Figure 19-5. Transmit Packet Structure (Excluding Vlan Tagged And Control Packets) - IBM PowerPC 405GP User Manual

• The IPG counter, which controls transmit deferral during the IPG between
back-to~back
transmits,
is started when transmit activity for the first packet ends, instead of when transmit and carrier
activity end. '
• SQE test is not performed.
• Collision indication is ignored.
19.3.3.7 Packet Content Configuration Options
EMAC can modify the content of the packet coming from MAL before issuing it to the Mil interface.
Figure 19-5 illustrates possible changes in the transmit packet format.
Packet as delivered to EMAC by MAL
DA
SA
LengthfType Data
(optional)
Packet as delivered by EMAC on the Mil
Preamble
SPD DA
SA
LengthfType
Data
Preamble - combination of alternative zeroes and ones (7 bytes)
SPD - Start Of Packet delimiter (1 byte)
DA - Destination Address (6 bytes)
SA - Source Address (6 bytes)
LengthfType - length of data field / type definition (Ethernet) (2 bytes)
Data - data field including padding if needed in short packets
FCS
(optional)
Padding
(if needed)
Padding (optional) - needed to fulfill the packet up to the minimum allowed size
FCS - Cycle Redundancy Check (4 bytes)
FCS
Figure 19·5. Transmit Packet Structure (Excluding VLAN Tagged and Control Packets)
The following options can be set for each packet, and can be provided as a part of command write
transactions from MAL (see "MAL TX Descriptor Control/Status Field" on page 19-7).
• Perform data padding automatically for short transmit packets. EMAC pads the transmit packet with
extra bytes between the data and the FCS field to reach a total length of 64 bytes (including FCS).
This feature is supported only when the packet coming from the Transmit FIFO does not contain
the FCS. Automatic padding enables software to avoid sending padding as a part of the packet
data field, and therefore, reduces the amount of data transferred on the system bus during the
transmission of the short packet.
• Source address insertion. When chosen, EMAC adds the source address (SA) field to the
transmitted packet. EMAC uses the content of the Individual Address High and Individual Address
Low registers for the source address value. This mode is mutually exclusive with source address
replacement mode.
19-10 PPC405GP User's Manual Preliminary