AX88796 datasheet(28/71 Pages) ASIX

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ASIX ELECTRONICS CORPORATION

AX88796 L

3-in-1 Local Bus Fast Ethernet Controller

REMOTE WRITE

A Remote Write transfer is used to move a block of data from the host into local buffer memory. The Remote

DMA will read data from the I/O port and sequentially write it to local buffer memory beginning at the Remote

Start Address. The DMA Address will be incremented and the Byte Counter will be decremented after each

transfer. The DMA is terminated when the Remote Byte Count Register reaches a count of zero.

4.2.4 Removing Packets from the Ring (Host read data from memory)

REMOTE READ

A Remote Read transfer is used to move a block of data from local buffer memory to the host. The Remote DMA

will sequentially read data from the local buffer memory, beginning at the Remote Start Address, and write data

to the I/O port. The DMA Address will be incremented and the Byte Counter will be decremented after each

transfer. The DMA is terminated when the Remote Byte Count Register reaches zero.

Packets are removed from the ring using the Remote DMA or an external device. When using the Remote DMA.

The Boundary Pointer can also be moved manually by programming the Boundary Register. Care should be

taken to keep the Boundary Pointer at least one buffer behind the Current Page Pointer. The following is a

suggested method for maintaining the Receive Buffer Ring pointers.

1. At initialization, set up a software variable (next_pkt) to indicate where the next packet will be read. At the

beginning of each Remote Read DMA operation, the value of next_pkt will be loaded into RSAR0 and

RSAR1.

2. When initializing the AX88796 set:

BNRY = PSTART

CPR = PSTART + 1

Next_pkt = PSTART + 1

3. After a packet is DMAed from the Receive Buffer Ring, the Next Page Pointer (second byte in AX88796

receive packet buffer header) is used to update BNRY and next_pkt.

Next_pkt = Next Page Pointer

BNRY = Next Page Pointer - 1

If BNRY < PSTART then BNRY = PSTOP – 1

Note the size of the Receive Buffer Ring is reduced by one 256-byte buffer; this will not, however, impede the

operation of the AX88796. The advantage of this scheme is that it easily differentiates between buffer full and

buffer empty: it is full if BNRY = CPR; empty when BNRY = CPR-1.

STORAGE FORMAT FOR RECEIVED PACKETS

The following diagrams describe the format for how received packets are placed into memory by the local DMA

channel. These modes are selected in the Data Configuration Register and setting the CPU[1:0] pins for ISA,

80186, MC68K or MCS-51 mode.

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