Motorola MPC8260 - Manual (Page 670)

24-2 MPC8260 PowerQUICC II UserÕs Manual MOTOROLA
Part IV. Communications Processor Module
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.
24.1 Ethernet on the MPC8260
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.
Figure 24-2. Ethernet Block Diagram
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