Sundance Technology
ST201
PRELIMINARY draft 2
received and transferred by RxDMA, a RxDMA- Complete interrupt will be generated for each frame.
HOST SYSTEM MEMORY
RFD 1
RFD 2
RFD n
FIGURE 5: RxDMA List Shown in Ring
Configuration
The host system must create a RxDMAList and the associated buffers prior to reception of a frame. One approach calls for the host system to allocate a block of full size (i.e. large enough to hold a max- imum size Ethernet frame of 1518 bytes) frame buffers in system data space and create RFDs that point to them. Another approach is for the host sys- tem to request the buffers from the protocol ahead of time.
After reset, the ST201 receive function is disabled. Once the RxEnable bit is set, frames will be received according to the matching mode pro- grammed in ReceiveMode register. Reception can be disabled by setting the RxDisable bit. If set while a frame is being received, RxDisable only takes effect after the active frame reception is fin- ished. The receive function begins with the RxDMA Logic in the idle state. The RxDMA Logic will begin processing a RxDMAList as soon as a
tions. Similar to TxDMA, the RxDMA Logic can be controlled by the RxDMAHalt and RxDMAResume bits. The host system should set the RxDMAHalt bit before modifying the list pointers in the RxD- MAList. The RxDMA Logic will return to the idle state when the RxDMAListPtr register is zero.
For RxDMA lists configured as a ring, the host sys- tem should clear the RxDMAComplete bit within the ReceiveFrameStatus field of the RFD from which the host system has finished reading data. If RxDMAPollPeriod is zero the host system should also issue a RxDMAResume in case the ST201 has halted due to detection of a set RxDMACom- plete bit within the ReceiveFrameStatus field of the next RFD in the ring. If the ST201 fetches a RxD- MAListPtr for a RFD that has already been used (a RFD in which the RxDMAComplete bit is set in ReceiveFrameStatus), the RxDMA Logic will either assert an implicit RxDMAHalt or, if the RxDMAPoll- Period register is set to a
The operation for adding RFDs into the RxDMAList starts with halting the RxDMA Logic by setting the RxDMAHalt bit within the DMACtrl register. The host system then updates RxDMANextPtr in the last RFD in the RxDMAList to point at the new RFD. The host system will also need to read the RxDMAListPtr, and if it was zero, write the address of the just added RFD into RxDMAListPtr and set the RxDMAResume bit within the DMACtrl register to
The ST201 can be configured to generate a RxD- MAComplete interrupt when RxDMA completes a frame reception. In response to a RxDMAComplete interrupt, the host system must examine the ReceiveFrameStatus field in the RFD of the received frame to determine the size of the frame and whether there were any errors. The host sys- tem must then copy the frame out of the receive buffers, if needed.
In general, when the host system enters its inter- rupt service routine, multiple frames may have been transferred by RxDMA. The host system can read RxDMAListPtr to determine which RFDs in the list have been used. The host system begins at the head of the RFD list, and traverses the list until it reaches the RFD whose address matches RxD- MAListPtr. However, since I/O operations are costly, it is more efficient to use the RxDMACom- plete bit in each RFD to determine which frames have been transferred by RxDMA.
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