ST75C540 datasheet(74/84 Pages) STMICROELECTRONICS

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XII - RECEIVING IN PARALLEL MODE (continued)

XII.7 - UART Mode

XII.7.1 - Description

In UART mode the ST75C530/540extracts from the

received Characters the Data information only. It re-

ports, troughthe DUAL Ram buffer,onlydatainforma-

tion charactervalidity. The mechanism is as follows :

- As long as the ST75C530/540 receives continu-

ous “1” nothing happens.

- When the ST75C530/540 receives the start bit

(“0”) it starts to compute the parity. As soon as the

number of data bit (defined by the FORM com-

mand) is received, the ST75C530/540 writes the

received character into the DTRBFx buffer and

update the receive Status word DTRBSx.

- The Reported errors are :

• Parity error (lowest priority): the receivedparity

is not equal to the computed parity. Some bits,

inside the character, are erroneous.

• Stop bit error (middle priority): the bit after the

parity was not a stop bit (“1”). Note that if the

two stop bit format was selected, only the first

stop bit will be checked.

• BreakDetection (highestpriority): the characteris

a breaksignal as definedin the transmit section.If

the duration of the break is longer than one char-

acter, only one break buffer will be reported.

XI.7.2 - Status Word Format

DTRBSx in UART Mode

Field

Pos.

Value

Definition

BUFF_LENG

3 .. 0

Other

Buffer empty.

1 character received

(DTRBFx[0]).

Not allowed.

BUFF_ERRS

5..4

No error.

Parity error

Stop bit error

Break signal detected

XIII - VOCODER DATA EXCHANGE

XIII.1 - Overview

The ST75C530/540 can receive (or transmit)

coded voice from (to) the telephone line or the

audio interface. The receiving mode is the CODER

mode while the transmit is the DECODER mode.

Two formats of Voice compression are provided: Low

bit rate and ADPCM. In all the formats and speed the

managementof the CodedVoice is exactly the same.

In any format a frame of all data equal to zero will

be synthesised (DECODER) as a frame of silence.

XIII.2 - Vocoder Buffer

A buffer area is reserved in the DUAL ram to

exchange Voice between the ST75C530/540 and

the Host processor. Thisareaisusedeitherforrecord-

ing (CODER) or playing back (DECODER) the voice

signal.

The DUAL Ram area associated with the VO-

CODER is as follows :

Name

Address

Description

VOCSTA

$1C

Vocoder Buffer Status

VOCDATA

$1D..$2E

Vocoder Buffer Data

VOCCORR

$2F..$30

Vocoder Buffer Corrector

The IT1 interrupt signal is dedicatedto the Vocoder

Buffer Management.

XIII.3 - Transmit (DECODER)

This mode is entered with the CONF DECODER

command.

If the ADPCM or Low bit rate without error correc-

tion mode (CONF_ERCOR = 0) are selected, the

user needs to feed the vocoder buffer with 18 bytes

of voice data, then set the VOCSTA byte with a

value different from zero.

In the low bit rate with error mode (CONF_ERCOR

= 1), the user needsto feed the vocoder buffer with

20 bytes of voice data, then set the VOCSTA byte

with a value different from zero.

Once the ST75C530/540 have read the buffer, it

clears the VOCSTAbyte and raise the IT1 interrupt.

The IT1 interrupt rate is as follows :

Mode

Interrupt

Time (ms)

Number of

Voice Samples

in the Buffer

(8kHz sampling)

ADPCM 32Kpbs

4.5

ADPCM 24Kpbs

ADPCM 16Kpbs

Low Bit Rate Nominal

(with and without

error correction)

240

Low Bit Rate

Fast/Slow Playback

Depends on

speed 15 to 45

Depends on

speed 120 to 360

Low Bit Rate Pause

A silence can be generatedby writing zero to all the

VOCDATA bytes (an d VOCCORR b ytes if

CONF_ERCOR = 1). The duration of the silence

will be the same as the other frames of signal.

As the buffer contains always a complete number

of samples representing the same duration, it is

easy to randomly advance forward/backward in a

message.

If the user does not feed the Buffer within the

Interrupt time, the ST75C530/540 will signal this

error by rising the ERR_VOCO in theSYSERR byte

and rising the IT0 Interrupt. In this case the pre-

vious frame will be re-transmited.

ST75C530 - ST75C540

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