132
Am79C971
Table 26.
Network Port Configuration.
5
DRTY
Disable Retry. When DRTY is set
to 1, the Am79C971 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 Am79C971 controller will
attempt 16 transmissions before
signaling a retry error.
Read/Write accessible only when
either the STOP or the SPND bit
is set.
4
FCOLL
Force Collision. This bit allows
the collision logic to be tested.
The Am79C971 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 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 is set in TMD1.
frame.
When APAD_XMT (CSR4, bit
11) 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.
The value of ADD_FCS is valid
only when STP is set in TMD1. 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
Am79C971 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
MENDECL, various loopback
modes are defined as follows in
Table 27. Refer to
Loop Back Op-
eration
section for more details.
PORTSEL [1:0]
XX
XX
XX
00
01
10
11
ASEL
(BCR2[1])
1
1
1
0
0
0
0
Link Status
(10BASE-T)
Fail
Pass
Don
’
t Care
Don
’
t Care
Don
’
t Care
Don
’
t Care
Don
’
t Care
MII Status
(BCR32[14])
0
0
1
Don
’
t Care
Don
’
t Care
Don
’
t Care
Don
’
t Care
Network
Port
AUI
10BASE-T
MII
AUI
10BASE-T
GPSI
MII
Table 27.
Loopback Configuration for AUI
LOOP
0
1
INTL
MENDECL
Loopback Mode
Non-loopback
External Loopback
Internal Loopback Include
MENDEC
Internal Loopback
Exclude MENDEC
X
0
X
X
1
1
0
1
1
1