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| 型號： | 78Q8430EBST#DB |
| 廠商： | Maxim Integrated Products |
| 文件頁數(shù)： | 20/88頁 |
| 文件大?。?/td> | 0K |
| 描述： | EVAL BOARD 78Q8430 |
| 標準包裝： | 1 |
| 系列： | * |
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DS_8430_001
78Q8430 Data Sheet
Rev. 1.2
27
The purpose of a static transmit QUE is to allow the host to configure a frame that will need to be
transmitted multiple times or transmitted at a later time without any interaction with the host. Transmit
QUE2 and QUE5 are static QUEs. Transmit QUE2 is best suited for MAC control pause frames as it can
be triggered to transmit by a main buffer watermark. Transmit QUE5 is best suited to Host Not
Responding (HNR) frames as it can be triggered to transmit by a host interrupt timeout.
When the MAC transmitter is idle and ready to transmit a frame, it determines which QUE to read from on
a priority basis. The lowest numbered QUE containing data that needs to be transmitted is selected by
the MAC, which means when more than one transmit QUE is ready, the one with the lowest number
always gets priority.
6.3
Host Interface
6.3.1
Reading Receive Data
The status of the frame at the top of the receive FIFO can be obtained by reading the Receive Packet
Status Register (RPSR). The 16 LSBs of the RPSR contain a count of the total number of bytes that
have entered the receive FIFO for this frame. A value of zero means that there are no new frames in the
receive FIFO. As frame bytes enter the FIFO, the count value is incremented. However, the count value
does not decrease the bytes read out of the read FIFO such that the final value will always be the final
frame size.
the DONE bit is set indicating that the count value contained in the total bytes field now contains the final
size in bytes of the frame and the error status and classification fields now contain the final frame status.
When the DONE bit is asserted, this also indicates that the status for this frame has been removed from
the receive status FIFO and future reads of the RPSR will refer to the next frame in the receive FIFO,
even if all of the data for the current frame has not been retrieved.
The frame data is read from the receive FIFO 32 bits at a time by successive reads to the Receive Data
Register (RDR). If the frame length is not an even multiple of 4 bytes then the final read of the RPDR
register for that frame will be padded with zeros.
6.3.2
Writing Transmit Data
A transmit QUE is initialized by writing to its Packet Control Word Register (PCWR). This will assign an
ID to the frame and select various transmission options. The frame size must then be set by writing to the
QUE Packet Size Register (PSZR). Transmit data is then written to the transmit FIFO 32 bits at a time via
successive writes to the Transmit Data Register (TDR).
If more bytes are written to the TDR than indicated n the PSZR, the excess bytes are ignored. Writes to
the TDR past the end of the frame, however, will trigger a transmit FIFO overrun interrupt condition.
Similarly, if a new frame is initialized by a write to the PCWR before the frame length counter is expired, a
transmit FIFO under-run interrupt condition will result and the previous frame will be aborted. If there is
any question, the PSZR can be queried for the remaining number of bytes expected in the previous frame
before a new frame is initialized.
In the event that the host wishes to terminate a frame early without triggering an under-run interrupt and
aborting the frame, or if the size of the frame is not initially known, the PSZR can be rewritten at any time
before the end of the frame’s transmission. As an example, no matter what the current value of the PSZR
is, if it is written with a value of one then the next write to the TDR will add one byte to the completed
frame. Conversely, if the frame byte counter is about to expire then writing a larger value to the PSZR will
extend the frame. It is an error to write a value of zero to the PSZR and the circuit behavior in this case is
undefined.
As each frame egresses the transmit FIFO, its status is placed in the transmit status FIFO. Transmit
frame status is recovered by reading the Transmit Packet Status Register (TPSR). The Packet ID field
from the PCWR is also placed in the TPSR such that the status can be associated with the exact frame to
which it belongs.
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