DP83936AVUL-20 National Semiconductor, DP83936AVUL-20 Datasheet - Page 31

• Manufacturer Part Number: DP83936AVUL-20
• Description: IC CTRLR ORIENT NETWORK 160PQFP
• Manufacturer: National Semiconductor

Specifications of DP83936AVUL-20

• Controller Type: Network Interface Controller (NIC)
• Interface: Twisted Pair
• Voltage - Supply: 4.75 V ~ 5.25 V
• Current - Supply: 140mA
• Operating Temperature: 0°C ~ 70°C
• Mounting Type: Surface Mount
• Package / Case: 160-MQFP, 160-PQFP
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant
• Other names: *DP83936AVUL-20

5 0 Buffer Management
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-T has reached the last receive descriptor in the list
meaning that the SONIC-T has detected EOL
system must supply additional descriptors for continued re-
ception 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-
2 Creating a separate list This requires an additional step
During this overflow condition the SONIC-T maintains own-
ership of the descriptor (RXpkt in use
the system to add additional descriptors to the list When
the system appends more descriptors the SONIC-T releas-
es ownership of the descriptor after writing 0000h to the
RXpkt in use field
Buffer Resources Exhausted This occurs when the
SONIC-T has detected that the Resource Read Pointer
(RRP) and Resource Write Pointer (RWP) registers are
equal (i e all RRA descriptors have been exhausted) The
RBE bit in the Interrupt Status register is set when the
SONIC-T finishes using the second to last receive buffer
and reads the last RRA descriptor Actually the SONIC-T is
not truly out of resources but gives the system an early
warning of an impending out of resources condition To con-
tinue reception after the last RBA is used the system must
supply additional RRA descriptor(s) update the RWP regis-
ter and clear the RBE bit in the ISR The SONIC-T 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-T reads the RRA to obtain another RBA Indication
of an RBA limit being exceeded is signified by the Receive
Buffer Area Exceeded (RBAE) interrupt being set (see Sec-
tion 6 4 6) An RDA will not be set up for the truncated pack-
et 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 5 4 2 )
5 5 TRANSMIT BUFFER MANAGEMENT
To begin transmission the system software issues the
Transmit command (TXP
er Management uses two areas in memory for transmitting
packets (Figure 5-12) the Transmit Descriptor Area (TDA)
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 RXpkt link field and setting EOL
reception the SONIC-T re-reads the last RXpkt link field
and updates its CRDA register to point to the next de-
scriptor
because the lists are not joined together and requires
that the CRDA register be loaded with the address of the
RXpkt link field in the new list
e
1 in the CR) The Transmit Buff-
i
(Continued)
e
00h) and waits for
0 At the next
e
1 The
31
and the Transmit Buffer Area (TBA) During transmission
the SONIC-T fetches control information from the TDA
loads its appropriate registers and then transmits the data
from the TBA When the transmission is complete the SON-
IC-T 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-
er
5 5 1 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
TXpkt status This field is written by the SONIC-T and pro-
vides status of the transmitted packet (See Section 5 5 1 2
for more details )
TXpkt config This field allows programming the SONIC-T
to one of the various transmit modes The SONIC-T reads
this field and loads the corresponding configuration bits
(PINTR POWC CRCI and EXDIS) into the Transmit Control
register (See Section 5 5 1 1 for more details )
TXpkt pkt size This field contains the byte count of the
entire packet
TXpkt frag count This field contains the number of frag-
ments the packet is segmented into
TXpkt frag ptr0 1 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
TXpkt frag size This field contains the byte count of the
packet fragment The minimum fragment size is 1 byte
TXpkt link 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-T
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 TXpkt frag count field is greater than one and
additional TXpkt frag ptr0 1 and TXpkt frag size fields
corresponding to each fragment are used The descriptor
format is shown in Figure 5-13 Note that in 32-bit mode the
upper word D
FIGURE 5-12 Overview of Transmit Buffer Management
k
31 16
l
is not used
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