ST75C540 datasheet(67/84 Pages) STMICROELECTRONICS

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XI.6 - HDLC Mode

XI.6.1 - Description

In HDLC mode the ST75C530/540 transmits the

data bytes contained into the DUAL Ram buffer

packed inside an HDLC frame. The mechanism is

as follows :

- While the Host has no frame to transmit, that is:

l o ng

a s

DTTBSx equals $00, t he

ST75C530/540 transmits the HDLC Flag $7E.

- When the Host wants to send some data, it feeds

the buffer with some data bytes to transmit (be-

tween 1 and 8) and set the BUFF_SFRM bit in

the

DTTBSx status buffer. At that time the

ST75C530/540 start sending data contained in

the Buffer, computin the CRC and performing

“zero intertion” if needed.

- When the host wants to send additional data

(within the same frame) it feeds the buffers just

like in synchronous mode. If an Underflow condi-

tion occurs, the ST75C530/540 will abort the

frame by sending 8 consecutive “1”, and the Host

must restart the whole parallel initialization.

- When the host wants to close a frame, it set the

BUFF_EFRM bit in the DTTBSx status buffer. At

that time the ST75C530/540 will send the con-

tents of the buffer, then send the CRC and an

HDLC closing flag $7E.

- If the Host, wants to abort a frame (while sending

a frame) it setthe

BUFF_FRAB bit in the DTTBSx

status buffer.At thattime, as soon as the last buffer

will be transmitted, the ST75C530/540 will send

8 consecutive “1” and wait for the next buffer.

XI.6.2 - Status Word Format

DTTBSx in HDLC Mode

Field

Pos. Value

Definition

BUFF_LENG 3 .. 0

other

Buffer empty.

1 Byte to transmit

(DTTBFx[0]).

2 Bytes to transmit

(

DTTBFx[0] and

DTTBFx[1]).

8 Bytes to transmit

(

DTTBFx[0 .. 7]).

Not allowed.

BUFF_SFRM

Data stream.

Start of frame : the buffer

is a beginning of frame.

BUFF_EFRM

Data stream.

End of frame : the buffer

will be followed by the

transmission of the CRC

and closing flag.

BUFF_FRAB

Data stream.

Abort frame :

8 consecutive “1” will be

t ransmit ted (whatever

BUFF_LENG is).

Other

Reserved, must be 0.

Notes : 1. A buffer can have

BUFF_SFRM and BUFF_EFRM set

in the same DTTBSx byte, this means that the frame

transmitted is short (between 1 and 8 Bytes long).

2. An ending frame (with

BUFF_EFRM set) must have at

least ONE byte of data to transmit.

XI - TRANSMITTING DATA IN PARALLEL MODE (continued)

TRANSMITTED

DATA

BUFF_FRAB

BUFF_SFRM

BUFF_EFRM

BUFF_LENG

(BUFF_DATA)

$7E

CRC

$7E

CRC

$7E

Figure 32

XI.6.3 - Single Short Frame (see Figure 32)

ST75C530 - ST75C540

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