DP83932C datasheet(20/98 Pages) NSC

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30 Buffer Management (Continued)

when its receive resources have been exhausted The sys-

tem should respond by replenishing the resources that have

been exhausted These overflow conditions (Descriptor Re-

sources Exhausted Buffer Resources Exhausted and RBA

Limit Exceeded) are indicated in the Interrupt Status register

and are detailed as follows

Descriptor Resources Exhausted

This occurs when the

SONIC has reached the last receive descriptor in the list

meaning that the SONIC has detected EOLe1 The system

must supply additional descriptors for continued reception

The system can do this in one of two ways 1) appending

descriptors to the existing list or 2) creating a separate list

1) Appending descriptors to the existing list This is the eas-

iest and preferred way To do this the system after cre-

ating the new list joins the new list to the existing list by

simply writing the beginning address of the new list into

the RXpktlink field and setting EOL e 0 At the next

reception the SONIC re-reads the last RXpktlink field

and updates its CRDA register to point to the next de-

scriptor

2) Creating a separate list This requires an additional step

because the lists are not joined together and requires

that the CRDA register be loaded with the address of the

RXpktlink field in the new list

During this overflow condition the SONIC maintains owner-

ship of the descriptor (RXpktin

use i 00h) and waits for

the system to add additional descriptors to the list When

the system appends more descriptors the SONIC releases

ownership of the descriptor after writing 0000h to the

RXpktin

use field

Buffer Resources Exhausted

This occurs when the

SONIC has detected that the Resource Read Pointer (RRP)

and Resource Write Pointer (RWP) registers are equal (ie

all RRA descriptors have been exhausted) The RBE bit in

the Interrupt Status register is set when the SONIC finishes

using the second to last receive buffer and reads the last

RRA descriptor Actually the SONIC is not truly out of re-

sources but gives the system an early warning of an im-

pending out of resources condition To continue reception

after the last RBA is used the system must supply addition-

al RRA descriptor(s) update the RWP register and clear

the RBE bit in the ISR The SONIC rereads the RRA after

this bit is cleared

RBA Limit Exceeded

This occurs when a packet does not

completely fit within the remaining space of the RBA This

can occur if the EOBC register is not programmed to a value

greater than the largest packet that can be received When

this situation occurs the packet is truncated and the SONIC

reads the RRA to obtain another RBA Indication of an RBA

limit being exceeded is signified by the Receive Buffer Area

Exceeded (RBAE) interrrupt being set (see Section 436)

An RDA will not be set up for the truncated packet and the

buffer space will not be re-used To rectify this potential

overflow condition the EOBC register must be loaded with a

value equal to or greater than the largest packet that can be

accepted (See Section 342)

35 TRANSMIT BUFFER MANAGEMENT

To begin transmission the system software issues the

Transmit command (TXPe1 in the CR) The Transmit Buff-

er Management uses two areas in memory for transmitting

packets

(Figure 3-11) the Transmit Descriptor Area (TDA)

and the Transmit Buffer Area (TBA) During transmission

the SONIC fetches control information from the TDA loads

its appropriate registers and then transmits the data from

the TBA When the transmission is complete the SONIC

writes the status information in the TDA From a single

transmit command packets can either be transmitted singly

or in groups if several descriptors have been linked togeth-

TLF10492 – 18

FIGURE 3-11 Overview of Transmit Buffer Management

351 Transmit Descriptor Area (TDA)

The TDA contains descriptors that the system has generat-

ed to exchange status and control information Each de-

scriptor corresponds to a single packet and consists of the

following 16-bit fields

TXpktstatus

This field is written by the SONIC and pro-

vides status of the transmitted packet (See Section 3512

for more details)

TXpktconfig

This field allows programming the SONIC to

one of the various transmit modes The SONIC reads this

field and loads the corresponding configuration bits (PINT

POWC CRCI and EXDIS) into the Transmit Control regis-

ter (See Section 3511 for more details)

TXpktpkt

size

This field contains the byte count of the

entire packet

TXpktfrag

count

This field contains the number of frag-

ments the packet is segmented into

TXpktfrag

ptr01

This field contains a 32-bit pointer

which locates the packet fragment to be transmitted in the

Transmit Buffer Area (TBA) This pointer is not restricted to

any byte alignment

TXpktfrag

size

This field contains the byte count of the

packet fragment The minimum fragment size is 1 byte

TXpktlink

This field contains a 15-bit pointer (A15 – A1) to

the next TDA descriptor The LSB the End Of List (EOL) bit

indicates the last descriptor in the list when set to a ‘‘1’’

When descriptors have been linked together the SONIC

transmits back-to-back packets from a single transmit com-

mand

The data of the packet does not need to be contiguous but

can exist in several locations (fragments) in memory In this

case the TXpktfrag

count field is greater than one and

additional TXpktfrag

ptr01 and TXpktfrag

size fields

corresponding to each fragment are used The descriptor

format is shown in

Figure 3-12 Note that in 32-bit mode the

upper word Dk3116l is not used

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