Xilinx XAPP169 fixpoint.tgz

MP3 NG: A Next Generation Consumer Platform

XAPP169 (v1.0) November 24, 1999 www.xilinx.com 13

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The RTOS provides process scheduling and memory allocation functions. T he RTOS could be

any of the commercially available packages. Probably more of a factor than any technical issue

is the licensing model for the product. Since this is a product that is targeted at the high-volum e,

cost sensitive, consumer market, an RTOS that is licensed on an up front fee basis with no unit

royalties is the most attractive.

The various BIOS components will be discussed later in the sections that describe the

hardware implementation for each interface. The key application modules a re as follows:

UI Manager

The User Interface (UI) manager is responsible for handling interaction b etween the user and

the system. This includes using the Screen BIOS to create the butto ns and menus that the user

sees, getting user input through the Touch BIOS and using this information to coordinate

activities such as downloading and playing MP3 files. The UI manager wo uld also spawn

separate processes for value added features such as an appointment calen dar, or a phone

book, as needed.

MP3 Decoder and Audio ISR

The MP3 decoder runs as an independent process, controlled by the UI mana ger. When

activated, it uses the FLASH BIOS to fetch MP3 file data, decompresses it and places the audio

data in a queue. The audio Interrupt Service Routine (ISR) is activated b y an interrupt from the

Audio DAC block in the FPGA. When activated, it reads data from this queue and writes it to

FIFOs in the Audio DAC block.

The key to getting optimal performance from the MP3 decoder on the RC32364 lies in taking

advantage of the MAC instruction supported by the processor. The instruction is part icularly

valuable in the implementation of the Discrete Cosine Transform (DCT) for sub-band synth esis.

There are several sources for MP3 decoder code. A fixed point dec oder (splay-0.81-

fixpoint.tgz) that was developed for the Linux ARM project can be downloaded from the

following URL:

ftp://ftp.netwinder.org/users/n/nico

Audio

BIOS

Touch

BIOS FLASH

BIOS

BIOS USB

BIOS MMU

BIOS

Screen

BIOS

System

Hardware

IRDA

Stack USB

Stack

RTOS

Audio

ISR

MP3

Decoder

Memory

Manager

Figure 14: System Software Architecture