Datasheet

5.0 Buffer Management (Continued)
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DP83815
5.2 Transmit Architecture
The following figure illustrates the transmit architecture of the DP83815 10/100 Ethernet Controller.
Figure 5-4 Transmit Architecture
When the CR:TXE bit is set to 1 (regardless of the current state), and the DP83815 transmitter is idle, then DP83815 will
read the contents of the current transmit descriptor into the TxDescCache. The DP83815’s TxDescCache can hold a
single fragment pointer/count combination.
5.2.1 Transmit State Machine
The transmit state machine has the following states:
The transmit state machine manipulates the following internal data spaces:
Inputs to the transmit state machine include the following events:
Transmit Descriptor
Current Tx Desc Ptr
Software/Memory Hardware
Tx Data FIFO
link
cmdsts
ptr
ptr
Tx DMA
cmdsts
Packet
TxHead
link
Tx Desc Cache
txIdle The transmit state machine is idle.
txDescRefr Waiting for the "refresh" transfer of the link field of a completed descriptor from the PCI bus.
txDescRead Waiting for the transfer of a complete descriptor from the PCI bus into the
TxDescriptorCache.
txFifoBlock Waiting for free space in the TxDataFIFO to reach TxFillThreshold.
txFragRead Waiting for the transfer of a fragment (or portion of a fragment) from the PCI bus to the
TxDataFIFO.
txDescWrite Waiting for the completion of the write of the cmdsts field of an intermediate transmit
descriptor (cmdsts.MORE == 1) to host memory.
txAdvance (transitory state) Examine the link field of the current descriptor and advance to the next
descriptor if link is not NULL.
TXDP A 32-bit register that points to the current transmit descriptor.
CTDD An internal bit flag that is set when the current transmit descriptor has been completed, and
ownership has been returned to the driver. It is cleared whenever TXDP is loaded with a
new value (either by the state machine, or the driver).
TxDescCache An internal data space equal to the size of the maximum transmit descriptor supported.
descCnt Count of bytes remaining in the current descriptor.
fragPtr Pointer to the next unread byte in the current fragment.
txFifoCnt Current amount of data in the txDataFifo in bytes.
txFifoAvail Current amount of free space in the txDataFifo in bytes (size of the txDataFifo - txFifoCnt).
CR:TXE Driver asserts the TXE bit in the command register (similar to SONIC).
XferDone Completion of a PCI bus transfer request.
FifoAvail TxFifoAvail is greater than TxFillThreshold.
Obsolete