MT9072AB datasheet(73/275 Pages) ZARLINK

73 / 275 page

MT9072

Data Sheet

Zarlink Semiconductor Inc.

9.1

HDLC Description

The HDLC handles the bit oriented protocol structure as per layer 2 of the switching protocol X.25 defined by

CCITT. It transmits and receives the packetized data serially while providing data transparency by zero insertion

and deletion. It generates and detects the flags, various link channel states and abort sequences. Further, it

provides a cyclic redundancy check on the data packets using the CCITT defined polynomial. In addition, it can

recognize a single byte, dual byte and all call address in the received frame. Access to Rx CRC and inhibiting of Tx

CRC for terminal adaptation is also provided. The HDLC controller has two 32 byte deep FIFO’s associated with it;

one for Transmit and one for Receive.

9.1.1

HDLC Frame Structure

A valid HDLC frame begins with an opening flag, contains at least 16 bits of address and control or information, and

ends with a 16 bit FCS followed by a closing flag. Data formatted in this manner is also referred to as a “packet”.

Refer to Table 33.

All HDLC frames start and end with a unique flag sequence “01111110”. 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.

The data field 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

CRC-CCITT standard generator 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.

9.1.2

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 bit is

inserted after all sequences of 5 contiguous 1 bits (including the last five bits of the FCS). Upon receiving five

contiguous 1s within a frame the receiver deletes the following 0 bit.

9.1.3

Invalid Frames

A frame is invalid if one of the following four conditions exists (Inserted zeros are not part of a valid count):

• 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.

• A short frame exists if there are less than 25 bits between the flags. Short frames are ignored by the receiver

and nothing is written to the receive FIFO.

Flag (7E)

Data Field

FCS

Flag (7E)

One Byte

01111110

n Bytes

≥ 2

Two Bytes

One Byte

01111110

Table 33 - HDLC Frame Format

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