RM0319 datasheet(165/368 Pages) STMICROELECTRONICS

165 / 368 page

RM0319

Fast Ethernet ports (RMII0/RMII1/MII1)

Doc ID 022640 Rev 3

165/368

the network control, the transmit buffer queue pointer register resets to point to the

beginning of the transmit queue.

Note:

Disabling receive does not have the same effect on the receive queue pointer.

Once the transmit queue is initialized transmit is activated by writing to bit 9, the Transmit

Start bit of the network control register. Transmit is halted when a buffer descriptor with its

used bit set is read or if a transmit error occurs or by writing to the transmit halt bit of the

network control register. Transmission is suspended if a pause frame is received while the

pause enable bit is set in the network configuration register. Rewriting the start bit while

transmission is active is allowed.

Transmission control is implemented with a tx_go variable which is readable in the transmit

status register at bit location 3. The tx_go variable is reset when:

●

Transmit is disabled

●

A buffer descriptor with its ownership bit set is read

●

A new value is written to the transmit buffer queue pointer register

●

Bit 10, tx_halt, of the network control register is written

●

There is a transmit error such as too many retries or a transmit underrun.

To set tx_go write to bit 9, tx_start, of the network control register. Transmit halt does not

take effect until any ongoing transmit finishes. If a collision occurs during transmission of a

multibuffer frame, transmission will automatically restart from the first buffer of the frame.

If a used bit is read mid way through transmission of a multibuffer frame this is treated as a

transmit error. Transmission stops, tx_er is asserted and the FCS will be bad.

If transmission stops due to a transmit error (that is. used bit read mid frame, transmit under-

run or too many retries), the transmit queue pointer resets to point to the beginning of the

transmit queue. Software needs to re-initialize the transmit queue after a transmit error.

If transmission stops due to a used bit being read at the start of the frame, the transmission

queue pointer is not reset and transmit starts from the same transmit buffer descriptor when

the transmit start bit is written.

Table 55.

Transmit buffer descriptor entry

Bit

Function

Word 0

[31:00]

Byte address of buffer

Word 1

[31]

Used - needs to be zero for the MACB to read data from the transmit buffer.

The MACB sets this to one for the first buffer of a frame once it has been successfully

transmitted. Software has to clear this bit before the buffer can be used again. (Note

this bit is only set for the first buffer in a frame - unlike receive where all buffers have

the Used bit set once used.)

[30]

Wrap - marks last descriptor in transmit buffer descriptor list.

[29]

Retry limit exceeded - transmit error detected

[28]

Transmit underrun - occurs either when hresp is not OK or the transmit data could not

be fetched in time or when buffers are exhausted in mid frame.

[27]

Buffers exhausted in mid frame

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