DP83902AVF datasheet(17/70 Pages) NSC

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70 Packet Reception (Continued)

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 ST-NIC Thus the packets previ-

ously received and still contained in the Ring will not be

destroyed

In heavily loaded network which cause overflows of the Re-

ceive Buffer Ring the ST-NIC may disable the local DMA

and suspend further receptions even if the Boundary regis-

ter is advanced beyond the Current register 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 Interrupt Status Register) The following procedure is

required to recover from a Receiver Buffer Ring Overflow

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

unpredictable manner It should also be noted that it is not

permissible 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 ST-NIC’s

overflow routine follows

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 ST-NIC

Command Register

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

plished by setting the STP bit in the ST-NIC’s Command

the ST-NIC

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

any transmission or reception that is in progress it is

necessary 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 ST-NIC’s Remote Byte Count registers

(RBCR0 and RBCR1)

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

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

jump to step 6

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

Transmit Error bit (TXE) is set to a 1 set the ‘‘Resend’’

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

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

step 6

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 ST-NIC’s ISR

is read to determine whether or not the packet was rec-

ognized by the ST-NIC If neither the PTX nor TXE bit

was set then the packet will essentially be lost and re-

transmitted only after a time-out takes place in the upper

level software By determining that the packet was lost at

the driver level a transmit command can be reissued to

the ST-NIC once the overflow routine is completed (as in

step 11) Also it is possible for the ST-NIC to defer indef-

initely when it is stopped on a busy network Step 5 also

alleviates this problem Step 5 is essential and should

not be omitted from the overflow routine in order for the

ST-NIC to operate correctly

Overflow Routine Flow Chart

TLF11157 – 57

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