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CHAPTER 3 FUNCTIONAL DESCRIPTION
User’s Manual S14054EJ4V0UM
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(6) Frame check sequence
The frame check sequence field is used to write 32-bit CRC (Cyclic Redundancy Check) to check transfer data.
A VLAN frame is slightly different from the normal frame in structure. In this frame, a 4-byte VLAN header is
inserted immediately after the source address field. The
µ
PD98431 has a VLAN frame detection function. If a
transmit or receive frame is detected as a VLAN frame, it performs packet processing based on the length of this
frame. For details, refer to
3.11 Operation for VLAN Frames
.
Figure 3-4. VLAN Frame Structure
7B
1B
6B
6B
2B
4B
B = byte
VLAN header
PA
SFD
DA
SA
TPID
2B
LEN
42B to 1500B
DATA
2B
TCI
FCS
TPID = Tag Protocol ID, TCI = Tag Control Information
3.4 Transmission Operation
The
µ
PD98431 creates a transmit data frame based on the data given by the host system to the transmit FIFO. If
a collision occurs, it executes a back-off algorithm and re-transmits the data in the transmit FIFO. Status information
for each transmit packet, such as the number of transmit bytes and occurrence of errors, is written to the TSVREG1
and TSVREG2 registers on completion of transmission.
3.4.1 Creating transmit packet
Usually, the transmit data written by the host system to the transmit FIFO includes a destination address and the
last valid data in the data field. The preamble, SFD, and FCS necessary for a transmit packet frame can be
automatically appended by the
µ
PD98431.
(1) Appending preamble and SFD
The
µ
PD98431 always appends a preamble and SFD to the transmit data in the transmit FIFO and outputs the
data to the network.
(2) Appending CRC
The
µ
PD98431 can automatically calculate the value of the CRC and append it to the FCS field at the end of a
packet. Whether a CRC is automatically appended or not is determined by the TXFDQ signal and the setting of
the CRCEN bit and the PADEN bit of the MACC1 register.
If appending a CRC is specified by the TXFDQ signal, a CRC is always automatically appended at the end of a
transmit packet. If appending a CRC is not specified by the TXFDQ signal, whether a CRC is automatically
appended depends on the setting of the CRCEN bit of the MACC1 register. If this CRCEN bit is 0, a CRC is not
automatically appended; if it is 1, a CRC is automatically appended.
Also when the PADEN bit specifying to append PAD is 1, the correct CRC is always automatically appended by
the
µ
PD98431, regardless of the specification by the TXFDQ signal or setting of the CRCEN bit.
If it is specified that a CRC is not automatically appended, the value of the FCS field must be appended at the
end of the transmit data stream that is written to the transmit FIFO by the upper layer.
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