DP83916 datasheet(18/96 Pages) NSC

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30 Buffer Management (Continued)

All RRA registers are concatenated with the URRA register

for generating the full 23-bit address

The resource descriptors that the system writes to the RRA

consists

four

fields

(1)

RXrsrcbuff

ptr0

(2)

RXrsrcbuff

ptr1

(3)

RXrsrcbuff

wc0

and

(4)

RXrsrcbuff

wc1 The fields must be contiguous (they can-

not straddle the end points) and are written in the order

shown in

Figure 3-8 The ‘‘0’’ and ‘‘1’’ in the descriptors

denote the least and most significant portions for the Buffer

Pointer and Word Count The first two fields supply the

23-bit starting location of the Receive Buffer Area (RBA)

and the second two define the number of 16-bit words that

the RBA occupies Note that a restriction applies to the

Buffer Pointer and Word Count The Buffer Pointer must be

pointing to a word boundary Note also that the descriptors

must be properly aligned in the RRA as discussed in Section

TLF11722 – 15

FIGURE 3-8 RRA Initialization

After configuring the RRA the RRA Read command (setting

RRRA bit in the Command register) may be given This

command causes the SONIC-16 to read the RRA descriptor

in a single block operation and load the following registers

(see Section 42 for register mnemonics)

CRBA0 register

wRXrsrcbuff ptr0

CRBA1 register

wRXrsrcbuff ptr1

RBWC0 register

wRXrsrcbuff wc0

RBWC1 register

wRXrsrcbuff wc1

When the command has completed the RRRA bit in the

Command register is reset to ‘‘0’’ Generally this command

is only issued during initialization At all other times the RRA

is automatically read as the SONIC-16 finishes using an

RBA

3443 Initializing The RDA

To accept multiple packets from the network the receive

packet descriptors must be linked together via the

RXpktlink fields Each link field must be written with a 15-bit

(A15 – A1) pointer to locate the beginning of the next de-

scriptor in the list The LSB of the RXpktlink field is the End

of List (EOL) bit and is used to indicate the end of the de-

scriptor list EOL e 1 for the last descriptor and EOL e0 for

the first or middle descriptors The RXpktin

use field indi-

cates whether the descriptor is owned by the SONIC-16

The system writes a non-zero value to this field when the

descriptor is available and the SONIC-16 writes all ‘‘0’s’’

when it finishes using the descriptor At startup the Current

Receive Descriptor Address (CRDA) register must be load-

ed with the address of the first RXpktstatus field in order for

the SONIC-16 to begin receive processing at the first de-

scriptor An example of two descriptors linked together is

shown in

Figure 3-9 The fields initialized by the system are

displayed in larger type The other fields are written by the

SONIC-16 after a packet is accepted The RXpktin

use

field is first written by the system and then by the SONIC-

16 Note that the descriptors must be aligned properly as

discussed in section 33 Also note that the URDA register is

concatenated with the CRDA register to generate the full

23-bit address

TLF11722 – 16

FIGURE 3-9 RDA Initialization Example

3444 Initializing the Lower Boundary of the RBA

A ‘‘false bottom’’ is set in the RBA by loading the End Of

Buffer Count (EOBC) register with a value equal to the maxi-

mum size packet in words (16 bits) that may be received

This creates a lower boundary in the RBA Whenever the

Remaining Buffer Word Count (RBWC01) registers decre-

ment below the EOBC register the SONIC-16 buffers the

next packet into another RBA This also guarantees that a

packet is always contiguously buffered into a single Receive

Buffer Area (RBA) The SONIC-16 does not buffer a packet

into multiple RBAs

After a hardware reset the EOBC register is automatically

initialized to 2F8h (760 words or 1520 bytes)

Sometimes it may be desired to buffer a single packet per

RBA When doing this it is important to set EOBC and the

buffer size correctly The suggested practice is to set EOBC

to a value that is at least 2 bytes less than the buffer size

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