am79c940 Advanced Micro Devices, am79c940 Datasheet - Page 60

• Manufacturer Part Number: am79c940
• Description: Media Access Controller For Ethernet Mace
• Manufacturer: Advanced Micro Devices

Bit 0 ASTRP RCV
Receive Frame Status (RCVFS)
The Receive Frame Status is a single byte location
which must be read by four read cycles to obtain the four
bytes (32-bits) of status associated with each receive
frame. Receive Frame Status can be read using either
the Register Direct or FIFO Direct access modes.
In Register Direct mode, access to the Receive FIFO will
be denied until all four status bytes for the completed
frame have been read from the Receive Frame Status
location. In FIFO Direct mode, the Receive Frame
Status is read through the Receive FIFO location, by
continuing to execute four read cycles after the comple-
tion of packet data (and assertion of EOF). The Receive
Frame Status can be read using either mode, or a com-
bination of both modes, however each status byte will be
presented only once regardless of access method.
Other register reads and/or writes can be interleaved at
any time, during the Receive Frame Status sequence.
The Receive Frame Status consists of the following four
bytes of information:
RFS0 Receive Message Byte Count
RFS1 Receive Status, (RCVSTS) [11–8]
RFS2 Runt Packet Count (RNTPC) [7–0]
RFS3 Receive Collision Count (RCVCC) [7–0]
RFS0—Receive Message Byte Count (RCVCNT)
Bit
Bit 7-0
60
RCVFS [31–00]
RCVCNT [7:0]
AMD
(RCVCNT) [7–0]
[7:0]
RCVCNT
Name
Auto Strip Receive. ASTRP RCV
enables the automatic pad strip-
ping feature. The pad and FCS
fields will be stripped from re-
ceive frames and not placed in
the FIFO. ASTRP RCV is set by
activation of the RESET pin or
the SWRST bit.
Description
The Receive Message Byte
Count indicates the number of
whole bytes in the received mes-
sage. If pad bytes were stripped
from
RCVCNT indicates the number
of bytes received less the num-
ber of pad bytes and less the
number of FCS bytes. RCVCNT
is 12 bits long. If a late collision is
detected (CLSN set in RCVSTS),
the count is an indication of the
length (in byte times) of the dura-
tion of the receive activity includ-
ing the collision. RCVCNT [10:8]
correspond to bits 3–0 in RFS1 of
the
RCVCNT [11–0] will be invalid
when OFLO is set.
Receive
the
received
Frame
(REG ADDR 6)
Status.
frame,
Am79C940
RFS1—Receive Status (RCVSTS)
Bit
Bit 7
Bit 6
Bit 5
Bit 4
OFLO CLSN FRAM FCS
OFLO
CLSN
FRAM
FCS
Name
Description
Overflow flag. Indicates that the
Receive FIFO over flowed due to
the inability of the host/controller
to read data fast enough to keep
pace with the receive serial bit
stream and the latency provided
by the Receive FIFO itself. OFLO
is indicated on the receive frame
that caused the overflow condi-
tion; complete frames in the Re-
ceive FIFO are not affected.
While the Receive FIFO is in the
overflow condition, it ignores ad-
ditional receive data on the net-
work.
detect logic will continue to oper-
ate and the Missed Packet Count
(MPC in register 24) will be incre-
mented for each packet which
passes
criteria, and complete without
collision.
Collision Flag. Indicates that the
receive operation suffered a colli-
sion during reception of the
frame. If CLSN is set, it indicates
that the receive frame suffered a
late collision, since a frame expe-
riencing collision within the slot
time will be automatically deleted
from the RCVFIFO (providing
LLRCV in the Receive Frame
Control register is cleared). Note
that if the LLRCV bit is enabled,
the late collision threshold is ef-
fectively moved from the normal
64-byte (512-bit) level to the
12-byte (96-bit) level. Runt pack-
ets suffering a collision will be
flushed from the RCVFIFO re-
gardless of the state of the RPA
bit (User Test Register). CLSN
will not be set if OFLO is set.
Framing Error flag. Indicates that
the received frame contained a
non-integer multiple of bytes and
an FCS error. If there was no
FCS error then FRAM will not be
set. FRAM is not valid during in-
ternal loopback. FRAM will not
be set if OFLO is set.
FCS Error flag. Indicates that
there is an FCS error in the
frame. The receive FCS is com-
puted and checked normally
when ASTRP RCV = 1, but is not
The
the
RCVCNT [10:8]
internal
address
address
match