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128
Am79C978
If
ADD_FCS is clear for a particular
frame, no FCS will be generated.
If ADD_FCS is set for a particular
frame, the state of DXMTFCS is
ignored and a FCS will be ap-
pended on that frame by the
transmit circuitry. See also the
ADD_FCS bit in TMD1.
DXMTFCS
is
set
and
This bit was called DTCR in the
LANCE (Am7990) device.
This bit is read/write accessible
only when either the STOP or the
SPND bit is set.
2
LOOP
Loopback Enable allows the
Am79C978 controller to operate
in full-duplex mode for test pur-
poses. The setting of the full-
duplex control bits in BCR9 have
no effect when the device oper-
ates in loopback mode. When
LOOP = 1, loopback is enabled.
In combination with INTL and
MIIILP, various loopback modes
are defined as follows in Table
30.
Refer to
Loopback Operation
section for more details.
This bit is read/write accessible
only when either the STOP or the
SPND bit is set. LOOP is cleared
by H_RESET or S_RESET and
is unaffected by STOP.
1
DTX
Disable
Am79C978 controller not access-
ing the Transmit Descriptor Ring
and, therefore, no transmissions
are attempted. DTX = 0, will set
TXON bit (CSR0 bit 4) if STRT
(CSR0 bit 1) is asserted.
Transmit
results
in
This bit is read/write accessible
only when either the STOP or the
SPND bit is set.
0
DRX
Disable Receiver results in the
Am79C978 controller not access-
ing the Receive Descriptor Ring
and, therefore, all receive frame
data are ignored. DRX = 0 will set
RXON bit (CSR0 bit 5) if STRT
(CSR0 bit 1) is asserted.
This bit is read/write accessible
only when either the STOP or the
SPND bit is set.
CSR16: Initialization Block Address Lower
Bit
Name
Description
31-16
RES
Reserved locations. Written as
zeros and read as undefined.
15-0
IADRL
This register is an alias of CSR1.
These bits are read/write acces-
sible only when either the STOP
or the SPND bit is set.
CSR17: Initialization Block Address Upper
Bit
Name
Description
31-16
RES
Reserved locations. Written as
zeros and read as undefined.
15-0
IADRH
This register is an alias of CSR2.
These bits are read/write acces-
sible only when either the STOP
or the SPND bit is set.
CSR18: Current Receive Buffer Address Lower
Bit
Name
Description
31-16
RES
Reserved locations. Written as
zeros and read as undefined.
15-0
CRBAL
Contains the lower 16 bits of the
current receive buffer address at
which the Am79C978 controller
will store incoming frame data.
These bits are read/write acces-
sible only when either the STOP
or the SPND bit is set. These bits
are unaffected by H_RESET,
S_RESET, or STOP.
CSR19: Current Receive Buffer Address Upper
Bit
Name
Description
31-16
RES
Reserved locations. Written as
zeros and read as undefined.
Table 30.
Loopback Configuration
LOOP
0
0
1
INTL
0
0
0
MIIILP
0
1
0
Function
Normal Operation
Internal Loop
External Loop