DP83916 datasheet(19/96 Pages) NSC

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30 Buffer Management (Continued)

An example would be EOBC e 759 words (1518 bytes) and

the buffer size set to 760 words (1520 bytes) The buffer

can be any size but as long as the EOBC is 1 word less

than the buffer size only one packet will be buffered in that

RBA

Note 1

It is possible to filter out most oversized packets by setting the buff-

er size to 759 words (1518 bytes) EOBC would be set to 758 words

(1516 bytes) for both cases With this configuration any packet over

1518 bytes will not be completely buffered because the packet will

overflow the buffer When a packet overflow occurs a Receive Buff-

er Area Exceeded interrupt (RBAE in the Interrupt Status Register

Section 436) will occur

Note 2

When buffering one packet per buffer it is suggested that the val-

ues in Note 1 above be used Since the minimum legal sized Ether-

net packet is 64 bytes however it is possible to set EOBC as much

as 64 bytes less than the buffer size and still end up with one packet

per buffer

Figure 3-10 shows this ‘‘range’’

345 Beginning of Reception

At the beginning of reception the SONIC-16 checks its in-

ternally stored EOL bit from the previous RXpktlink field for

a ‘‘1’’ If the SONIC-16 finds EOLe1 it recognizes that after

the previous reception there were no more remaining re-

ceive packet descriptors It re-reads the same RXpktlink

field to check if the system has updated this field since the

last reception If the SONIC-16 still finds EOLe1 reception

ceases (See Section 35 for adding descriptors to the list)

Otherwise the SONIC-16 begins storing the packet in the

RBA starting at the Current Receive Buffer Address

(CRBA01) registers and continues until the packet has

completed Concurrent with the packet reception the Re-

maining Buffer Word Count (RBWC01) registers are decre-

mented after each word is written to memory This register

determines the remaining words in the RBA at the end of

reception

346 End of Packet Processing

At the end of a reception the SONIC-16 enters its end of

packet processing sequence to determine whether to ac-

cept or reject the packet based on receive errors and pack-

et size At the end of reception the SONIC-16 enters one of

the following two sequences

Successful reception sequence

Buffer recovery for runt packets or packets with errors

3461 Successful Reception

If the SONIC-16 accepts the packet it first writes 5 words of

descriptor information in the RDA beginning at the address

pointed to by the Current Receive Descriptor Address

(CRDA) register It then reads the RXpktlink field to ad-

vance the CRDA register to the next receive descriptor The

SONIC-16 also checks the EOL bit for a ‘‘1’’ in this field If

EOLe1 no more descriptors are available for the SONIC-

16 The SONIC-16 recovers the address of the current

RXpktlink field (from a temporary register) and generates a

‘‘Receive Descriptors Exhausted’’ indication in the Interrupt

Status register (See Section 347 on how to add descrip-

tors) The SONIC-16 maintains ownership of the descriptor

not writing to the RXpktin

use field Otherwise if

EOLe0 the SONIC-16 advances the CRDA register to the

next descriptor and resets the RXpktin

use field to all

‘‘0’s’’

The SONIC-16 accesses the complete 7 word RDA descrip-

tor in a single block operation

The SONIC-16 also checks if there is remaining space in the

RBA The SONIC-16 compares the Remaining Buffer Word

Count (RBWC01) registers with the static End Of Buffer

Count (EOBC) If the RBWC is less than the EOBC a maxi-

mum sized packet will no longer fit in the remaining space in

the RBA hence the SONIC-16 fetches a resource descrip-

tor from the RRA and loads its registers with the pointer and

word count of the next available RBA

3462 Buffer Recovery for Runt Packets or

Packets with Errors

If a runt packet (less than 64 bytes) or packet with errors

arrives and the Receive Control register has been config-

ured to not accept these packets the SONIC-16 recovers

its pointers back to the original positions The CRBA01 reg-

isters are not advanced and the RBWC01 registers are not

decremented The SONIC-16 recovers its pointers by main-

taining a copy of the buffer address in the Temporary Re-

ceive Buffer Address registers (TRBA01) The SONIC-16

recovers the value in the RBWC01 registers from the Tem-

porary Buffer Word Count registers (TBWC01)

347 Overflow Conditions

When an overflow condition occurs the SONIC-16 halts its

DMA operations to prevent writing into unauthorized memo-

ry The SONIC-16 uses the Interrupt Status register (ISR) to

indicate three possible overflow conditions that can occur

TLF11722 – 17

Range of EOBC e (RXrsrcwc01 b2 to RXrsrcwc01 b32)

FIGURE 3-10 Setting EOBC for Single Packet RBA

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