
186
Am79C976
8/01/00
P R E L I M I N A R Y
5
DRTY
Disable Retry. When DRTY is set
to 1, the Am79C976 controller will
attempt only one transmission. In
this mode, the device will not pro-
tect the first 64 bytes of frame
data in the Transmit FIFO from
being overwritten, because auto-
matic retransmission will not be
necessary. When DRTY is set to
0, the Am79C976 controller will
attempt 16 transmissions before
signaling a retry error.
Read/Write accessible.
4
FCOLL
Force Collision. This bit allows
the collision logic to be tested.
The Am79C976 controller must
be in internal loopback for FCOLL
to be valid. If FCOLL = 1, a colli-
sion will be forced during loop-
back
transmission
which will result in a Retry Error.
If FCOLL = 0, the Force Collision
logic will be disabled. FCOLL is
defined after the initialization
block is read.
attempts,
Read/Write accessible.
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 the transmit descriptor.
frame.
When the auto padding logic,
which
is
enabled
APAD_XMT bit (CSR4, bit11),
adds padding to a frame, a valid
FCS field is appended to the
frame, regardless of the state of
DXMTFCS.
by
the
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 the transmit de-
scriptor.
DXMTFCS
is
set
and
This bit was called DTCR in the
LANCE (Am7990) device.
Read/Write accessible.
2
LOOP
Loopback Enable allows the
Am79C976 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:.
Refer to
Loop Back Operation
section for more details.
Read/Write accessible. LOOP is
cleared by H_RESET or
S_RESET and is unaffected by
STOP.
1
DTX
Disable
Am79C976 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
Read/Write accessible.
0
DRX
Disable Receiver results in the
Am79C976 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.
Read/Write accessible.
,-
Bit
Name
Description
31-0
RES
Reserved locations. Written as
zeros and read as undefined.
LOOP
MIIILP
Function
0
0
Normal Operation
0
1
Internal Loop
1
0
External Loop