Sundance Spas ST201 - Manual (Page 14)

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Sundance Technology ST201 PRELIMINARY draft 2
TXDMA AND FRAME TRANSMISSION
The TxDMA block transfers frame data from a host
system to the ST201 based on a linked list of frame
descriptors called TFDs. The frame to be transmit-
ted is divided into data fragments (or buffers) within
the host system’s memory. The host system cre-
ates a list of TFDs, also in system memory, where
each TFD contains the memory locations of one or
more fragments of a frame as shown in Figure 2.
The TFD format is covered in the Registers and
Data Structures section.
The resulting linked list of TFDs is referred to as
the TxDMAList, as shown in Figure 3.
After reset, the ST201 MAC transmission is dis-
abled, until the TxEnable bit is set. Any data trans-
ferred by TxDMA Logic into the TxFIFO will stay in
the TxFIFO waiting for the TxEnable bit to be set
enabling transmission. After enabling data trans-
mission, the TxDMA Logic is in the idle state. In the
simple case of a single frame, the host system
must create a TFD within the host system memory.
This TFD must contain the addresses and lengths
of the fragments containing the data to be transmit-
ted. The host system must write zero into TxD-
MANextPtr since this is the only frame. The host
starts the TxDMA Logic by writing the memory
location (a non-zero address) of the TFD into TxD-
MAListPtr register. The TxDMA Logic, if is not in
the TxDMAHalt state, begins transferring data into
the ST201 when:
• The TxFIFO has 16 or more bytes of free space,
AND
• The fragment length is less than or equal to the
amount of free space in the TxFIFO,
OR
• The amount of free space in the TxFIFO is
greater than or equal to the value (in bytes) of
32 * TxDMABurstThresh register
If these conditions hold, the TxDMA Logic fetches
the fragment addresses and fragment lengths from
the TFD and writes them one at a time into the
ST201 registers, which are used to control the data
transfer operations. If the TxDMA Logic transfers
more data than can fit into the TxFIFO, an overrun
will occur.
If the host system disables data transmission (by
resetting the TxDisable bit) while a frame transmis-
sion is in progress, the current frame transmission
will complete before data transmission is disabled.
Next TFD Ptr.
TxFrameControl
1st TxDMAFragAddr
1st TxDMAFragLen
2nd TxDMAFragAddr
2nd TxDMAFragLen
Last TxDMAFragAddr
Last TxDMAFragLen
1st Data Frag (Buffer)
2nd Data Frag (Buffer)
Last Data Frag (Buffer)
HOST SYSTEM MEMORY
TFD
FIGURE 2: TxDMA Data Structure
TFD 1
TFD 2
HOST SYSTEM MEMORY
FIGURE 3: TxDMA List