Am79C973/Am79C975
139
P R E L I M I N A R Y
troller must be in internal loop-
back for FCOLL to be valid. If
FCOLL = 1, a collision will be
forced during loopback transmis-
sion attempts, which will result in
a Retry Error. If FCOLL = 0, the
Force Collision logic will be dis-
abled. FCOLL is defined after the
initialization block is read.
Read/Write accessible only when
either the STOP or the SPND bit
is set.
3
DXMTFCS
Disable Transmit CRC (FCS).
When DXMTFCS is set to 0, the
transmitter will generate and ap-
pend an FCS to the transmitted
frame. When DXMTFCS is set to
1, no FCS is generated or sent
with
the
transmitted
DXMTFCS is overridden when
ADD_FCS and ENP bits are set
in TMD1.
frame.
When APAD_XMT bit (CSR4,
bit11) is set to 1, the setting of
DXMTFCS has no effect on
frames shorter than 64 bytes.
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.
Read/Write accessible only when
either the STOP or the SPND bit
is set.
2
LOOP
Loopback Enable allows the
Am79C973/Am79C975 controller
to operate in full-duplex mode for
test purposes. The setting of the
full- duplex control bits in BCR9
have no effect when the device
operates in loopback mode.
When LOOP = 1, loopback is en-
abled. In combination with INTL
and MIIILP, various loopback
modes are defined as follows in
Table 24.
Refer to
Loop Back Operation
section for more details.
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
Am79C973/Am79C975 controller
not accessing the Transmit De-
scriptor Ring and, therefore, no
transmissions are attempted.
DTX = 0, will set TXON bit (CSR0
bit 4) if STRT (CSR0 bit 1) is as-
serted.
Transmit
results
in
Read/Write accessible only when
either the STOP or the SPND bit
is set.
0
DRX
Disable Receiver results in the
Am79C973/Am79C975 controller
not accessing the Receive De-
scriptor 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.
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.
Read/Write accessible only when
either the STOP or the SPND bit
is set.
Table 24. Loopback Configuration
LOOP
0
0
1
INTL
0
0
0
MIIILP
0
1
0
Function
Normal Operation
Internal Loop
External Loop