11-4
MCF5272 User’s Manual
FEC Frame Transmission
11.4 FEC Frame Transmission
Figure 11-3. Ethernet Frame Format
The Ethernet transmitter is designed to work with almost no host intervention. As soon as
the driver enables the FEC transmitter by setting ECNTRL[ETHER_EN] and
X_DES_ACTIVE[24], the FEC fetches the first transmit buffer descriptor (TxBD). If the
user has a frame ready to transmit, DMA of the transmit data buffer(s) begins immediately.
A collision window (512 bits) of transmit data is sent as a DMA to the transmit FIFO before
off-chip transmission begins.
When the transmit FIFO contains 512 bits of data, the FEC asserts E_TxEN and starts
transmitting the preamble sequence, the start-of-frame delimiter, and then the frame data.
However, if the line is busy, the controller defers the transmission (carrier sense is active).
Before transmitting, the controller waits for carrier sense to become inactive. When carrier
sense goes inactive, the controller waits to verify that it stays inactive for 60 bit times. If so,
the transmission begins after waiting an additional 36 bit times (96 bit times after carrier
sense originally went inactive).
If a collision occurs during the transmit frame, the FEC follows the specified backoff
procedures and attempts to retransmit the frame until the retry limit is reached. The FEC
stores the first 64 bytes of the transmit frame in internal RAM, so they need not be retrieved
Receive enable
E_RxDV
Receive data
E_RxD[0]
Unused, configure as PB14
E_RxER
Unused input, tie to ground
E_CRS
Unused, configure as PB[13:11]
E_RxD[3:1]
Unused output, ignore
E_TxER
Unused, configure as PB[10:8]
E_TxD[3:1]
Unused, configure as PB15
E_MDC
Input after reset, connect to ground
E_MDIO
Table 11-2. Seven-Wire Mode Configuration (Continued)
Signal Description
MCF5272 Pin
Preamble
Start Frame
Data
Delimiter
Destination
Address
Type/
Length
Source
Address
Frame Check
Sequence
7 Bytes
1 Byte
6 Bytes
6 Bytes
2 Bytes
46–1500 Bytes
4 Bytes
Stored in Receive Buffer
Stored in Transmit Buffer
NOTE: Short Tx frames are padded automatically by the MCF5272
(Pads)
Содержание DigitalDNA ColdFire MCF5272
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Страница 90: ...2 42 MCF5272 User s Manual Exception Processing Overview ...
Страница 96: ...3 6 MCF5272 User s Manual MAC Instruction Execution Timings ...
Страница 158: ...5 46 MCF5272 User s Manual Motorola Recommended BDM Pinout ...
Страница 184: ...7 12 MCF5272 User s Manual Interrupt Controller Registers ...
Страница 338: ...13 44 MCF5272 User s Manual Application Examples ...
Страница 414: ...18 6 MCF5272 User s Manual PWM Programming Model ...
Страница 452: ...19 38 MCF5272 User s Manual Power Supply Pins ...
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Страница 492: ...21 10 MCF5272 User s Manual Non IEEE 1149 1 Operation ...
Страница 548: ...INDEX Index 12 MCF5272 User s Manual ...