-4 (dsc) [652B]
Get:2 ftp://ftp.es.debian.org testing/main transfig 1:3.2.4-rel-4 (tar) [326kB]
Get:3 ftp://ftp.es.debian.org testing/main transfig 1:3.2.4-rel-4 (diff) [13.0kB]
Fetched 340kB in 1m51s (3062B/s)
dpkg-source: extracting transfig in transfig-3.2.4-rel
$vi transfig-3.2.4-rel/fig2dev/dev/texfonts.h$ cd transfig-3.2.4-rel/
$dpkg-buildpackage -rfakeroot -uc -us ... [lots of output] ...$ cd ..
\$ su
Password:
# dpkg -i transfig_3.2.4-rel-4_i386.deb
Selecting previously deselected package transfig.
(Reading database ... 67734 files and directories currently installed.)
Unpacking transfig (from transfig_3.2.4-rel-4_i386.deb) ...
Setting up transfig (3.2.4-rel-4) ...
# exit


If you find this method cumbersome, you can have a look at this directory where a style file meant to produce big fonts for big pages (posters) can be found.

C.E.C. Artime is a GNU fan and a free software advocator since 2000.
J.A. Baro is a Linux user and a Perl hacker since 1996.

Copyright © 2003, C.E.C. Artime, J.A. Baro. Copying license http://www.linuxgazette.com/copying.html
Published in Issue 96 of Linux Gazette, November 2003

 ...making Linux just a little more fun! Servo motors, Linux/TRAI and other fun stuff By Pramode C.E

Servo motors, Linux/RTAI and other fun stuff

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## Introduction

In last month's article, I had explored the problems associated with writing timing sensitive code under Linux and looked at how RTAI solves them in an elegant manner. In this issue, I present an interesting application developed by a few of my students as part of a Computer Vision project. I would also like to share with you a few programs which I wrote in my effort to understand RTAI better:

• A simple program which measures pulse width of a PWM signal
• A frequency counting program which uses interrupts under RTAI
• A program which tries to demonstrate the effect of priority inversion in multithreaded code and the use of resource semaphores under RTAI (The reader is expected to have an understanding of basic semaphore operations to follow this part).

## The Project

In his article Creating a web page controlled 2-axis movable camera with RTLlinux/Pro, Cort Dougan presents the software and hardware involved in the construction of an interesting device which allows him to always keep an eye on his pet cat, Kepler. An ordinary web cam is controlled by two servo motors - one makes the cam move up and down and the other makes it sweep 180 degrees. The movement of both the servos is controlled by real time tasks.

Motivated by the article, a few of my students tried their hand at designing such a system. Here is what they came up with:

The top and bottom servo's should be clearly visible - the one at the bottom serves to rotate the platform resting on its axis (the platform on which the webcam is mounted). The whole thing is made out of transparent plastic. Here is a closer view of the bottom part:

### Controlling the servo

A hobby servo motor(like the Futaba S2003 used in this project) runs off 5V DC, has high torque for its size and can be positioned at points along a 180 degree arc (or a little bit more - the servo doesn't rotate the full 360 degrees, there is some kind of mechanical stop' built into it - which can of course be removed if you are seriously into servo hacking). The control wire (normally white colour) of the servo should be continuously fed with a digital signal whose period is about 20ms - the period need not be very accurate, and can be lesser than 20ms, but the on time should be precisely controlled, it is what decides where the servo will move to - in this case, it was seen that the servo moves for about 170 degrees for an on time from 2.2ms down to 0.5ms. Here is a picture of the control pulse:

Generating this control pulse with normal' Linux is a difficult issue - other activities going on in the system can seriously disturb the waveform - with the result that the servo will start reacting violently. So, hard real time RTAI tasks are used.

Two real time tasks are used to control the servos - one controlling the top servo and the other one, the bottom servo. The servo position is communicated to the real time tasks from user space with the help of two real time fifos. Let's first look at some macro/variable definitions (the actual rotation angles have not been accurately measured - so users of the Futaba S2003 need not be worried about any numerical discrepancy):


#define BOTTOM_SERVO_FIFO 0
#define TOP_SERVO_FIFO 1

#define FIFO_SIZE 1024

#define BOTTOM_SERVO 0
#define TOP_SERVO 1

#define BOTTOM_SERVO_PIN 2 /* Bit D1 of parport o/p register */
#define TOP_SERVO_PIN 4 /* Bit D2 of parport o/p register */

#define TICK_PERIOD 1000000 /* 1 ms */
#define STACK_SIZE 4096

#define MIN_ON_PERIOD 500000 /* .5 ms */
#define NSTEPS 35
#define STEP_PERIOD 50000 /* 50 micro seconds */

#define TOTAL_PERIOD 20000000 /* 20ms */

#define ONE_SHOT

RTIME on_time[2], off_time[2];
RT_TASK my_task1, my_task2;


The bottom servo is connected to pin 3 of the parallel port and the other one, to pin 4 - the macro's BOTTOM_SERVO_PIN and TOP_SERVO_PIN define the values to be written to the parallel port data register to set these pins (bit D0 of parallel port data register controls pin number 2, bit D1 pin number 3 and so on - the macro's are definitely poorly named and meant to confuse readers). The minimum on period of the control pulse is defined to be 500000 nano seconds (.5 ms). Because we get a servo rotation of 170 degree for pulse widths from 0.5ms to 2.2ms, a 50 micro second change in pulse width gives us 5 degree of rotation. We define the zeroth step' of the servo as the position to which it moves when it sees a pulse of 0.5ms duration and the thirty-fourth step' as the position it moves to when it sees a pulse whose on time is 0.5ms + (50 micro second * 34), ie, 2.2ms. So, in a total of 35 steps, each step about 5 degree, we get full 170 degree rotation. What the user program communicates with the real time tasks through the fifos is this step number.

The total period is defined to be 20ms. The zeroth element of the on_time array stores the current on time of the pulse which controls the bottom servo and the first element, that of the top servo. Similar is the case with the off_time array.

Let's now look at the code for the tasks which control the servo's:


static void servo_task(int pin)
{
while(1) {
outb(inb(0x378) | pin, 0x378);
rt_sleep(on_time[(pin >> 1) - 1]);
outb(inb(0x378) & ~pin, 0x378);
rt_sleep(off_time[(pin >> 1) - 1]);
}
}


The argument to the task is BOTTOM_SERVO_PIN or TOP_SERVO_PIN, depending on which servo it controls. The servo number can be obtained from these values by shifting them right once and subtracting one. First, the corresponding parallel port pin is made high and a sleep is executed - then, the pin is made low and the task goes to sleep once again.

Let's now look at the fifo handler code - user programs communicate with the real time tasks through two fifo's - one for each servo. The fifo handler code, which gets executed when either of them is written to from a user space program, is the same. User programs communicate a step number', ie, an integer in the range 0 to 34.


int fifo_handler(unsigned int fifo)
{
int n, r;
unsigned int on;
r = rtf_get(fifo, &n, sizeof(n));
rt_printk("fifo = %d, r = %u, n = %u\n", fifo, r, n);
if((n < 0) && (n > 34)) return -1;
on = MIN_ON_PERIOD + n*STEP_PERIOD;
on_time[fifo] = nano2count(on);
off_time[fifo] = nano2count(TOTAL_PERIOD - on);
return 0;
}


The code should be easy to understand, it simply reads a step number, converts it into a corresponding on time' and stores it into the proper slot in the on_time array. The argument to the handler is the number of the fifo to which data was written to from the user space program.

We now come to the module initialization part, the code should be easy to understand.


int init_module(void)
{
RTIME tick_period;
RTIME now;

rtf_create(BOTTOM_SERVO_FIFO, FIFO_SIZE);
rtf_create_handler(BOTTOM_SERVO_FIFO, fifo_handler);
rtf_create(TOP_SERVO_FIFO, FIFO_SIZE);
rtf_create_handler(TOP_SERVO_FIFO, fifo_handler);

#ifdef ONE_SHOT
rt_set_oneshot_mode();
#endif
rt_task_init(&my_task1, servo_task, BOTTOM_SERVO_PIN, STACK_SIZE, 0, 0, 0);
rt_task_init(&my_task2, servo_task, TOP_SERVO_PIN, STACK_SIZE, 0, 0, 0);
tick_period = start_rt_timer(nano2count(TICK_PERIOD));
on_time[BOTTOM_SERVO] = nano2count(MIN_ON_PERIOD);
on_time[TOP_SERVO] = on_time[BOTTOM_SERVO];
off_time[BOTTOM_SERVO] = nano2count(TOTAL_PERIOD - MIN_ON_PERIOD);
off_time[TOP_SERVO] = off_time[BOTTOM_SERVO];
now = rt_get_time() + tick_period;
rt_task_make_periodic(&my_task1, now, tick_period);
rt_task_make_periodic(&my_task2, now, tick_period);

return 0;
}



And here comes the module cleanup:


void cleanup_module(void)
{
stop_rt_timer();
rt_busy_sleep(10000000);
rtf_destroy(BOTTOM_SERVO_FIFO);
rtf_destroy(TOP_SERVO_FIFO);
rt_task_delete(&my_task1);
rt_task_delete(&my_task2);
}



The real time tasks were found to control the servo's perfectly. The system was heavily loaded by running multiple kernel compiles, copying and untarring large files and several other tricks - but the real time tasks were found to perform satisfactorily.

## Using Interrupts

It is easy to measure the frequency of a low-frequency square wave. Simply apply it to the parallel port interrupt pin, write an interrupt service routine and increment a count within the routine. This count may be transferred to a user space program through a real time fifo.

A real time application may not tolerate interrupts getting missed or the interrupt handling code getting executed a long time after the interrupt is asserted. With a normal' Linux kernel, this is a real problem. The Linux kernel may disable interrupts when it executes critical sections of code - during this time, the system remains unresponsive to external events. In an RTAI patched kernel, even if Linux asks for interrupts to be disabled, interrupts don't really get disabled; only thing is RTAI does not let the Linux kernel see the interrupt. A real time task will still be able to handle interrupts undisturbed.

Here is a small program which counts the number of interrupts coming on the parallel port interrupt input pin. I tested it out by using a function generator to generate a square wave at various low frequencies. A simple 555 timer based circuit should also do the job.


#include <linux/module.h>
#include <rtai.h>
#include <rtai_sched.h>
#include <rtai_fifos.h>
#include <asm/io.h>

#define FIFO_R 0
#define FIFO_W 1
#define TIMERTICKS 1000000000 /* 1 second */
#define STACK_SIZE 4096
#define FIFO_SIZE 1024

static int prev_total_count = 0;
static int new_total_count = 0;
static RT_TASK my_task;

static void fun(int t)
{
while(1) {
prev_total_count = new_total_count;
new_total_count = 0;
rt_task_wait_period();
}
}

int fifo_handler(unsigned int fifo)
{
char c;
rtf_get(FIFO_R, &c, sizeof(c));
rtf_put(FIFO_W, &prev_total_count, sizeof(prev_total_count));
return 0;
}

static void handler(void)
{
new_total_count++;
}

int init_module(void)
{
RTIME tick_period, now;

rt_set_periodic_mode();
rt_task_init(&my_task, fun, 0, STACK_SIZE, 0, 0, 0);
tick_period = start_rt_timer(nano2count(TIMERTICKS));
now = rt_get_time();
rt_task_make_periodic(&my_task, now + tick_period, tick_period);

rtf_create(FIFO_R, FIFO_SIZE);
rtf_create(FIFO_W, FIFO_SIZE);
rtf_create_handler(FIFO_R, fifo_handler);

rt_request_global_irq(7, handler);
rt_enable_irq(7);
outb(0x10, 0x37a);
return 0;
}

void cleanup_module(void)
{
stop_rt_timer();
rt_busy_sleep(10000000);
rt_task_delete(&my_task);
rt_disable_irq(7);
rt_free_global_irq(7);
rtf_destroy(FIFO_R);
rtf_destroy(FIFO_W);
}


The rt_request_global_irq and rt_enable_irq functions together instruct the RTAI kernel to service IRQ 7 (which is the parallel port interrupt). The interrupt handler simply increments a count. Every 1 second (the frequency of our input doesn't change fast, and we are only interested in observing a long cumulative count) a real time task wakes up and stores this count value to another variable (called prev_total_count) and also clears the counter. User programs can access prev_total_count through a FIFO.

A user program reads the count by writing a dummy value to FIFO_R and reading from FIFO_W. Writing to FIFO_R will result in fifo_handler getting executed, which will place the count onto FIFO_W; it can then be read by the user program. There should surely be a better way to do this - only trouble is, I don't know how. Here is the user program:


/* User space test program */

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <assert.h>

#define FIFO_W "/dev/rtf0"
#define FIFO_R "/dev/rtf1"

main()
{
int fd1, fd2, dat,r;

fd1 = open(FIFO_W, O_WRONLY);
fd2 = open(FIFO_R, O_RDONLY);
assert(fd1 > 2);
assert(fd2 > 2);

write(fd1, "a", 1);
r = read(fd2, &dat, sizeof(dat));
assert(r == 4);
printf("interrupt count = %d\n", dat);
}


## Measuring Pulse Width

Let's say we wish to measure the off-time of a pulse of total width 3ms with an accuracy of not more than 0.1ms. We start a periodic task with period 0.1 ms. At each awakening' of this periodic task, it checks whether the signal is low or high. The number of times the signal is low, out of a total of 30 samples, is recorded. Here is the code which implements this procedure:

#include <linux/module.h>
#include <rtai.h>
#include <rtai_sched.h>

#define LPT1_BASE 0x378
#define LPT1_STATUS 0x379
#define ACK 6

#define STACK_SIZE 4096
#define TIMERTICKS 100000 /*  0.1 milli second */
#define TOTAL_SAMPLES 30 /* Take 30 samples at 0.1 ms each */

static RT_TASK my_task;
static int old_off_samples, new_off_samples;

static void fun(int t)
{
static int count = 0;
while(1) {
new_off_samples = new_off_samples + ((inb(LPT1_STATUS) >> ACK) & 0x1);
if(++count == TOTAL_SAMPLES) {
count = 0;
old_off_samples = new_off_samples;
new_off_samples = 0;
}
rt_task_wait_period();
}
}

int init_module(void)
{
RTIME tick_period, now;

rt_set_periodic_mode();
rt_task_init(&my_task, fun, 0, STACK_SIZE, 0, 0, 0);
tick_period = start_rt_timer(nano2count(TIMERTICKS));
now = rt_get_time();
rt_task_make_periodic(&my_task, now + tick_period, tick_period);
return 0;
}

void cleanup_module(void)
{
stop_rt_timer();
rt_printk("old = %u, new = %u\n", old_off_samples, new_off_samples);
rt_busy_sleep(10000000);
rt_task_delete(&my_task);
}



The periodic task checks the D6th bit of the parallel port status register. It is 1 if the input signal on the 10th pin is low. The count could be, as usual, transferred to a user space program through a fifo.

## Semaphores and Priority inversion

Programmer's use semaphore's to synchronize the activity of multiple threads. RTAI has a function:

void rt_sem_init(SEM *sem, int value);

Which can be used to create and initialize semaphore objects. There is an interesting problem called priority inversion' associated with the use of locking mechanisms in an an environment which allows preemptive execution of tasks with varying priorities. It seems that the problem was brought to the attention of the real time design community by certain glitches encountered during the Mars Pathfinder Mission (a Google search would yield more information). I shall try to describe the problem first (the description is based on information obtained from the Net on the Pathfinder mission failure).

Suppose we have a very high priority task, Task-H which periodically accesses a buffer to write data to it (or read data from it). Another task, a very low priority and very infrequently running task, which always runs only for a short amount of time (let's call it Task-L), also might need to access the buffer to write to or read from it. Both these tasks would have to successfully grab a lock before one of them is able to access the buffer; the lock can be acquired by only one task at a time. Let's say Task-L grabs the lock and starts accessing the buffer. In between, suppose an interrupt causes the scheduling of the very high priority task, Task-H. Now, Task-H also would try to grab the lock, but blocks because it is currently held by Task-L. In the normal case, Task-L would finish very soon and release the lock, allowing Task-H to continue. But suppose a medium priority task, Task-M gets scheduled. Now, as long as Task-M does not finish, the OS will not allow Task-L to continue. Only if Task-L finishes doing whatever it has to do and releases the lock will Task-H be able to continue - the result is Task-H gets delayed by the time Task-M would take to complete. Suppose the system designer overlooks this and builds a watchdog timer which would time out and reboot the system if Task-H does not run for a short amount of time - the result would be system reboots whenever Task-M comes in between Task-H and Task-L.

Here is a small program which tries to demonstrate this problem.


#include <linux/module.h>
#include <rtai.h>
#include <rtai_sched.h>

#define LPT1_BASE 0x378
#define STACK_SIZE 4096
#define TIMERTICKS 100000000 /*  .1 second */

#define HIGH_PRIO 0
#define MEDIUM_PRIO 1
#define LOW_PRIO 2

#define PORTB 0x61
#define PIT_CTRL 0x43
#define PIT_DATA 0x42

static RT_TASK my_task1, my_task2, my_task3;
SEM flag;
RTIME tick_period;
unsigned char c = 0;

void speaker_on(void)
{
unsigned char c;
c = inb(PORTB)|0x3;
outb(c, PORTB);
}

void speaker_off(void)
{
unsigned char c;
c = inb(PORTB)&~0x3;
outb(c, PORTB);
}

void generate_tone(void)
{
/* Counter 2, low and high, mode 3, binary */
outb(0xb6, PIT_CTRL);
outb(152, PIT_DATA);
outb(10, PIT_DATA);
}

static void my_delay(unsigned int i)
{
while(i--);
}

/* Highest priority */
static void info_bus_task(int t)
{
speaker_on();
rt_sem_wait(&flag);
rt_printk("info bus task got mutex...\n");
rt_sem_signal(&flag);
speaker_off();

}

/* Medium Priority */
static void comm_task(int t)
{
my_delay(0xffffffff);
my_delay(0xffffffff);
my_delay(0xffffffff);
}

/* Low priority */
static void weather_task(int t)
{
rt_sem_wait(&flag);
rt_sleep(30*tick_period);
rt_sem_signal(&flag);
}

int init_module(void)
{
RTIME  now;

//rt_typed_sem_init(&flag, 1, RES_SEM);
rt_sem_init(&flag, 1);
rt_set_periodic_mode();
generate_tone();
rt_task_init(&my_task1, info_bus_task, 0, STACK_SIZE, HIGH_PRIO, 0, 0);
rt_task_init(&my_task2, comm_task, 0, STACK_SIZE, MEDIUM_PRIO, 0, 0);
rt_task_init(&my_task3, weather_task, 0, STACK_SIZE, LOW_PRIO, 0, 0);

tick_period = start_rt_timer(nano2count(TIMERTICKS));
now = rt_get_time();
rt_task_make_periodic(&my_task1, now + 2*tick_period, tick_period);
rt_task_make_periodic(&my_task2, now + 3*tick_period, tick_period);
rt_task_make_periodic(&my_task3, now + tick_period, tick_period);
return 0;
}

void cleanup_module(void)
{
stop_rt_timer();
rt_busy_sleep(10000000);
rt_sem_delete(&flag);

rt_task_delete(&my_task1);
rt_task_delete(&my_task2);
rt_task_delete(&my_task3);
}

The information bus, communication and weather tasks are respectively the high, medium and low priority tasks. RTAI lets us set priorities to tasks during rt_task_init. The weather task starts first. It grabs a semaphore (the semaphore is initialized to 1 in rt_sem_init) and then goes to sleep for 30 ticks (3 seconds, as each tick is 0.1 second). The information bus task runs at the next timer tick (the second argument to rt_task_make_periodic is the point of time at which the task is to be started); it turns on the PC speaker and then attempts to do a down' operation on the semaphore and in the process, gets blocked. We expect the weather task to complete its sleep, release the semaphore and let the information bus task run to completion, thereby stopping the noise coming out of the speaker. But the trouble is that before the weather task comes out of its sleep, a medium priority communication' task gets scheduled and starts executing a series of busy loops (which on my Athlon XP system generates a combined delay of about 17 seconds when compiled with -O2 - it would be better if you time the delay loop in a userland program before you plug it into the kernel - remember, as long as that delay loop is running, your machine will be in an unusable state - so be careful with what you do). The operating system can't bring the weather taks out of its sleep even after 3 seconds is over because the medium priority task is busy executing a loop - after about 17 seconds, the communication task would end, thereby letting the weather task continue with its execution. The weather task perform an up' operation on the semaphore bringing the information bus task out of the block and letting it stop the speaker. We note that the high priority information bus task is getting delayed by the communication task.

RTAI supports resource semaphores' - they can be used to solve the above problem. Had we initialized our semaphore like this:

rt_typed_sem_init(&flag, 1, RES_SEM);

the task which originally acquired' the semaphore (the weather task) would have `inherited' the priority of the high priority information bus task blocked on it. This would have resulted in the RTAI scheduler preempting the communication task and giving control back to the weather task exactly after 3 seconds of sleep.

## Acknowledgements

The webcam-servo motor setup was implemented by Krishna Prasad, Mahesh and friends as part of a Computer Vision project; they wish to acknowledge the influence of Cort Dougan's implementation of the idea on RTLinux. RTAI comes with good documentation, and lots of example code. I would like to thank all those people who took the pains not only to build a great system, but also document it well.

## About the Author

I have been teaching GNU/Linux and elementary Computer Science since 1997. If you are sure that you really wish to waste your time, you might drop in on my home page pramode2.tripod.com

I am an instructor working for IC Software in Kerala, India. I would have loved becoming an organic chemist, but I do the second best thing possible, which is play with Linux and teach programming!

Copyright © 2003, Pramode C.E. Copying license http://www.linuxgazette.com/copying.html
Published in Issue 96 of Linux Gazette, November 2003