Chapter 11. Ethernet Module
11-5
FEC Frame Transmission
from system memory in case of a collision. This improves external bus use and reduces
latency whenever the backoff process results in an immediate retransmission.
See Figure 11-27 on page 11-36 for the following discussion. When the end of the last
transmit buffer in the current frame is reached, the 32-bit frame check sum is appended (if
TxBD[TC] is set) and transmission is disabled (E_TxEN is negated). Following the
transmission of the check sum, the FEC writes the frame status bits into the buffer
descriptor and clears the ready bit. When the end of the current BD is reached but it is not
the last buffer in the frame, then only the ready bit is cleared. Short frames are automatically
padded by the transmit logic.
If the transmit frame length exceeds the value programmed in the maximum frame length
register, the BABT interrupt is asserted. However, the entire frame is transmitted and is not
truncated. See Section 11.5.14, “Maximum Frame Length Register (MAX_FRM_LEN).”
Both buffer and frame interrupts may be generated as determined by the I_MASK register
settings.
Setting the graceful transmit stop bit, X_CNTRL[GTS], pauses transmission. The FEC
transmitter stops immediately if no transmission is in progress. Otherwise it continues
transmission until the current frame finishes normally or terminates with a collision. When
X_CNTRL[GTS] is cleared, the FEC resumes transmission with the next frame.
The FEC transmits bytes lsb first.
11.4.1 FEC Frame Reception
The FEC receiver is designed to work with almost no intervention from the host and can
perform address recognition, CRC, short frame checking, and maximum frame length
checking.
When the FEC receiver is enabled by setting ECNTRL[ETHER_EN] and
R_DES_ACTIVE[24] it immediately starts processing receive frames. Received frame
processing proceeds as follows:
•
When E_RxDV asserts, the receiver first checks for a valid header comprised of a
preamble and start-of-frame delimiter (PA/SDF).
•
If the PA/SFD is valid, it is stripped off and the frame processed further by the
receiver. If a valid PA/SFD is not found, the frame is ignored.
•
In serial mode, the first 16 bit times of E_RxD0 following assertion of E_RxDV
(RENA) are ignored.
•
After the first 16 bit times, the data sequence is checked for alternating I/0.
•
If a 11 or 00 data sequence is detected during bit times 17 to 21, the remainder of the
frame is ignored.
Содержание DigitalDNA ColdFire MCF5272
Страница 1: ...MCF5272UM D Rev 0 02 2001 MCF5272 ColdFire Integrated Microprocessor User s Manual ...
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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 ...
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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 ...