STA014T datasheet(8/45 Pages) STMICROELECTRONICS

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XTI2DSPCLK

XTI2OCLK

PFD

VCO

Switching

Circuit

OCLK

DCLK

Update FRAC

FRAC

XTI

Disable PLL

Figure 5. PLL and Clocks Generation System

2.2 - PLL & Clock Generator System

When STA014 receives the input clock, as de-

scribed in Section 2.1, and a valid layer III input

bitstream, the internal PLL locks, providing to the

DSP Core the master clock (DCLK), and to the

Audio Output Interface the nominal frequencies of

the incoming compressed bit stream. The STA014

PLL block diagram is described in Figure 5.

The audio sample rates are obtained dividing the

oversampling clock (OCLK) by software programma-

ble factors. The operation is done by STA014 em-

bedded software and it is transparent to the user.

The STA014 PLL can drive directly most of the com-

mercial DACs families, providing an over sampling

clock, OCLK, obtained dividing the VCO frequency

with a software programmable dividers.

2.3 - STA014 Operational Modes

The device can be configured in 4 different op-

erational modes. To select one specific mode a

dedicated CHIP_MODE registers is available. For

proper operation the following steps must be is-

sued to switch between different modes:

- issue a software reset (SOFT_RESET register)

- select the desired mode (CHIP_MODE register)

- run the device (RUN register)

Hereby is a short description of each available

mode

ADPCM Encoder

This mode can be used to encode the incom-

ing bitstream with 4 different compression al-

gorithms. Moreover different sample frequen-

cies and word size are supported. For a

detailed description of this features refer to the

related registers.

ADPCM Decoder

This mode can be used when an ADPCM

compressed bitstream must be decoded.

The input interface handling and control flow is

the same as in the MP3 Mode.

BYPASS mode

Using this mode it’s possible to use the em-

bedded post-processing controls (volume and

tone controls) to process an incoming uncom-

pressed stereo audio stream. In this configura-

tion ADC input is the only supported interface.

This could be useful, for instance, to process

audio data coming from an external tuner or

some other auxiliary source.

MP3 mode

In MP3 Mode (default mode) STA014 decodes

the incoming bitstream, acting as a master of

the data communication from the source to it-

self.

This control is done by a specific buffer man-

agement, controlled by STA014 embedded

software. The data coming from the serial in-

terface are stored in the input buffer, a 256

bytes long FIFO.

The feedback line DATA_REQ actually is the

result of the h/w comparison between the writ-

ing address of the FIFO and the constant

value 252. This means that if the buffer is filled

up with more than 252 bytes the DATA_REQ

line goes low, requesting MCU to stop trans-

mission: the maximum time to stop transmit-

ting is given by the time required to transmit 4

bytes (this time, in turn, depends on the bit-

stream speed used to send MP3 data).

The input interface can receive data with a

speed up to 20Mbit/s. The speed at which the

FIFO is emptied is equal to the MP3 nominal

bitrate. Provided the FIFO is filled up with 252

bytes the time required to empty it (in worst

condition, which is 320kbit/s mpeg stream) is

about 6ms. So if no more data is received in

this time the buffer will be emptied and this will

badly affect the output audio.

STA014-STA014B-STA014T

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