MT9074
Advance Information
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.
Frame Abort
The transmitter will abort a current packet by
substituting a zero followed by seven contiguous 1s
in place of the normal packet. The receiver will abort
upon reception of seven contiguous 1s occurring
between the flags of a packet which contains at least
26 bits.
The FCS field, which precedes the closing flag,
consists of two bytes. A cyclic redundancy check
utilizing the CRC-CCITT standard generator
16
12
5
polynomial “X +X +X +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.
Note that should the last received byte before the
frame abort end with contiguous 1s, these are
included in the seven 1s required for a receiver
abort. This means that the location of the abort
sequence in the receiver may occur before the
location of the abort sequence in the originally
transmitted packet. If this happens then the last data
written to the receive FIFO will not correspond
exactly with the last byte sent before the frame abort.
Interframe Time Fill and Link Channel States
Data Transparency (Zero Insertion/Deletion)
When the HDLC transmitter is not sending packets it
will wait in one of two states
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.
•
Interframe Time Fill state: This is a
continuous series of flags occurring between
frames indicating that the channel is active
but that no data is being sent.
•
Idle state: An idle Channel occurs when at
least 15 contiguous 1s are transmitted or
received.
In both states the transmitter will exit the wait state
when data is loaded into the transmitter FIFO.
Invalid Frames
Go-Ahead
A frame is invalid if one of the following four
conditions exists (Inserted zeros are not part of a
valid count):
A go ahead is defined as the pattern "011111110"
(contiguous 7Fs) and is the occurrence of a frame
abort sequence followed by a zero, outside of the
boundaries of a normal packet. Being able to
•
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.
distinguish
a
proper (in packet) frame abort
sequence from one occurring outside of a packet
allows a higher level of signalling protocol which is
not part of the HDLC specifications.
•
•
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.
Packets which are at least 25 bits in length
but less than 32 bits between the flags are
also invalid. In this case the data is written to
the FIFO but the last byte is tagged with a
“bad packet” indication.
HDLC Functional Description
The HDLC transceiver can be reset by either the
power reset input signal or by the HRST Control bit
in the test control register (software reset). When
reset, the HDLC Control Registers are cleared,
resulting in the transmitter and receiver being
disabled. The Receiver and Transmitter can be
enabled independent of one another through Control
Register 1. The transceiver input and output are
enabled when the enable control bits in Control
Register 1 are set. Transmit to receive loopback as
•
If a frame abort sequence is detected the
packet is invalid. Some or all of the current
packet will reside in the receive FIFO,
assuming the packet length before the abort
sequence was at least 26 bits long.
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MITEL [ MITEL NETWORKS CORPORATION ]
