IDT Ethernet Interfaces
Address Recognition Logic
79RC32438 User Reference Manual
11 - 7
November 4, 2002
Notes
Ethernet FIFO Transmit Threshold Register
Figure 11.3 Ethernet FIFO Transmit Threshold Register (ETH[0|1]FIFOTT)
Address Recognition Logic
Ethernet frames contain the address of the source and of the destination. Both addresses are 48-bits in
length and are typically represented as a series of six bytes separated by hyphens in the order that they are
transmitted (left to right) on the wire. The bits within bytes are transmitted on the wire from right to left (that
is, least significant bit first and most significant bit last). These addresses are referred to as Medium Access
Control (MAC) addresses.
An example of a MAC address, and the order in which its bits are transmitted on the wire, is shown in
Figure 11.4.
Figure 11.4 Representation of MAC Address
Based on the destination address in a received Ethernet frame, the address recognition logic deter-
mines if the packet should be accepted by the Ethernet interface and passed to the DMA Controller or if the
frame should be rejected.
TTH
Description:
Transmit Threshold.
This field contains the number of words which must be present in the
Ethernet output FIFO in order for the MAC to start transmitting the frame. The MAC will begin
transmitting the frame before the threshold is reached if the last byte of a packet is written into
the FIFO.
For correct operation of the Ethernet interface, this field should be set to a value greater than or
equal to the number of words programmed in the COLWIN field in the ETH[0|1]CLRT register
minus two words (that is, do not count SFD or preamble).
Care should be exercised in determining the value selected for the transmit threshold, since mis-
configuration could lead to a deadlock. For example, if this field is set to 125 words and the trans-
mit FIFO contains 120 words, then further DMA transmit requests will not be generated since the
remaining space in the transmit FIFO is not at least 16 words. In addition, the Ethernet MAC will
not start transmitting the frame since the transmit threshold has not been reached, thus resulting
in a deadlock.
Initial Value:
Undefined
Read Value:
Previous value written
Write Effect:
Modify value
ETH[0|1]FIFOTT
0
31
TTH
25
7
0
Byte 0
AC
Byte 1
DE
Byte 2
48
Byte 3
00
Byte 4
00
Byte 5
80
Binary Representation:
0 0 1 1 0 1 0 1
0 1 1 1 1 0 1 1
0 0 0 1 0 0 1 0
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 1
Universally/Locally Administered Address Bit (2nd bit on the wire)
Individual/Group Address Bit (1st bit on the wire)
MAC Address:
AC-DE-48-00-00-80