DP8390D datasheet(9/56 Pages) NSC

9 / 56 page

70 Packet Reception (Continued)

Received Packet Aborted if It Hits Boundary Pointer

TLF8582 – 8

Buffer Ring Overflow

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

Boundary Pointer Address reception of the incoming pack-

et will be aborted by the NIC Thus the packets previously

received and still contained in the Ring will not be de-

stroyed

In a heavily loaded network environment the local DMA may

be disabled preventing the NIC from buffering packets from

the network To guarantee this will not happen a software

reset must be issued during all Receive Buffer Ring over-

flows (indicated by the OVW bit in the Interrupt Status Reg-

ister) The following procedure is required to recover

from a Receiver Buffer Ring Overflow

If this routine is not adhered to the NIC may act in an unpre-

dictable manner It should also be noted that it is not per-

missible to service an overflow interrupt by continuing to

empty packets from the receive buffer without implementing

the prescribed overflow routine A flow chart of the NIC’s

overflow routine can be found at the right

Note

It is necessary to define a variable in the driver which will be called

‘‘Resend’’

1 Read and store the value of the TXP bit in the NIC’s

Command Register

2 Issue the STOP command to the NIC This is accom-

plished be setting the STP bit in the NIC’s Command

the NIC

Note

If the STP is set when a transmission is in progress the RST bit may

not be set In this case the NIC is guaranteed to be reset after the

longest packet time (1500 bytes e 12 ms) For the DP8390D (but not

for the DP8390B) the NIC will be reset within 2 microseconds after

the STP bit is set and Loopback mode 1 is programmed

3 Wait for at least 16 ms Since the NIC will complete any

transmission or reception that is in progress it is neces-

sary to time out for the maximum possible duration of an

Ethernet transmission or reception By waiting 16 ms this

is achieved with some guard band added Previously it

was recommended that the RST bit of the Interrupt

Status Register be polled to insure that the pending

transmission or reception is completed This bit is not a

reliable indicator and subsequently should be ignored

4 Clear the NIC’s Remote Byte Count registers (RBCR0

and RBCR1)

TLF8582 – 95

Overflow Routine Flow Chart

5 Read the stored value of the TXP bit from step 1 above

If this value is a 0 set the ‘‘Resend’’ variable toa0and

jump to step 6

If this value is a 1 read the NIC’s Interrupt Status Regis-

ter If either the Packet Transmitted bit (PTX) or Trans-

mit Error bit (TXE) is set to a 1 set the ‘‘Resend’’ vari-

able to a 0 and jump to step 6 If neither of these bits is

set placea1inthe ‘‘Resend’’ variable and jump to step

This step determines if there was a transmission in prog-

ress when the stop command was issued in step 2 If

there was a transmission in progress the NIC’s ISR is

read to determine whether or not the packet was recog-

nized by the NIC If neither the PTX nor TXE bit was set

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