MT9075AP datasheet(17/78 Pages) MITEL

17 / 78 page

Preliminary Information

MT9075A

4-145

HDLC0 Functions

When connected to the Data Link (DL) HDLC0 will

operate at a selected bit rate of 4, 8, 12, 16 or 20

kbits/sec. HDLC0 can be selected by setting the

control bit HDLC0 (bit 7) to one in page 01H, address

14H. When this bit is zero all interrupts from HDLC0

are masked. For more information refer to following

sections.

HDLC1 Functions

This controller may be connected to time slot 16

under Common Channel Signalling (CCS) mode. It

should be noted that the AIS16S function (page 03H,

address 19H) will always be active and the TAIS16

function (page 01H, address 16H) will override all

other transmit signalling.

HDLC1 can be selected by setting the control bit

HDLC1 (bit 6) to one in page 01H, address 14H.

When this bit is zero all interrupts from HDLC1 are

masked.

HDLC Overview

The HDLC handles the bit oriented packetized data

transmission as per X.25 level two protocol deﬁned

by CCITT. It provides ﬂag and abort sequence

generation

and

detection,

zero

insertion

and

deletion,

and

Frame

Check

Sequence

(FCS)

generation and detection. A single byte, dual byte

and all call address in the received frame can be

recognized. Access to the receive FCS and inhibiting

of transmit FCS for terminal adaptation are also

provided. Each HDLC controller has a 128 byte deep

FIFO associated with it. The status and interrupt

ﬂags are programmable for FIFO depths that can

vary from 16 to 128 bytes in steps of 16 bytes. These

and other features are enabled through the HDLC

control registers on page 0BH and 0CH.

HDLC Frame Structure

A valid HDLC frame (also referred as “packet”)

begins with an opening ﬂag, contains at least 16 bits

of data ﬁeld, and ends with a 16 bit FCS followed by

a closing ﬂag (Table 9).

All HDLC frames start and end with a unique ﬂag

sequence

“011111102” (7EH). The transmitter

generates these ﬂags and appends them to the

packet to be transmitted. The receiver searches the

incoming data stream for the ﬂags on a bit-by-bit

basis to establish frame synchronization.

Table 9 - HDLC Frame Format

The data ﬁeld usually consists of an address ﬁeld,

control ﬁeld and information ﬁeld. The address ﬁeld

consists of one or two bytes directly following the

opening ﬂag. The control ﬁeld consists of one byte

directly following the address ﬁeld. The information

ﬁeld immediately follows the control ﬁeld and

consists of

n bytes of data. The HDLC does not

distinguish between the control and information

ﬁelds and a packet does not need to contain an

information ﬁeld to be valid.

The FCS ﬁeld, which precedes the closing ﬂag,

consists of two bytes. A cyclic redundancy check

utilizing

the

CCITT

standard

polynomial

“X16+X12+X5+1” produces the 16-bit FCS. In the

transmitter the FCS is calculated on all bits of the

address and data ﬁeld. The complement of the FCS

is transmitted, most signiﬁcant bit ﬁrst, in the FCS

ﬁeld. The receiver calculates the FCS on the

incoming packet address, data and FCS ﬁeld and

compares the result to “F0B8”. If no transmission

errors are detected and the packet between the ﬂags

is at least 32 bits in length then the address and data

are entered into the receive FIFO minus the FCS

which is discarded.

Data Transparency (Zero Insertion/Deletion)

Transparency ensures that the contents of a data

packet do not imitate a ﬂag, go-ahead, frame abort

or idle channel. The contents of a transmitted frame,

between the ﬂags, is examined on a bit-by-bit basis

and a 0 is inserted after all sequences of 5

contiguous 1s (including the last ﬁve bits of the

FCS). Upon receiving ﬁve contiguous 1s within a

frame the receiver deletes the following 0.

Invalid Frames

A frame is invalid if one of the following four

conditions exists:

•

If the FCS pattern generated from the

received data does not match the “F0B8”

pattern then the last data byte of the packet

is written to the received FIFO with a ‘bad

packet’ indication.

Opening

Flag (7EH)

Data

Field

FCS

Closing

Flag (7EH)

One Byte

01111110

n Bytes

≥ 2

Two Bytes

One Byte

01111110

MT9075AP Datasheet(PDF) 17 Page - Mitel Networks Corporation

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