AX88196BLF datasheet(22/86 Pages) ASIX

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ASIX ELECTRONICS CORPORATION

AX88196BLF

4.2.3 Filling Packet to Transmit Buffer (Host fill data to memory)

The Remote DMA channel is used to both assembles packets for transmission, and to remove received packets from

the Receive Buffer Ring. It may also be used as a general-purpose slave DMA channel for moving blocks of data or

commands between host memory and local buffer memory. There are two modes of operation, Remote Write and

Remote Read Packet.

Two register pairs are used to control the Remote DMA, a Remote Start Address (RSAR0, RSAR1) and a Remote

Byte Count (RBCR0, RBCR1) register pair. The Start Address Register pair points to the beginning of the block to

be moved while the Byte Count Register pair is used to indicate the number of bytes to be transferred. Full

handshake logic is provided to move data between local buffer memory (Embedded Memory) and a bi-directional

data port.

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 AX88196B 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 AX88196B

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 AX88196B. The advantage of this scheme is that it easily differentiates between buffer full and

buffer empty.

It is full when BNRY

= CPR.

It is empty when BNRY = CPR-1.

AX88196BLF Datasheet(PDF) 22 Page - ASIX Electronics Corporation

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