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III - INTERNAL CIRCUIT DESCRIPTION

A general block diagram for the STi5500 is shown

is Figure 1.

The performance offered by the ST20 32-bit micro-

core allows the following operations to be per-

formed in software :

1 Device drivers for Video, Audio and Sub-picture

Decoders

2 Audio/Video/Subpicture synchronisation

3 System management functions

4 Electronic program guide

5 Conditional access module

The use of a 32-bit CPU enables advanced graph-

ics routines to be employed for on-screen display

functions, allowing fast turnaround system up-

grades.

III.1 - The ST20 32-bit CPU

The ST20 micro-core family has been developed by

SGS-THOMSON Microelectronics to provide the

tools and building blocks to enable the development

of highly integrated application-specific 32-bit de-

vices at the lowest cost and fastest time to market.

The ST20 macrocell library includes the ST20Cx

family of 32-bit VL-RISC (variable length reduced

instruction set computer) micro-cores, embedded

memories, standard peripherals, I/O, controllers

and ASICs.

The STi5500 uses the ST20 macrocell library to

provide all of the dedicated hardware modules

required in a set top box or DVD system.

These include :

- High performance internal SRAM and cache sub-

system,

- I

- UART serial I/O interface to modem and auxiliary

ports,

- Interrupt controller for internal and external interrupts,

- DMA to MPEG audio and video device(s),

- External memory interface supporting DRAM,

EPROM and peripherals,

- PWM/timer modulefor control of system clock VCXOs,

- Programmable I/O pins,

- Smart card interfaces.

III.2 - MPEG-2 Video/Audio Decoder

The video decoder implemented in the STi5500

uses a patented memory reduction/bandwidth re-

duction scheme to offer the user the best band-

width/memory size compromise.

The algorithm is lossless and uses "on-the-fly"

decoding to reduce the memory requirements to

2 frame buffers in memory size reduction mode.

When used in bandwidth reduction mode the mem-

ory usage is the normal three buffers but the band-

width required by the decoder is significantly re-

duced over a classical implementation.

In summary the features of this decoder core are :

- Video decoder fully supports MPEG-2 Main Pro-

file/Main Level (MP@ML),

- Memory reduction architecture allows sharing of

single 16MBit SDRAM between MPEG decoding,

micro and transport functions - memory expand-

able to 32Mbits of SDRAM,

- Letterbox filter,

- Horizontal and vertical image re-sizing,

- 2 to 8 bit OSD (6-bit luma resolution, 4-bit chroma

resolution),

- Accepts MPEG-2 Program Streams, PES and

MPEG-1 system streams,

- Automatic error concealment.

The output from the video decoder is fed directly to

a PAL/NTSC encoder unit generating simultane-

ously a composite video signal, component Y/C

and RGB for each of the two standards.

The signals can be optionally encoded following

the Macrovision

has a licence to use the technology. The digital

encoder cell is also capable of encoding into the

composite signal teletext and closed caption infor-

mation.

The audio decoder performs MPEG levels 1 and

II decoding and is functionally equivalent to the

STi3520A audio decoder. The STi5500 has a dedi-

cated interface to an external Dolby AC3 decoder

whilst allowing audio buffering to be performed in

the 16 Mbit SDRAM.

III.3 - High Quality Graphics

The graphics performance of the STi5500 supports

the

new requirements for intelligent program

guides and interactive applications.

The display interface supports up to 256 colours for

each OSD region and a transparency feature al-

lows mixing of video with the OSD. Fast access

graphics and many other additional features are

available and are supported by a graphics library.

Excellent system performance is obtained by

closely coupling the high performance RISC proc-

essor and cache with the MPEG audio/video core

and display memory.

Low latency RISC access and DMA engines allow

rapid construction of bit maps. DVD graphics are

also supported by an integrated sub picture de-

coder. Pan and scan and letterbox output are pro-

vided for 16:9 applications.

STi5500

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