MT9074
Advance Information
flag followed by the data and closing flag is sent and
zero insertion still included, but no CRC. That is, the
FCS is injected by the microprocessor as part of the
data field. This is used in V.120 terminal adaptation
for synchronous protocol sensitive UI frames.
RQ9
RQ8
Byte status
last byte (bad packet)
firstbyte
1
0
1
0
1
1
0
0
last byte (good packet)
packetbyte
HDLC Receiver
The end-of-packet-detect (EOPD) interrupt indicates
that the last byte written to the Rx FIFO was an EOP
byte (last byte in a packet). The end-of-packet-read
(EopR) interrupt indicates that the byte about to be
read from the Rx FIFO is an EOP byte (last byte in a
packet). The Status Register should be read to see if
the packet is good or bad before the byte is read.
After initialization and enabling, the receiver clocks in
serial data, continuously checking for Go-aheads (0
1111 1110), flags (0111 1110), and Idle Channel
states (at least fifteen ones). When a flag is
detected, the receiver synchronizes itself to the
serial stream of data bits, automatically calculating
the FCS. If the data length between flags after zero
removal is less than 25 bits, then the packet is
ignored so no bytes are loaded into Rx FIFO. When
the data length after zero removal is between 25 and
31 bits, a first byte and bad FCS code are loaded into
the Rx FIFO (see definition of RQ8 and RQ9 below).
For an error-free packet, the result in the CRC
register should match the HEX pattern of ’F0B8’
when a closing flag is detected.
A minimum size packet has an 8-bit address, an 8-bit
control byte, and a 16-bit FCS pattern between the
opening and closing flags (see Section 9.3.2). Thus,
the absence of a data transmission error and a frame
length of at least 32 bits results in the receiver
writing a valid packet code with the EOP byte into Rx
FIFO. The last 16 bits before the closing flag are
regarded as the FCS pattern and will not be
transferred to the receiver FIFO. Only data bytes
(Address, Control, Information) are loaded into the
Rx FIFO.
If address recognition is required, the Receiver
Address Recognition Registers are loaded with the
desired address and the Adrec bit in the Control
Register 1 is set high. Bit 0 of the Address Registers
is used as an enable bit for that byte, thus allowing
either or both of the first two bytes to be compared to
the expected values. Bit 0 of the first byte of the
address received (address extension bit) will be
monitored to determine if a single or dual byte
address is being received. If this bit is 0 then a two
byte address is being received and then only the first
six bits of the first address byte are compared. An all
call condition is also monitored for the second
address byte; and if received the first address byte is
ignored (not compared with mask byte). If the
address extension bit is a 1 then a single byte
address is being received. In this case, an all call
condition is monitored for in the first byte as well as
the mask byte written to the comparison register and
the second byte is ignored. Seven bits of address
comparison can be realized on the first byte if this is
a single byte address by setting the Seven bit of
Control Register 2.
In the case of an Rx FIFO overflow, no clocking
occurs until a new opening flag is received. In other
words, the remainder of the packet is not clocked into
the FIFO. Also, the top byte of the FIFO will not be
written over. If the FIFO is read before the reception
of the next packet then reception of that packet will
occur. If two beginning of packet conditions
(RQ9=0;RQ8=1) are seen in the FIFO, without an
intermediate EOP status, then overflow occurred for
the first packet.
The receiver may be enabled independently of the
transmitter. This is done by setting the RXEN bit of
Control Register 1. Enabling happens immediately
upon writing to the register. Disabling using RXEN
will occur after the present packet has been
completely loaded into the FIFO. Disabling can occur
during a packet if no bytes have been written to the
FIFO yet. Disabling will consist of disabling the
internal receive clock. The FIFO, Status, and
Interrupt Registers may still be read while the
receiver is disabled. Note that the receiver requires a
flag before processing a frame, thus if the receiver is
enabled in the middle of an incoming packet it will
ignore that packet and wait for the next complete
one.
The following two Status Register bits (RQ8 and
RQ9) are appended to each data byte as it is written
to the Rx FIFO. They indicate that a good packet has
been received (good FCS and no frame abort), or a
bad packet with either incorrect FCS or frame abort.
The Status and Interrupt Registers should be read
before reading the Rx FIFO since status and
interrupt information correspond to the byte at the
output of the FIFO (i.e. the byte about to be read).
The Status Register bits are encoded as follows:
The receive CRC can be monitored in the Rx CRC
Registers. These registers contain the actual CRC
sent by the other transmitter in its original form; that
is, MSB first and bits inverted. These registers are
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