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3.7.2
Rate Adaption of Ethernet Packet Data Streams
The MC92603 supports applications in which the device is used to transmit and receive
IEEE Std. 802.3-2002, PCS, PMA, type 1000BASE-X packet streams. When the MC92603
is being operated in reference clock mode, as described in Section 3.6.2, "Reference Clock
Timing Mode (RCCE = Low)," rate adaption is performed to account for frequency offset
between the transmitter and receiver. In backplane applications (see Section 3.6.2,
"Reference Clock Timing Mode (RCCE = Low)"), rate adaption is accommodated by
adding K28.5 IDLE code groups to, or deleting K28.5 IDLE code groups from, the data
stream to match the incoming data rate to the receiver data rate as defined by its reference
clock frequency. The indiscriminate addition or deletion of K28.5 IDLE code groups from
an 802.3 packet stream would interfere with proper system operation.
The MC92603 compatibility mode (COMPAT input is high) allows for rate adaption using
methods compatible with Ethernet packet streams and does not interfere with proper system
operation. The following are the features of the compatibility mode:
• Context-sensitive rate adaption during receipt of configuration, IDLE, and data code
groups
• Tolerates up to +100 ppm frequency offset
• Supports Jumbo frame lengths of up to 14 Kbytes (if JPACK is high)
• Supports frame bursting
• Internal or external 8B/10B encoding/decoding may be used
• Compatible with IEEE Std. 802.3-2002 Clause 4 specification [4] of media access
control function
• Compatible with Clause 36 of the specification [4] of physical coding sublayer
(PCS) and physical medium attachment (PMA) sublayer function
• Compatible with Clause 37 of the specification [4] of auto-negotiation function
3.7.2.1
Rate Adaption Method
The MC92603 utilizes a FIFO in its receiver to act as an elastic buffer for the receive data
interface. The elastic buffer allows for proper operation of the interface in the presence of
jitter and frequency offset. However, frequency offset will eventually lead to elastic buffer
overrun or underrun. In order to prevent underruns and overruns, one or more code groups
must be added to or deleted from the packet stream.
The MC92603 must determine the proper type of code groups to add or delete and do it at
an appropriate time to ensure compatibility with the packet data streams. The code group
type and timing is determined by the current context of the packet stream. There are three
contexts considered: configuration, idle, and data transmission.
MOTOROLA
Ethernet Compliant Applications Modes (COMPAT = High)
Chapter 3. Receiver
3-19
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