DM9008F datasheet(41/68 Pages) ETC

41 / 68 page

DM9008

ISA/Plug & Play Super Ethernet Contoller

Final

Version :DM9008-DS-F02

June 14, 2000

Buffer Ring Overflow

If the Buffer Ring has been filled and the DMA reaches the

Boundary Pointer Address, reception of the incoming packet

will be aborted by the ENC. Thus, the packets previously

received and still contained in the Ring will not be erased.

In a heavily loaded network environment, the local DMA may

be disabled, preventing the DM9008 from buffering packets

from the network. To guarantee this will not happen, a software

reset must be issued during all Receive Buffer Ring overflows

(indicated by the OVW bit in the ISR). The following procedure

is used to recover from a Receiver Buffer Ring Overflow.

1. Issue

the

STOP

mode

command

(Command

DM9008

may not

immediately

enter

the STOP mode. If it is currently processing a packet,

ENC will enter STOP mode only after finishing the

packet. DM9008 indicates that it has entered STOP

mode by setting the RST bit in theInterrupt Status

2. Clear the Remote Byte Counter Registers (RBCR0,

RBCR1). The DM9008 requires these registers to be

cleared before it sets the RST bit.

3. Poll the Interrupt Status Register for the RST bit.

When set, the ENC is in Stop mode.

4. Place DM9008 in LOOPBACK (mode 1 or 2) by writing

02H or 04H to the Transmit Configuration Register.

This step is required to properly enable DM9008 to be

used on an active network.

5. Issue

the

START

mode

command

(Command

because DM9008 is in LOOPBACK.

6. Remove at least one packet from the Receive Buffer

Ring to accommodate additional incoming packets.

7. Take DM9008 out of LOOPBACK by programming the

Transmit

Configuration

to its

original

and

resume normal operation.

Note: If the Remote DMA channel is not used, step 6

may be eliminated and packets can be removed

from the Receive Buffer Ring after step 1. This will

reduce or eliminate the polling time incurred in

step 3.

End of Packet Operations

At the end of the packet, DM9008 determines whether the

received packet is to be accepted or rejected. It branches

either to a routine to store Buffer Header, or to another routine

that recovers the buffers used to store the packet.

Successful Reception

If the packet is successfully received, the DMA is restored to

the first buffer used to store the packet (pointed to by CURR).

The DMA then stores the Receive Status, a pointer indicating

where the next packet will be stored, and the number of

received bytes. Note that the remaining bytes in the last buffer

are discarded and reception of the next packet begins on the

next empty 256 byte buffer boundary. CURR is then initialized

to the next available buffer in the Buffer Ring.

Buffer Recovery for Rejected Packets

If the packet is a runt packet or contains CRC or Frame

Alignment errors, it is rejected. The buffer management logic

resets the DMA back to the first buffer page used to store the

packet (pointed to by CURR), recovering all buffers that had

been used to store the rejected packet. This operation will not

be performed if DM9008 is programmed to accept either runt

packets or packets with CRC or Frame Alignment errors. The

received CRC is always stored in buffer memory after the last

byte of data for the packet is received.

Error Recovery

If the packet

is rejected, DM9008 resotres DMA by

reprogramming the DMA starting address pointed to by CURR.

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