am79c940 Advanced Micro Devices, am79c940 Datasheet - Page 57

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

USER ACCESSIBLE REGISTERS
The following registers are provided for operation of the
MACE device. All registers are 8-bits wide unless other-
wise stated. Note that all reserved register bits should
be written as zero.
Receive FIFO (RCVFIFO)
This register provides a 16-bit data path from the Re-
ceive FIFO. Reading this register will read one word/
byte from the Receive FIFO. The RCVFIFO should only
be read when Receive Data Transfer Request
(RDTREQ) is asserted. If the RCVFIFO location is read
before 64-bytes are available in the RCVFIFO (or
12-bytes in the case that LLRCV is set in the Receive
Frame Control register), DTV will not be returned. Once
the 64-byte threshold has been achieved and RDTREQ
is asserted, the de-assertion of RDTREQ does not pre-
vent additional data from being read from the RCVFIFO,
but indicates the number of additional bytes which are
present, before the RCVFIFO is emptied, and
subsequent reads will not return DTV (see the FIFO
Sub-System section for additional details). Write opera-
tions to this register will be ignored and DTV will not be
returned.
Byte transfers from the RCVFIFO are supported, and
will be fully aligned to the target memory architecture,
defined by the BSWP bit in the BIU Configuration Con-
trol register. The Byte Enable inputs (BE1–0) will define
which half of the data bus should be used for the trans-
fer. The external host/controller will be informed that the
last byte/word of data in a receive frame is being read
from the RCVFIFO, when the MACE device asserts the
EOF signal.
Transmit FIFO (XMTFIFO)
This register provides a 16-bit data path to the Transmit
FIFO. Byte/word data written to this register will be
placed in the Transmit FIFO. The XMTFIFO can be writ-
ten at any time the Transmit Data Transfer Request
(TDTREQ) is asserted. The de-assertion of TDTREQ
does not prevent data being written to the XMTFIFO, but
indicates the number of additional write cycles which
can take place, before the XMTFIFO is filled, and
subsequent writes will not return DTV (see the FIFO
Sub-System section for additional details). Read opera-
tions to this register will be ignored and DTV will not be
returned.
Byte transfers to the XMTFIFO are supported, and ac-
cept data from the source memory architecture to en-
sure the correct byte ordering for transmission, defined
by the BSWP bit in the MAC Configuration Control regis-
ter. The Byte Enable inputs (BE1–0) will define which
half of the data bus should be used for the transfer. The
RCVFIFO [15–0]
XMTFIFO [15–0]
(REG ADDR 0)
(REG ADDR 1)
Am79C940
use of byte transfers have implications on the latency
time provided by the XMTFIFO (see the FIFO Sub-
System section for additional details). The external host/
controller must indicate the last byte/word of data in a
transmit frame is being written to the XMTFIFO, by as-
serting the EOF signal.
Transmit Frame Control (XMTFC)
The Transmit Frame Control register is latched inter-
nally on the last write to the Transmit FIFO for each indi-
vidual packet, when EOF is asserted. This permits
automatic transmit padding and FCS generation on a
packet-by-packet basis.
Bit
Bit 7
Bit 6–4 RES
Bit 3
DRTRY
RES
DRTRY
DXMTFCS Disable Transmit FCS. When
Name
RES
RES
Description
Disable Retry. When DRTRY is
set, the MACE device will provide
a single transmission attempt for
the packet, all further retries will
be suspended. In the case of a
collision during the attempt, a
Retry Error (RTRY) will be re-
ported in the Transmit Status.
With DRTRY cleared, the MACE
device will attempt up to 15 re-
tries (16 attempts total) before in-
dicating a Retry Error. DRTRY is
cleared by activation of the RE-
SET pin or SWRST bit. DRTRY is
sampled during the transmit
process when a collision occurs.
DRTRY should not be changed
whilst data remains in the Trans-
mit FIFO since this may cause an
unpredictable retry response to a
collision. Once the Transmit
FIFO is empty, DRTRY can be
reprogrammed.
Reserved. Read as zeroes. Al-
ways write as zeroes.
DXMTFCS = 0 the transmitter
will generate and append an FCS
to the transmitted frame. When
DXMTFCS = 1, no FCS will be
appended to the transmitted
frame, providing that APAD XMT
is also clear. If APAD XMT is set,
the calculated FCS will be ap-
pended to the transmitted mes-
sage regardless of the state of
DXMTFCS.
DXMTFCS for each frame is pro-
grammed when EOF is asserted
to transfer the last byte/word for
the transmit packet to the FIFO.
DXMTFCS
DXMTFCS
RES
is
The
(REG ADDR 2)
RES
cleared
value
AMD
APAD XMT
57
by
of