Freescale Semiconductor POWERPC MPC860T - Manual (Page 22)

3-2 MPC860T (Rev. D) Fast Ethernet Controller Supplement MOTOROLA
PRELIMINARYÑSUBJECT TO CHANGE WITHOUT NOTICE
3.2 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
Þrst 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 speciÞed backoff
procedures and tries retransmitting the frame until the retry limit threshold is reached. The
FEC stores the Þrst 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
Table 3-2. Serial Mode Connections to the External Transceiver
Signal Description FEC Signal Name
Transmit clock TX_CLK
Transmit enable TX_EN
Transmit data TXD0
Collision COL
Receive clock RX_CLK
Receive enable RX_DV
Receive Data RXD0
Unused 860T inputsÑTie to ground RX_ER, CRS, RXD[3:1]
Unused 860T outputsÑIgnore TX_ER, TXD[3:1], MDC, MDIO
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Freescale Semiconductor, Inc.
For More Information On This Product,
Go to: www.freescale.com
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