DP83916 datasheet(21/96 Pages) NSC

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30 Buffer Management (Continued)

TLF11722 – 19

FIGURE 3-12 Transmit Descriptor Area

3511 Transmit Configuration

The TXpktconfig field allows the SONIC-16 to be pro-

grammed into one of the transmit modes before each trans-

mission At the beginning of each transmission the SONIC-

16 reads this field and loads the PINTR POWC CRCI and

EXDIS bits into the Transmit Control register (TCR) The

configuration bits in the TCR correspond directly with the

bits in the TXpktconfig field as shown in

Figure 3-13 See

Section 434 for the description on the TCR

PINTR POWC CRCI EXDIS

7654321

XXXXXXX

Note

x e don’t care

FIGURE 3-13 TXpktconfig Field

3512 Transmit Status

At the end of each transmission the SONIC-16 writes the

status bits (k100l) of the Transmit Control Register (TCR)

and the number of collisions experienced during the trans-

mission into the TXpktstatus field

(Figure 3-14 res e re-

served) Bits NC4-NC0 indicate the number of collisions

where NC4 is the MSB See Section 434 for the descrip-

tion of the TCR

NC4

NC3

NC2

NC1

NC0

EXD

DEF

NCRS

CRSL

EXC

OWC

res

PMB

BCM

PTX

FIGURE 3-14 TXpktstatus Field

352 Transmit Buffer Area (TBA)

The TBA contains the fragments of packets that are defined

by the descriptors in the TDA A packet can consist of a

single fragment or several fragments depending upon the

fragment count in the TDA descriptor The fragments also

can reside anywhere within the full 23-bit address range

and be aligned to any byte boundary When an odd byte

boundary is given the SONIC-16 automatically begins read-

ing data at the corresponding word boundary The SONIC-

16 ignores the extraneous bytes which are written into the

FIFO during odd byte alignment fragments The minimum

allowed fragment size is 1 byte

Figure 3-11 shows the rela-

tionship between the TDA and the TBA for single and multi-

fragmented packets

353 Preparing To Transmit

All fields in the TDA descriptor and the Current Transmit

Descriptor Address (CTDA) register of the SONIC-16 must

be initialized before the Transmit Command (setting the

TXP bit in the Command register) can be issued If more

than one packet is queued the descriptors must be linked

together with the TXpktlink field The last descriptor must

have EOLe1 and all other descriptors must have EOLe0

To begin transmission the system loads the address of the

first TXpktstatus field into the CTDA register Note that the

upper 8-bits of address are loaded in the Upper Transmit

Descriptor (UTDA) register The user performs the following

transmit initialization

1) Initialize the TDA

2) Load the CTDA register with the address of the first

transmit descriptor

3) Issue the transmit command

Note that if the Source Address of the packet being trans-

mitted is not in the CAM the Packet Monitored Bad (PMB)

bit in the TXpxtstatus field will be set (see Section 434)

3531 Transmit Process

When the Transmit Command (TXP e 1 in the Command

mation in the TDA descriptor loads its appropriate registers

(shown below) and begins transmission (See Section 42

for register mnemonics)

TCR

wTXpktconfig

TPS

wTXpktpkt size

TFC

wTXpktfrag count

TSA0

wTXpktfrag ptr0

TSA1

wTXpktfrag ptr1

TFS

wTXpktfrag size

CTDA

wTXpktlink

(CTDA is loaded after all fragments have been read and

successfully transmitted If the halt transmit command is is-

sued (HTX bit in the Command register is set) the CTDA

During transmission the SONIC-16 reads the packet de-

scriptor in the TDA and transmits the data from the TBA If

TXpktfrag

count is greater than one the SONIC-16 after

finishing transmission of the fragment fetches the next

TXpktfrag

ptr01 and TXpktfrag

size fields and transmits

the next fragment This process continues until all frag-

ments of a packet are transmitted At the end of packet

transmission status is written in to the TXpktstatus field

The SONIC-16 then reads the TXpktlink field and checks if

EOL e 0 If it is ‘‘0’’ the SONIC-16 fetches the next de-

scriptor and transmits the next packet If EOL e 1 the SON-

IC-16 generates a ‘‘Transmission Done’’ indication in the

Interrupt Status register and resets the TXP bit in the Com-

mand register

In the event of a collision the SONIC-16 recovers its pointer

in the TDA and retransmits the packet up to 15 times The

SONIC-16 maintains a copy of the CTDA register in the

Temporary Transmit Descriptor Address (TTDA) register

The SONIC-16 performs a block operation of 6 3 or 2 ac-

cesses in the TDA depending on where the SONIC-16 is in

the transmit process For the first fragment it reads the

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