Motorola MPC8260 User Manual

24-2

MOTOROLA

a random period of time, called a backoff, before trying to retransmit. Once the backoff time
expires, the station waits for silence on the LAN before retransmitting, which is called a
retry. If the frame cannot be sent within 15 retries, an error occurs

10-Mbps Ethernet transmits at 0.8 µs per byte. The preamble plus start frame delimiter is
sent in 6.4 µs. The minimum 10-Mbps Ethernet interframe gap is 9.6 µs and the slot time
is 52 µs.

Setting GSMR[MODE] to 0b1100 selects Ethernet. The SCC performs the full set of IEEE
802.3/Ethernet CSMA/CD media access control and channel interface functions.

The MPC8260 Ethernet controller requires an external serial interface adaptor (SIA) and
transceiver function to complete the interface to the media. 

Although the MPC8260 contains DPLLs that allow Manchester encoding and decoding,
these DPLLs were not designed for Ethernet rates. Therefore, the MPC8260 Ethernet
controller bypasses the on-chip DPLLs and uses the external system interface adaptor on
the EEST instead. The on-chip DPLL cannot be used for low-speed (1-Mbps) Ethernet
either because it cannot properly detect start-of-frame or end-of-frame.

Note that the CPM of the MPC8260 requires a minimum system clock frequency of 24
MHz to support Ethernet.

Shifter

Rx

Receiver

Control

Unit

FIFO

Shifter

Tx

Transmitter

Control

Unit

FIFO

Slot Time

Clock

Generator

Internal Clocks

Control

Registers

TXD

RXD

Peripheral Bus

Data

Data

RCLK

TCLK

and Defer

Counter

RTS = TENA

Random No.

CD = RENA

CTS = CLSN

CTS = CLSN

CD = RENA

REJECT

RSTRT

60x Bus