
Chapter 4
Registers
325
24674
Rev. 3.00
April 2003
AMD-8111 HyperTransport I/O Hub Data Sheet
AMD Preliminary Information
CMD2 is a command-style register. This register is reset by RESET_L.
8
DXMTFCS. Disable Transmit CRC (FCS)
. When DXMTFCS is cleared to 0, the transmitter
generates and appends an FCS to the transmitted frame. When DXMTFCS is set to 1, no FCS is
generated or sent with the transmitted frame. DXMTFCS is overridden when ADD_FCS and ENP bits
are set in the transmit descriptor. When the auto padding logic, which is enabled by the APAD_XMT bit
(CMD2, bit6), adds padding to a frame, a valid FCS
fi
eld is appended to the frame, regardless of the
state of DXMTFCS. If DXMTFCS is set and ADD_FCS is clear for a particular frame, no FCS is
generated. If ADD_FCS is set for a particular frame, the state of DXMTFCS is ignored and a FCS is
appended on that frame by the transmit circuitry. See also the ADD_FCS bit in the transmit descriptor.
This bit was called DTCR in the Am7990 device.
VALBIT0
.
Value bit for byte 1
. Read-write. The value of this bit is written to any bits in the CMD2
register that correspond to bits in the CMD2[6:0] bit map
fi
eld that are set to 1.
APAD_XMT. Auto Pad Transmit
. When set, APAD_XMT enables the automatic padding feature.
Transmit frames are padded to extend them to 64 bytes including FCS. The FCS is calculated for the
entire frame, including pad, and appended after the pad
fi
eld. When the auto padding logic modi
fi
es a
frame, a valid FCS
fi
eld is appended to the frame, regardless of the state of the DXMTFCS bit (CMD2,
bit 8) and of the ADD_FCS bit in the transmit descriptor.
DRTY. Disable Retry
. When DRTY is set to 1, the controller attempts only one transmission. In this
mode, the device does not protect the
fi
rst 64 bytes of frame data in the Transmit FIFO from being
overwritten, because automatic retransmission will not be necessary. When DRTY is cleared to 0, the
controller attempts 16 transmissions before signaling a retry error.
INLOOP. Internal Loopback
. When this bit is set, the transmitter is internally connected to the
receiver so that the data output and control signals are connected internally to the data input and
control signals. The device is forced into full duplex mode so that collisions can not occur. The
INLOOP and EXLOOP bits should not be set at the same time.
EXLOOP. External Loopback
. When this bit is set, the device is forced into full duplex mode so that
collisions can not occur during loop back testing. If the PHY interface output signals are connected
externally to the PHY interface inputs, then transmitted frames will also be received. This connection
can be made by attaching an external jumper or by programming an attached PHY to loopback mode.
The INLOOP and EXLOOP bits should not be set at the same time.
Reserved.
REX_UFLO. Retransmit on Under
fl
ow
. When this bit is set to 1, if the transmitter is forced to abort a
transmission because the transmit FIFO under
fl
ows, the transmitter does not discard the frame.
Instead, it automatically waits until the entire frame has been loaded into the transmit FIFO and then
restarts the transmission process. When this bit is cleared to 0, if the transmitter is forced to abort a
transmission because the FIFO under
fl
ows, the transmitter discards the frame. In either case, the
XmtUnderrunPkts counter is incremented.
Reserved.
7
6
5
4
3
2
1
0
Bits
Description