mt9072av2 Zarlink Semiconductor, mt9072av2 Datasheet - Page 74

• Manufacturer Part Number: mt9072av2
• Description: Octal T1/e1/j1 Framer
• Manufacturer: Zarlink Semiconductor

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9.1.4
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 25 bits.
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.
9.1.5
When the HDLC transmitter is not sending packets it will wait in one of two states
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In both states the transmitter will exit the wait state when data is loaded into the transmitter FIFO.
9.1.6
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 distinguish a proper (in packet)
frame abort sequence from one occurring outside of a packet allows a higher level of signaling protocol which is not
part of the HDLC specifications.
9.1.7
The HDLC transceiver can be reset by either the power reset input signal or by the HRST Control bit in the HDLC
Test control register (YF3). 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 HDLC
control(YF2 bits RXEN and TXEN). The transceiver input and output are enabled when the enable control bits in
HDLC control are set. Transmit to receive loopback as well as a receive to transmit loopback are also supported.
Transmit and receive bit rates and enables can operate independently.
Received packets from the serial interface, are sectioned into bytes by an HDLC receiver that detects flags, checks
for go-ahead signals, removes inserted zeros, performs a cyclical redundancy check (CRC) on incoming data, and
monitors the address if required. Packet reception begins upon detection of an opening flag. The resulting bytes are
concatenated with two status bits (RQ9, RQ8 in HDLC status register Y1D) and placed in a receiver first-in-first-out
(Rx FIFO); a buffer register that generates status and interrupts for microprocessor read control.
In conjunction with the control circuitry, the microprocessor writes data bytes into a Tx buffer register (Tx FIFO) that
generates status and interrupts. Packet transmission begins when the microprocessor writes a byte to the Tx FIFO.
Two status bits are added to the Tx FIFO for transmitter control of frame aborts (FA) and end of packet (EOP) flags.
Packets have flags appended, zeros inserted, and a CRC, also referred to as frame checking sequence (FCS),
added automatically during serial transmission. When the Tx FIFO is empty and finished sending a packet,
Interframe Time Fill bytes (continuous flags (7E hex)), or Mark Idle (continuous ones) are transmitted to indicate
that the channel is idle.
Packets which are at least 26 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.
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 25 bits long.
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.
Frame Abort
Interframe Time Fill and Link Channel States
Go-Ahead
Functional Description
Zarlink Semiconductor Inc.
MT9072
74
Data Sheet