am79c940 Advanced Micro Devices, am79c940 Datasheet - Page 53

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

Receive Function Programming
The Receive Frame Control register allows program-
ming of the automatic pad field stripping feature and the
configuration of the Match/Reject (M/R) pin. ASTRP
RCV and M/R must be static when the receive function
is enabled (ENRCV = 1). The receiver should be dis-
abled before (re-) programming these options.
The EADI port can be used to permit reception of frames
to commence whilst external address decoding takes
place. The M/R bit defines the function of the EAM/R pin,
and hence whether frames will be accepted or rejected
by the external address comparison logic.
The programming of additional receive attributes are
distributed between the FIFO and MAC Configuration
Control registers, and the User Test Register.
All receive frames can be accepted by setting the PROM
bit (bit 7) in the MAC Configuration Control register.
When PROM is set, the MACE device will attempt to re-
ceive all messages, subject to minimum frame enforce-
ment. Setting PROM will override the use of the EADI
port to force the rejection of unwanted messages. See
the sections External Address Detection Interface for
more details.
The point at which RDTREQ is asserted in relation to the
number of bytes of a frame that are present in the Re-
ceive FIFO (RCVFIFO) is controlled by the RCVFW bits
in the FIFO Configuration Control register, or the
LLRCV bit in the Receive Frame Control register.
RDTREQ will be asserted when one of the following
conditions is true:
(i) There are at least 64 bytes in the RCVFIFO.
(ii) The received packet has passed the 64 byte mini-
(iii) A receive packet has completed, and part or all of it
(iv) The LLRCV bit has been set and greater than
Note that if the RCVFW is set below the 64-byte limit, the
MACE device will still require 64-bytes of data to be re-
ceived before the initial assertion of RDTREQ. Subse-
quently, RDTREQ will be asserted at any time the
RCVFW threshold is exceeded. The only times that the
RDTREQ will be asserted when there are not at least an
initial 64-bytes of data in the RCVFIFO are:
(i) When the ASTRP RCV bit has been set in the Re-
mum criteria, and the number of bytes in the
RCVFIFO is greater than or equal to the threshold
set by the RCVFW (16 or 32 bytes).
is present in the RCVFIFO.
12-bytes of at least 8 bytes have been received.
ceive Frame Control register, and the pad is auto-
matically stripped from a minimum length packet.
Am79C940
(ii) When the RPA bit has been set in the User Test
(iii) When the LLRCV bit has been set in the Receive
No preamble/SFD bytes are loaded into the Receive
FIFO. All references to bytes past through the receive
FIFO are received after the preamble/SFD sequence.
Depending on the bus latency of the system, RCVFW
can be set to ensure that the RCVFIFO does not over-
flow before more data is read. When the entire frame is
in the RCVFIFO, RDTREQ will be asserted regardless
of the value in RCVFW. The default value of RCVFW is
64-bytes after hardware or software reset.
The receive operation of the MACE device can be halted
at any time by clearing the ENRCV bit in the MAC Con-
figuration Control register. Note that any receive frame
currently in progress will be accepted normally, and the
MACE device will disable the receive process once the
message has completed. The Missed Packet Count
(MPC) will be incremented for subsequent packets that
would have normally been passed to the host, and are
now ignored due to the disabled state of the receiver.
Note that clearing the ENRCV bit disables the assertion
of RDTREQ. If ENRCV is cleared during receive activity
and remains cleared for a long time and if the tail end of
the receive frame currently in progress is longer than the
amount of space available in the Receive FIFO, Receive
FIFO overflow will occur. However, even with RDTREQ
deasserted, if there is valid data in the Receive FIFO to
be read, successful slave reads to the Receive FIFO
can be executed (indicated by valid DTV). It is the host’s
responsibility to avoid the overflow situation.
Automatic Pad Stripping
During reception of a frame the pad field can be stripped
automatically. ASTRP RCV = 1 enables the automatic
pad stripping feature. The pad field will be stripped be-
fore the frame is passed to the FIFO, thus preserving
FIFO space for additional frames. The FCS field will also
be stripped, since it is computed at the transmitting sta-
tion based on the data and pad field characters, and will
be invalid for a receive frame that has the pad charac-
ters stripped.
The number of bytes to be stripped is calculated from
the embedded length field (as defined in the IEEE 802.3
definition) contained in the packet. The length indicates
the actual number of LLC data bytes contained in the
message. Any received frame which contains a length
field less than 46 bytes will have the pad field stripped.
Register, and a runt packet of at least 8 bytes has
been received.
Frame Control register, and at least 12-bytes (after
SFD) has been received.
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