Fast Ethernet Controller (FEC)
MPC885 PowerQUICC Family Reference Manual, Rev. 2
45-4
Freescale Semiconductor
45.2.2
FEC Frame Transmission
FEC transmissions require almost no host intervention. When the software driver sets the ETHER_EN bit
in the Ethernet control register (ECNTRL) and the X_DES_ACTIVE bit in the CSR TxBD active register
(X_DES_ACTIVE), the FEC is enabled and fetches the first TxBD. If the user has a frame ready to
transmit, a DMA transfer of the transmit data buffers begins immediately.
A 512-bit collision window of transmit data is sent to the transmit FIFO before transmission begins. If the
line is not busy, the MAC transmit logic asserts TX_EN and sends the preamble sequence, the start frame
delimiter (SFD), and then the frame information. If the line is busy, the controller waits for the carrier sense
signal, CRS, to remain inactive for 60 bit times. Transmission begins after an additional 36 bit times (96
bit times after CRS became inactive).
If a collision occurs during the transmit frame, the FEC follows the specified backoff procedures and tries
retransmitting the frame until the retry limit threshold is reached. The FEC stores the first 64 bytes of the
transmit frame in internal RAM so that they do not have to be retrieved from system memory in case of a
collision. This improves bus usage and latency in case the backoff timer output causes a need for an
immediate retransmission.
When the end of the current BD is reached and TxBD[L] is set, the frame check sequence (32-bit CRC) is
appended (if TxBD[TC] = 1) and TX_EN is negated. After the frame check sequence is sent, the FEC
writes the frame status bits into the BD and clears the R bit. When the end of the current BD is reached
and the L bit is not set (a frame consists of multiple buffers), only the R bit is cleared. Short frames are
automatically padded by the transmit logic.
A transmit frame length exceeding the value set for MAX_FRAME_LENGTH in the receive hash register
(R_HASH) generates a babbling transmit interrupt (I_EVENT[BABT] = 1); however, the entire frame is
sent (no truncation). Whether buffer or frame interrupts can be generated is determined by I_MASK
settings.
To pause transmission, set the graceful transmit stop bit, X_CNTRL[GTS]. When GTS is set, the FEC
transmitter stops immediately if no transmission is in progress or continues transmission until the current
frame either finishes or terminates with a collision. The GRA interrupt occurs when the graceful transmit
stop operation completes. When GTS is cleared, the FEC resumes transmission with the next frame.
The FEC transmits bytes lsb first.
Collision
COL
—
Receive clock
RX_CLK
—
Receive enable
RX_DV
CRS_DV
Receive data
RXD0
RXD0
Unused MPC885 inputs—Tie to ground
RX_ER, CRS, RXD[3:1]
RX_ER, RXD[1]
Unused MPC885 outputs—Ignore
TX_ER, TXD[3:1], MDC, MDIO
TXD[1], MDC, MDIO
Table 45-3. Serial Mode Connections to the External Transceiver (continued)
Signal Description
FEC MII Signal Name
FEC MII Signal Name
Summary of Contents for PowerQUICC MPC870
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