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Backplane Applications Modes (COMPAT = Low)
Overrun may occur when an appropriate code group to remove cannot be identified. In this
situation, one code group is replaced in the data stream (to report the error), and a second
data code group is removed from the packet stream. The received packet stream continues
normally thereafter. The MAC sublayer should detect the repeated code group as a CRC
error.
Underrun may occur when an appropriate code group to insert cannot be identified. In this
situation, one data code group is repeated in the packet stream. The received packet stream
continues normally thereafter. The repeated code group is reported as an underrun error as
described in Section 3.7.1.2, "TBI Operation," and Table 3-11. The MAC sublayer should
detect the repeated code group as a CRC error.
3.7.3.1
Jumbo Frame Considerations
It is recommended if Jumbo frames are to be supported, that the 'recovered clock mode'
(RCCE is high) be used to reduce power and latency. It should be noted that it is possible
for the MC92603 transmit controller to shorten the IPG by two to achieve even/odd
alignment and for the receiver to remove four code groups in the IPG to perform rate
adaption if in 'reference clock mode.' This means that if the IPG is only 12 code groups
originally, then the receiver could present an IPG as small as 6 code groups (with maximum
frequency offset).
If at least eight code groups are required in the IPG in the user's application the alternatives
are the following:
• Run in 'recovered clock mode'
• Furnish an initial IPG of at least 14 code groups
• Ensure that the total of the frequency offset and packet length does not require more
than two code groups to be deleted
• Maximum number of code groups dropped per packet = frequency offset ∗ packet
length
3.8
Backplane Applications Modes (COMPAT = Low)
3.8.1
Byte Mode (Uncoded Data)
Backplane byte mode is enabled by negating both the TBIE and COMPAT signals low. The
received data is a byte (8 bits) of data when in this mode. The internal 8B/10B decoder is
used to decode the 8-bit data from the 10-bit code group received. The received byte is
presented on the RECV_x_7–RECV_x_0 signals.
The RECV_x_ERR is negated low when the receiver is operating normally and asserted
high when received data contains an error or the receiver is in an error state. When
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MC92603 Quad Gigabit Ethernet Transceiver Reference Manual
MOTOROLA
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