This DIY documentation explains the software and hardware aspects of how to build a portable MP3 player from scratch, the design being simple and cost effective.

COMPONENTS & COSTS INVOLVED:

AVR Butterfly (from Atmel) = $ 20 USD
PCB hardware (single sided)= $ 20 USD
VS1001K decoder(from VLSI solution Oy) = $ 20 USD
Other misc. costs = $ 20 USD
TOTAL=$ 100 USD (APPROX.)

Concept
Most of the needed components come fused within the Atmel AVR butterfly. It comes built in with the ATmega169, display and a joystick. Add to this the VS1001k, which is a decoder used to decode the MP3 files and you have most of the functional blocks of the MP3 player.
The choice of components is such that it helps in simplifying the design as well as reducing component count.

LCD
The standard LCD module can be used or optionally the LCD display of NOKIA 3310 phone can be used. This LCD, which is controlled by a Phillips controller, is easy to work with and easily available. Using this option frees up some of the I/O on the AVR butterfly, which can be used for other useful purposes such as adding up ATA/ CF memory.

Hardware
The PCB used for this project can be seen above. A summarization of all the components and a bird’s eye view of the connections is given in the picture.

Battery & charging
Lithium polymer batteries are best for this application given their high performance and compact size. Charging is accomplished by plugging into the USB port of a PC. The Maxim 1811 charger IC ensures that the battery is safely recharged. A full recharge from dead-battery takes around 2 hours . At an average current consumtion of 20 mW and firmware stand-by modes ensuring reduced consumption, this device can last for around 20 hours.

Software and firmware
The software and firmware versions can be found here
It includes routines that deal with communication with the various components that include the 3310 LCD display, MP3 decoder, MMC card and a basic FAT16 implementation. The FAT16 system will do long filenames and support directories. The software also has the ability to accurately measure and display battery power and it can shutdown the device when battery power drops below a critical level.

Drawbacks
There are few drawbacks which are to be viewed more as future development avenues:
- No fast forward and rewind controls
- No playlist support
- FAT implementation only looks at primary partition 1 (some MMCs use partition 2 in MBR!)
- ID3 info is occasionally ignored

SOURCES: http://www.brokentoaster.com/butterflymp3/index.html