DP83901A datasheet(13/62 Pages) NSC

13 / 62 page

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 SNIC Thus the packets previously

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

stroyed

In heavily loaded network which cause overflows of the Re-

ceive Buffer Ring the SNIC may disable the local DMA and

suspend further receptions even if the Boundary register 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 SNIC 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 SNIC’s

overflow routine can be found on the next page

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 SNIC’s

Command Register

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

plished by setting the STP bit in the SNIC’s Command

the SNIC

3 Wait for at least 16 ms Since the SNIC will complete

any transmission or reception that is in progress it is

necessary to time out for the maximum possible dura-

tion 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 SNIC’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 toa0and

jump to step 6

If this value is a 1 read the SNIC’s Interrupt Status Reg-

ister 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 SNIC’s ISR is

read to determine whether or not the packet was recog-

nized by the SNIC If neither the PTX nor TXE bit was

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

mitted only after a time-out takes place in the upper lev-

el software By determining that the packet was lost at

the driver level a transmit command can be reissued to

the SNIC once the overflow routine is completed (as in

step 11) Also it is possible for the SNIC to defer indefi-

nitely 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

SNIC to operate correctly

6 Place the SNIC in either mode 1 or mode 2 loopback

This can be accomplished by setting bits D2 and D1 of

the Transmit Configuration Register to ‘‘01’’ or ‘‘10’’

respectively

7 Issue the START command to the SNIC This can be

accomplished by writing 22H to the Command Register

This is necessary to activate the SNIC’s Remote DMA

channel

8 Remove one or more packets from the receive buffer

ring

9 Reset the overwrite warning (OVW overflow) bit in the

Interrupt Status Register

10 Take the SNIC out of loopback This is done by writing

the Transmit Configuration Register with the value it

contains during normal operation (Bits D2 and D1

should both be programmed to 0)

11 If the ‘‘Resend’’ variable is set to a 1 reset the ‘‘Re-

send’’ variable and reissue the transmit command This

is done by writing a value of 26H to the Command Reg-

ister If the ‘‘Resend’’ variable is 0 nothing needs to be

done

Note 1

If Remote DMA is not being used the SNIC does not need to be

started before packets can be removed from the receive buffer ring Hence

step 8 could be done before step 7

Note 2

When the SNIC is in STOP mode the Missed Talley Counter is

disabled

Received Packet Aborted If It Hits Boundary

TLF10469 – 11

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