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CHAPTER 3 FUNCTIONAL DESCRIPTION
User’s Manual S14054EJ4V0UM
32
(3) Occurrence of late collision
If the number of collision windows exceeds the collision window set by the LCOL field of the CLRT register, it is
assumed as a late collision and transmission is aborted. If a late collision occurs, the LCOL bit of the TSVREG1
register is set to 1.
(4) Transmission excessive defer
If transmission is not started 24288-bit time after the
µ
PD98431 tried to start transmission, it is assumed to be an
excessive defer and transmission is aborted. If an excessive defer occurs, the TEDFR bit of the TSVREG1
register is set to 1.
(5) If attempt is made to transmit packet exceeding maximum packet length
If an attempt is made to transmit a packet with a length exceeding the maximum packet length set by the LMAX
register, the
µ
PD98431 continues transmission until the packet length reaches the value of the LMAX register
and aborts transmission if the packet length exceeds the value of the LMAX register. If the HUGEN bit of the
MACC1 register is set to 1, however, the transmit packet length limit by the LMAX register is canceled. If
transmission is aborted under this condition, the TGNT bit of the TSVREG2 register is set to 1.
(6) Occurrence of transmit FIFO underrun
If all the data in the transmit FIFO has been transmitted and transmit data from the system cannot be written to
the FIFO in time, the transmit FIFO underruns and transmission is aborted. If transmission is aborted because
the transmit FIFO underruns, the TUDR bit of the FSVREG register is set to 1.
3.5 Reception Operation
The
µ
PD98431 supplies receive data to the host system from the receive data stream sent from the PHY device.
It detects the preamble and SFD, checks the length field, and executes CRC check. Status information on each
received packet, such as the number of received bytes and occurrence of errors, is written to the RSVREG register
after reception has been completed. This status information can be appended to the data stream output from the
receive FIFO to the host system, depending on the setting of the MACC3 register. In addition, packet filtering can be
also set depending on the address condition.
3.5.1 Detecting preamble and SFD
In the MII mode, each port recognizes data on the RXDn signal as receive data if the RXDVn signal goes high. In
the 10 Mbps serial mode, the port recognizes data on RXDn[0] as receive data if the CRSn signal goes high. When
recognized as receive data, the RXD data, which is serial data, is converted into parallel data inside the
µ
PD98431.
When a preamble pattern (1, 0, 1, 0 ...) is detected from the data converted into parallel data, each port waits until (1,
1) and SFD that follow the preamble pattern are detected. When the SFD is detected, the beginning of a receive
packet is recognized, and storing data in the receive FIFO is started. The preamble and SFD are eliminated from the
receive packet and are not stored in the receive FIFO.
3.5.2 Length field check
Each port of the
µ
PD98431 counts the length of a receive packet, assumes the 2 bytes following the source
address field to be the length field, and checks the data field length. The packet regarded as a VLAN frame regards
the 2 bytes following the VLAN header as a length field. For information on how the VLAN frame is detected, refer to
3.11.1 Detecting VLAN frames
. The result of checking is reported to the host system as status information.
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