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CHAPTER 3 FUNCTIONAL DESCRIPTION
User’s Manual S14054EJ4V0UM
28
3.2.9 Operating clock
The
µ
PD98431 requires four operating clocks: transmit clock and receive clock provided by a PHY device, a clock
for register access, and a clock to transfer data with the FIFO bus.
The transmission block of the MAC module reads transmit data from the transmit FIFO in synchronization with the
transmit clock input from the PHY device, generates a transmit frame, and outputs it to the PHY device. In the
meantime, the reception block of the MAC module receives a receive frame in synchronization with the receive clock
input from the PHY device, and writes the data to the host system to the receive FIFO. As the transmit clock and
receive clock, 25 MHz clocks are input for the 100 Mbps operation and 2.5 MHz clocks are input for the 10 Mbps
operation.
The clock to access a register is provided in the HCLK signal. The frequency of this signal can be set in the range
66 to 25 MHz. The MDC signal used for the MII management interface is generated by dividing this HCLK signal.
The clock provided to the FIFO bus is the FCLK signal. Data is written from the host system to the internal FIFOs
of the
µ
PD98431, or read from the FIFOs to the host system in synchronization with this FCLK signal. The frequency
of the FLCK signal is in the range 66 to 25 MHz.
3.3 Frame Format
In an Ethernet network, information is transmitted or received in the form of a packet or frame. The frame format
used for Ethernet consists of preamble (PA), start frame delimiter (SFD), destination address (DA), source address
(SA), type/length field (TYPE/LEN), data field (DATA), and frame check sequence (FCS) (refer to
Figure 3-3
).
The frame length is defined to be 64 bytes at minimum and 1518 bytes at maximum, excluding the preamble and
SFD.
Figure 3-3. Ethernet/IEEE802.3 Frame Structure
7B
1B
6B
6B
2B
4B
B = byte
PA
SFD
DA
SA
/LEN
TYPE
FCS
46B to 1500B
DATA
(1) Preamble and SFD
The preamble and SFD consists of the repetition of “10” for 62 bits and “11”, and indicate the beginning of each
frame.
(2) Destination address
The destination address field indicates the MAC address of the destination. A unicast address, multicast
address, or broadcast address is written to this field.
(3) Source address
The MAC address at the transmission source is written to the source address field.
(4) Type/length field
This field of an Ethernet frame is used to indicate a protocol type. In an IEEE802.3 frame, this field is used to
indicate the length of the data field.
(5) Data field
The data field consists of 46 to 1500 bytes.
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