STA015T datasheet(9/55 Pages) STMICROELECTRONICS

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STA015 STA015B STA015T

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) STA015 decodes the incoming bitstream, acting as a master of the data

communication from the source to itself. This control is done by a specific buffer management,

controlled by STA015 embedded oftware.

The data coming from the serial interface 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 writing 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 transmission: the maximum time to stop

transmitting is given by the time required to transmit 4 bytes (this time, in turn, depends on the bitstream

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.

In this mode the fractional part of the PLL is disabled and the audio clocks are generated at nominal rates.

Fig. 6 describes the default DATA_REQ signal behaviour. Programming STA015 it is possible to invert the

polarity of the DATA_REQ line (register REQ_POL).

In order to allow proper operation of the device in broadcast applications a special BRAODCAST MP3 de-

coding mode is available. When configured in BROADCAST mode the device will operate as a slave de-

coder and no more feedback will be generated to the data source.

The output PCM clock will be automatically adjusted by the embedded DSP in order to follow the incoming

bitstream rate and to avoid input buffer underrun/overrun. A special configuration file must be used to en-

able this operational mode: the file must be downloaded via I2C link after device power-on. Please contact

your local ST branch to have more information about.

Figure 6. DATA_REQ control line

2.4 STA015 Decoding States

There are three different decoder states: Idle, Init, and Decode. Commands to change the decoding

states are described in the STA015 I2C registers description.

Idle Mode

IIn this mode (entered after a S/W or H/W reset) the decoder is waiting for the RUN command. This mode

should be used to initialize the configuration registers of the device. The DAC connected to STA015 can

be initialized during this mode (set MUTE to 1).

MUTE to 1).

PLAY

MUTE

Clock State

PCM Output

Not Running

Running

SOURCE SEND DATA TO STA015

DATA_REQ

SOURCE STOPS TRANSMITTING DATA

D00AU1144

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