Tag Archives: programming

Best book to learn C?

If you could recommend one book to a total beginner who wants to learn how to program in C, what would it be?

It shouldn’t be microcontroller-specific (more general is better), but it does need to make sense to someone with zero programming experience (in any language).

(Myself, I started with K&R, although I had some experience with BASIC (on the C64 and Apple II), Logo, and shell programming before that.)

Building a Wifi Radio – Part 8, Adding a Tuning Control

This is the eighth part of an ongoing series about building a low cost, open source streaming internet radio based on the ASUS WL-520gU Wireless Router.  If you haven’t already, check out the previous parts (see the links at the end of this article) for some background about the project.

In part seven, we added an LCD status display for the radio that shows the stream name as well as the artist and title of the current track.  In this part, we’ll add a tuning knob that lets us change stations without using a computer.

It turns out that this is mostly a software exercise, made simple by taking advantage of the analog to digital converter function of the Atmel ATmega168 AVR that is controlling the LCD display.  The addition of the tuner control turns the display circuit into a very simple user interface.  Turn the knob and the station changes.  The position of the knob determines what station the radio is “tuned” to, and when combined with a calibrated scale it will make it easy to change to any one of the several streaming radio stations stored as presets (favorites?) in the router.

To give you an idea of how this works, here is a demo of the tuner control changing between ten preset stations I have set on the router.  The tuner control is in the upper right hand corner of the breadboard.  As I adjust the control, the music changes and LCD display updates to show the name of each new station.

Is that cool or what?

If you are interested in adding this functionality to the radio, keep reading and I’ll show you how.

Changes to the hardware:

You will need:

  • The completed AVR-based LCD display from part seven
  • A 1k-10k trimmer or potentiometer, linear taper
  • Some hookup wire


Here is an updated schematic of the AVR circuit showing the potentiometer connected to ADC4 (pin 27).

Wifi Radio User Interface Schematic
click to enlarge


The AVR firmware has been significantly expanded, slightly reworked and cleaned up in some areas.

The most important changes are:

  • The addition of a serial transmit function so the AVR can talk to the router (based on the uart_putchar function)
  • New code supporting the analog to digital converter (ADC) which reads the value of a potentiometer connected to ADC4.
  • A new Timer1 overflow interrupt has been added, which occurs roughly every 0.5 seconds.  The interrupt service routine (ISR) checks the position of the tuner control, and if it has changed, sends the value to the router.  The ISR is towards the top of the file, see the SIGNAL (TIMER1_OVF_vect) section.

The ADC range of the ATmega is 0 – 1024 for an input voltage from 0 to 5V.  The AVR sends serial data in the format “Tuner: Value” back to the router when the tuner position changes by more than ADC_SENS counts (default is 5).  The AVR waits for an “AVR Start!” command from the router before sending any data, this avoids filling up the serial receive buffer on the router before it’s ready to start processing data.  An important consequence of this is that the AVR must be reset before running the control script on the router.

You can download the source code and compiled .hex file here.  Flash it to the AVR using any compatible ISP programmer and you should be good to go.  The source is commented fairly well so if you’re interested in learning how the interface works, take a look.  You will need a copy of the ATmega168 datasheet to understand the register names and other architecture-specific parts of the code.  Feel free to post in the comments with any questions.

Modifying the circuit:

This part is pretty simple – just wire the potentiometer as shown in the schematic.  Most potentiometers have three terminals.  The left terminal goes to ground, the right one to +5V, and the middle terminal to ADC4 on the AVR (pin 27).

Tuner control on breadboard

Tweaks to the OpenWrt configuration:

To make bidirectional communication with the AVR work, we have to change a couple config files on the router and disable some services that would otherwise get in the way.


The first change is to modify the /etc/config/network file so that we can always telnet or ssh into the router on a LAN port using the IP  The ability to access the router via ethernet is helpful in case we screw something up and lose the wireless connection or the router loses it’s IP address, etc.

Modify the LAN section of /etc/config/network to look like this (changes in bold):

#### LAN configuration
config interface lan
#option type     bridge
option ifname    "eth0.0"
option proto    static
option ipaddr
option netmask

Save changes, restart the router, and connect an ethernet crossover cable (straight cable might work on some computers) to the router.  Configure your desktop/laptop computer with a static IP, like  Try to open a telnet connection (or ssh if you have set a password on the router) and see if you can log in.  If not, don’t continue with the next steps until you can get this working.


We previously used the router’s serial port to get a login shell.  Now that we’re trying to receive data from the AVR on the same serial port, we need to disable the login shell or it will capture the data before we can get to it.

Edit /etc/inittab to look like this (changes in bold):

::sysinit:/etc/init.d/rcS S boot
::shutdown:/etc/init.d/rcS K stop
#tts/0::askfirst:/bin/ash --login
#ttyS0::askfirst:/bin/ash --login
tty1::askfirst:/bin/ash --login


Sysrq is a fascinating and very low level debugging feature of the Linux kernel.  It can be used to perform troubleshooting operations and reboot the system.  Usually it is invoked with a magic key combination on a desktop computer, but in this case I found that it is easy to accidentally trip over the serial port when using an AVR.  (The “break” RS-232 code triggers Sysrq, this probably has something to do with it.)

Fortunately, it’s easy to disable by editing the /etc/sysctl.conf file and adding these lines:

# Disables the magic SysRq key
kernel.sysrq = 0

Reboot the router to apply the changes.  Now we can get on with the good stuff!

Shell scripting magic:

The real action happens on the router, where a shell script waits for input from the router and changes the station accordingly.

This script is called interface.sh and can be downloaded to the router using wget as shown:

root@OpenWrt:~# cd ~
root@OpenWrt:~# wget http://mightyohm.com/files/wifiradio/interface.sh
root@OpenWrt:~# chmod ugo+x interface.sh

The interface script calls an updated version of the display script from part 7, called display2.sh:

root@OpenWrt:~# wget http://mightyohm.com/files/wifiradio/display2.sh
root@OpenWrt:~# chmod ugo+x display2.sh

Once both scripts are downloaded, executable and located in /root you can launch interface.sh as follows:

root@OpenWrt:~# ./interface.sh
volume: 60%   repeat: on    random: off
volume: 60%   repeat: on    random: off
adding: http://relay3.slayradio.org:8000/
adding: http://scfire-dtc-aa01.stream.aol.com:80/stream/1046

… more stations here …

Tuner Position:  0
New station...
[playing] #1/10   0:00/0:00 (100%)
volume: 60%   repeat: on    random: off

The interface script adds ten presets to the router, shows the playlist, and then waits for valid tuner data from the AVR.  Once it receives a “Tuner: value” line (which should occur shortly after the AVR receives a go signal from the script), the script prints the received tuner positon and changes to the requested station.  It will then wait for new tuner data from the AVR and change the station when necessary.

As you can see in the video, this works very well.  Over a fast Wi-Fi connection, the time to change stations is almost instantaneous – very satisfying!

That’s it for part eight.  In part nine, I’ll add some finishing touches to the router configuration and start talking about enclosures.  Stay tuned!

Update: There is a new Wifi Radio Discussion Forum, hop over there to ask questions about the project or see what other people are working on!  (4/12/09)

Update 2: Part nine is now available.

Merry Book-mas

This Christmas I received a bunch of great books from friends and family.  My O’Reilly collection in particular doubled in size!

Here’s a list of what I’ll be reading in 2009:

Building a Wifi Radio – Part 7, Building an LCD Display

This is the seventh part of an ongoing series about building a low cost, open source streaming internet radio.  If you haven’t already, check out the previous parts (see the links at the end of this article) for some background about the project.

In part six, we used OpenWrt’s UNIX-style shell commands to interface with mpd, the music player daemon, and redirect song and stream information to our ASUS WL-520gU wireless router’s serial port.  In this part, we’ll use a Sparkfun 16×2 LCD display and a handful of other components to build an LCD status display for the radio.

The Atmel AVR Microcontroller:

After much thought, I decided to use an Atmel ATmega168 AVR microcontroller to drive the display.  I realize that this raises the technical level of this project significantly, but I have been wanting to feature an AVR project on the site and this is a great opportunity.  The truth is that an Arduino would work just as well and it shouldn’t be too difficult to port this program to an Arduino sketch.  (The Arduino is built with the same ATmega168 microcontroller, after all.)  If anyone does this, let me know and I’ll post a link to your version of the display.

If you’re an AVR veteran, you can skip over this part and straight to the bill of materials below.

If you are new to the AVR, don’t be intimidated.  There are a number of tutorials online to help you learn how to use this inexpensive and powerful microcontroller.  I recommend starting with this one or maybe this one, but see my note about AVR MacPack below if you’re using a Mac.  If you’ve never programmed in C before, you’ll have an additional hurdle to get over, although for this project you won’t need any actual knowledge of programming or C to burn the code to the AVR and get things working.

You will need to install some software to work with the AVR, I recommend:

  • AVR MacPack for OS X (the Adafruit tutorial recommends OSX-AVR, use this instead)
  • WinAVR for Windows

I recommend following a tutorial or two and getting a simple blinking LED example working on your AVR before building the LCD display.  That way you can be sure your programmer, development environment, breadboard, etc are working first.

Building the display:

Bill of Materials:

You will need:

also nice, but not required:

  • a Sparkfun ISP breakout board to simplify connecting the programmer to the breadboard
  • a normally-open pushbutton switch to reset the circuit


Here is the schematic of the LCD display (click to enlarge):

Wifi Radio LCD Display Schematic


You can download the source code and compiled .hex file here.

Special thanks to Peter Fleury for his excellent LCD library, which saved me from reinventing the wheel!  He also has another page about interfacing LCD displays to an AVR.

Assembling the circuit:

Assembling the circuit on the breadboard is pretty straightforward.  Here’s a photo showing all components of the setup.  The router is shown above with the serial port wired to the breadboard (the RX line is floating as we’re not using it yet).  The USB AVR programmer is on the right, where it is also functioning as a 5V power supply for the circuit. Make sure the 2-pin jumper on the USBTinyISP is installed, this enables the +5V supply.  The LCD is shown displaying the current stream name (DI.fm).

Wifi Radio LCD Display

Here is a closeup of the components installed on the breadboard to show how I did things, feel free to experiment with the placement of components.  As long as you follow the schematic the circuit should still work.

Wifi Radio LCD Display

Here is a closeup of the serial port connection to the router, including the 4-pin female 0.1″ header.  I soldered wires to the pins of the female header (not the pins on the board).

Wifi Radio LCD Display

Here is a closeup of the AVR, crystal, and the Sparkfun ISP breakout board showing the pinout:

Wifi Radio LCD Display

Flashing the AVR:

Once the circuit is assembled on the breadboard, we need to flash the AVR microcontroller with the main.hex file included with the firmware above.

If you’re using AVR MacPack and OS X, this should be easy (since that’s what I’m using).  PC guys will need to figure this out for themselves but hopefully the process is similar (please let me know if the Makefile works).

Connect the USBTinyISP to your computer with the USB cable and to the breadboard with the ISP cable.  The green light on the programmer should be on, indicating it is ready, and the backlight of the LCD should be lit, indicating that the breadboard is getting power.

Open a terminal window and create a new directory, I used ~/temp.  Unzip the firmware into a directory somewhere, and execute ‘make flash’, as shown:

macbook:temp jkeyzer$ unzip ./AVR_wifiradio_display.zip
Archive:  ./AVR_wifiradio_display.zip
  inflating: lcd.c
  inflating: lcd.h
  inflating: main.c
  inflating: main.hex
  inflating: Makefile
macbook:temp jkeyzer$ make flash
avr-gcc -Wall -Os -DF_CPU=16000000 -mmcu=atmega168  -c main.c -o main.o

a few warnings later …

avr-gcc -Wall -Os -DF_CPU=16000000 -mmcu=atmega168  -c lcd.c -o lcd.o
avr-gcc -Wall -Os -DF_CPU=16000000 -mmcu=atmega168  -o main.elf main.o lcd.o
rm -f main.hex
avr-objcopy -j .text -j .data -O ihex main.elf main.hex
avrdude -c usbtiny -p atmega168 -U flash:w:main.hex:i
avrdude: AVR device initialized and ready to accept instructions
Reading | ################################################## | 100% 0.01s
avrdude: Device signature = 0x1e9406
avrdude: NOTE: FLASH memory has been specified, an erase cycle will be performed
         To disable this feature, specify the -D option.
avrdude: erasing chip
avrdude: reading input file "main.hex"
avrdude: writing flash (1326 bytes):
Writing | ################################################## | 100% 3.32s
avrdude: 1326 bytes of flash written
avrdude: verifying flash memory against main.hex:
avrdude: load data flash data from input file main.hex:
avrdude: input file main.hex contains 1326 bytes
avrdude: reading on-chip flash data:
Reading | ################################################## | 100% 0.68s
avrdude: verifying ...
avrdude: 1326 bytes of flash verified
avrdude: safemode: Fuses OK
avrdude done.  Thank you.

If everything went well, the LCD display firmware is now loaded into the ATmega168 and the circuit is ready to go.  If not, double check your connections and take a look at the help! page for the USBTinyISP.

Testing the display:

Telnet or ssh into the router.  Start mpd and connect to a stream using mpc (we covered this in part five).

Once the stream starts playing, execute the display.sh script we created in part six.  Within a few seconds, if everything is working, you should see the stream name on the display, followed by the artist and name of the current song.  Congratulations!

Here is a video of the LCD display in action, including the horizontal scrolling feature to show information that is too wide to fit within the visible area of the display:

That’s it for part seven!  In part eight, we’ll start working on the input side of the user interface.

Like what you’re seeing?  Have suggestions about what could be improved?  Leave a comment or contact me directly.

Update: Part eight, in which I add a tuning control to the radio, is now available.

Update 2: There is a new Wifi Radio Discussion Forum, hop over there to ask questions about the project or see what other people are working on!  (4/12/09)