DP83936AVF datasheet(29/104 Pages) NSC

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50 Buffer Management (Continued)

All RRA registers are concatenated with the URRA register

for generating the full 32-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 5-9 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

32-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 two restrictions apply to the Buffer Pointer and

Word Count First in 32-bit mode since the SONIC-T al-

ways writes long words an even count must be written to

RXrsrcbuff

wc0 Second the Buffer Pointer must either

be pointing to a word boundary in 16-bit mode (A0e0) or a

long word boundary in 32-bit mode (A0A1e00) Note also

that the descriptors must be properly aligned in the RRA as

discussed in Section 53

TLF12597 – 20

FIGURE 5-9 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-T to read the RRA descriptor

in a single block operation and load the following registers

(see Section 62 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-T finishes using an RBA

5443 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

descriptor list EOL e 1 for the last descriptor and EOL e 0

for the first or middle descriptors The RXpktin

use field

indicates whether the descriptor is owned by the SONIC-T

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

descriptor is available and the SONIC-T 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-T to begin receive processing at the first de-

scriptor An example of two descriptors linked together is

shown in

Figure 5-10 The fields initialized by the system are

displayed in bold type The other fields are written by the

SONIC-T after a packet is accepted The RXpktin

use

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

Note that the descriptors must be aligned properly as dis-

cussed in Section 53 Also note that the URDA register is

concatenated with the CRDA register to generate the full

32-bit address

TLF12597 – 21

FIGURE 5-10 RDA Initialization Example

5444 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-T 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-T does not buffer a packet

into multiple RBAs Note that in 32-bit mode the SONIC-T

holds the LSB always low so that it properly compares with

the RBWC01 registers

After a hardware reset the EOBC reset the EOBC register

is automatically initialized to 2F8h (760 words or 1520

bytes) For 32-bit applications this is the suggested value for

EOBC EOBC defaults to 760 words (1520 bytes) instead of

759 words (1518 bytes) because 1518 is not a double word

(32-bit) boundary (see Section 5421) If the SONIC-T is

used in 16-bit mode then EOBC should be set to 759 words

(1518 bytes) because 1518 is a word (16-bit) boundary

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 4 bytes in 32-bit mode or 2 bytes

in 16-bit mode less than the buffer size An example of this

for 32-bit mode is to set EOBC to 760 words (1520 bytes)

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