83C795 Datasheet, PDF (86/136 Pages) List of Unclassifed Manufacturers – Ethernet System Controller

LAN CONTROLLER OVERVIEW
83C795
• Data field
• CRC field
E ach of these fields is explained in the following
s ections .
7.4.9.1 Start of Frame
T he preamble field is us ed to train the Manches ter
decoder and to detect carrier. If carrier is detected,
preamble pas s es through the receive deserializer
whi ch di s car ds i t whil e s ear chi ng for the
S tart-of-F rame Delimiter (S F D) s ymbol. On
detecting a good S F D, a VALID_F R AME s ignal is
as serted and the receive F IF O is cleared to accept
the received frame. T he receive F IF O is loaded by
the des erializer with octets (bytes ) s tarting with the
firs t bit after S FD.
While the destination addres s (DA field) is being
checked for recognition, the receive DMA is
dis abled. If the addres s is recognized, the DMA is
enabled and trans fer to memory begins when the
F IF O fills to the programmed burs t level. If the
frame’s addres s is not recognized, the receive unit
clears out the F IF O, s tops fillingit, andwaits for the
s tart of the next frame.
T he s ource addres s and data fields are pas sed to
buffer memory. In some protocols , the firs t 2 bytes
of thedatafielddenoteaframelength. T hes ebytes
are not interpreted by the S MC795. T hey are
treated as ordinary data.
7.4.9.2 End of Frame
If there is a los s of carrier s ense, 3 dribble clocks
(receive clocks that occur after the los s of carrier
s ens e) are needed to ens ure the s ynchronizations
of all lines ignals tothereceiver circuits . When using
the internal Manchester decoder (either 10BAS E -T
or AUI interfaces ), this decoder automatically
s upplies s ufficient dribble clocks to the receiver to
complete proces s ing of the frame. When the
Manches ter decoder is bypass ed, it is neces s ary to
s upply dribble clocks at the XR XC pin after XCR S
terminates .
T he CR C from the received frame is s ent to
memory with the frame via DMA and is included in
the byte count pos ted in the buffer header.
If the receive unit detects errors in the frame (s uch
as an incorrect CR C, an alignment problem, a
fores hortened frame), it can abort reception
dependingon the configuration of theS aveE rrored
Packets and Accept R unt F rames bits of the
R eceive Configuration R egis ter - R CON.S E P and
R CON.R UNT S res pectively. Certain other types of
errors (includingFIFOoverflowandR eceiver Buffer
Overwrite) always abort reception.
If reception is aborted, the DMA controller s tops
s ending bytes to the buffer, the R eceive S tatus
R egis ter (R S R ) and the Interrupt S tatus R egis ter
(IS R ) areupdated, andthereceiveunit waits for the
next frame to begin.
No buffer header will be pos ted for the frames that
have not been accepted; the previous contents of
the header location will remain unchanged.
T he received packet length s hould be les s than
32,764 bytes , including DA, S A, data, and CR C.
T he receiver does not reject longer frames but it
may be hard to fit the contents into available buffer
s pace. T he buffer ring mus t always have enough
s pace to contain the entire frame with a 4-byte
header. Packets larger than the available buffer
s pace will not be received, regardles s of the S E P
bit in the R CON R egis ter. S uch frames will be
posted as ring overwrites and caus es the OVW
interrupt to be s et.
R eceiver interrupts (R XE for frames with errors and
PR X for frames without errors ) indicate the DMA
has completely pos ted the frame to memory. If the
DMA aborts , thes e interrupts are not set for the
current frame. If s et previous ly, they remain
unchanged. Packets s horter than 64 bytes will be
r eceived only when the Accept R unts bit
(R CON.R UNT S ) is enabled.
7.4.10 Receiver Blinding
T he R eceiver Carrier S ens e function is blinded for
a period of 4.0 µs ec starting at the end of (XCR S +
XCOL) when the device has trans mitted a frame.
T his allows the heartbeat to be detected without
res etting the deference timer and ens ures that an
improperly-s paced frame will not interfere with
proper pos ting of s tatus for a new reception.
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