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CHAPTER 3 FUNCTIONAL DESCRIPTION
User’s Manual S14054EJ4V0UM
30
(3) Appending PAD
If the data length of one packet written to the transmit FIFO is less than the minimum frame length of 64 bytes
(68 bytes in the case of a VLAN frame), the
µ
PD98431 automatically appends PAD to extend the data length to
the minimum frame length. If the PADEN bit of the MACC1 register is set to 1 or if the appending CRC is
specified by the TXFDQ signal, the PAD auxiliary function is enabled.
Whether PAD is appended to a VLAN frame depends on the APD and VPD bits of the MACC2 register. For
details, refer to
3.11 Operation for VLAN Frames
.
3.4.2 Starting packet transmission
Transmitting a packet to the network is started if either of the following conditions is satisfied:
•
If the quantity of the data in the transmit FIFO exceeds a predetermined threshold level
•
If an attribute indicating the end of data is given to TXFDQ or FDQ at the same time as the transmit data that is
to be written to the transmit FIFO
The transmission start threshold level is written to the TFDWL field of the TFIC register.
If transmission is started during half-duplex operation, carrier sense is first performed to check to see if any other
station on the network is transmitting data. If no other station is transmitting data, and if the gap between packets
that was determined in advance when the previous data transmission was completed, or inter-packet gap is satisfied,
outputting a transmit data stream to the PHY device is immediately started. If any other station is transmitting data,
the
µ
PD98431 waits until the communication ends and postpones its transmission until the inter-packet gap is
satisfied.
Because the carrier sense is ignored during full-duplex operation, outputting transmit data to the PHY device is
started as soon as the transmission start condition has been satisfied.
3.4.3 Setting inter-packet gap
With the
µ
PD98431, the inter-packet gap (IPG) must be set by using a register. This gap may be back-to-back
IPG that is applied when data is successively transmitted with the minimum gap width, or non-back-to-back IPG that
is used in the other cases.
(1) Back-to-back IPG
The back-to-pack IPG is set by the IPGT register. The back-to-back IPG is used to transmit data successively
with the minimum gap width after station transmission. At this time, the minimum gap width is written to the IPGT
register. If the next transmission start condition is satisfied within the time of the minimum gap width after
transmission of the station has been completed, the
µ
PD98431 assumes back-to-back transmission and
immediately outputs the next transmit data stream in the PHY device after the IPG time set in the IPGT register
has elapsed.
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