DP83932C datasheet(21/98 Pages) NSC

21 / 98 page

30 Buffer Management (Continued)

TLF10492 – 19

FIGURE 3-12 Transmit Descriptor Area

3511 Transmit Configuration

The TXpktconfig field allows the SONIC to be programmed

into one of the transmit modes before each transmission At

the beginning of each transmission the SONIC reads this

field and loads the PINT 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

PINT

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 writes the status

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

the number of collisions experienced during the transmis-

sion 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 32-bit address range

and be aligned to any byte boundary When an odd byte

boundary is given the SONIC automatically begins reading

data at the corresponding word boundary in 16-bit mode or

a long word boundary in 32-bit mode The SONIC ignores

the extraneous bytes which are written into the FIFO during

odd byte alignment fragments The minimum allowed frag-

ment size is 1 byte

Figure 3-11 shows the relationship be-

tween 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 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 up-

per 16-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 634)

3531 Transmit Process

When the Transmit Command (TXP e 1 in the Command

tion 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 reads the packet descriptor

in the TDA and transmits the data from the TBA

TXpktfrag

count is greater than one the SONIC after fin-

ishing 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 then reads the TXpktlink field and checks if

EOL e 0 If it is ‘‘0’’ the SONIC fetches the next descriptor

and transmits the next packet If EOL e 1 the SONIC gen-

erates a ‘‘Transmission Done’’ indication in the Interrupt

Status register and resets the TXP bit in the Command reg-

ister

In the event of a collision the SONIC recovers its pointer in

the TDA and retransmits the packet up to 15 times The

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

rary Transmit Descriptor Address (TTDA) register

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

es in the TDA depending on where the SONIC is in the

transmit process

For the first fragment

it reads the

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