Sundance Spas ST201 - Manual (Page 16)

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Sundance Technology ST201 PRELIMINARY draft 2
are independent of each other in general. A special
case is when a transmit under run occurs. In this
case the current frame being transmitted is the only
frame in the TxFIFO. When a transmit under run
occurs, the ST201 stops TxDMA operation and
generates an interrupt with a TxUnderrun error
flagged in TxStatus. The host system can deter-
mine which is the under run error frame by examin-
ing the current value of TxDMAListPtr. The host
system can assume that all frames in the TxDMAL-
ist ahead of the under run error frame have been
transmitted successfully. To recover from an under
run, the host system should halt the TxDMA Logic
by setting the TxDMAHalt bit, wait until TxDMAIn-
Prog and TxInProg are cleared, then issue a TxRe-
set to reset the under run (TxFIFO and Transmit
MAC). Transmission needs to be enabled (by
TxEnable) again and all transmit-related thresholds
(TxStartThresh in particular) should be restored.
To re-transmit the frame, the host system writes
the value of the under run frame’s TFD into the
TxDMAListPtr register.
FRAME RECEPTION AND RXDMA
The frame RxDMA mechanism is similar to the
TxDMA mechanism. RxDMA is structured around a
linked list of frame descriptors, called RFDs. RFDs
contain pointers to the fragment buffers into which
the ST201 is to place receive data, as shown in
Figure 4.
The RFD format is covered in the Registers and
Data Structures section.
Similar to TFDs, the resulting linked list of RFDs is
referred to as the RxDMAList. One option available
to RxDMA that differs from TxDMA is that the RxD-
MAList can be formed into a ring as shown in Fig-
ure 5. A host system can allocate a number of full
size frame buffers, create a RFD for each one, and
link the RFDs into a circular list. As frames are
Next RFD Ptr.
RxFrameStatus
1st RxDMAFragAddr
1st RxDMAFragLen
2nd RxDMAFragAddr
2nd RxDMAFragLen
Last RxDMAFragAddr
Last RxDMAFragLen
1st Data Frag (Buffer)
2nd Data Frag (Buffer)
Last Data Frag (Buffer)
HOST SYSTEM MEMORY
RFD
FIGURE 4: RxDMA Data Structure