DP83934CVUL20 National Semiconductor, DP83934CVUL20 Datasheet - Page 27

• Manufacturer Part Number: DP83934CVUL20
• Manufacturer: National Semiconductor

Specifications of DP83934CVUL20

• Operating Supply Voltage (typ): 5V
• Operating Supply Voltage (min): 4.75V
• Operating Supply Voltage (max): 5.25V
• Operating Temperature Classification: Commercial
• Mounting: Surface Mount
• Pin Count: 160
• Lead Free Status / RoHS Status: Not Compliant

5 0 Buffer Management
5 4 2 Receive Buffer Area (RBA)
The SONIC-T stores the actual data of a received packet in
the RBA The RBAs are designated by the Resource De-
scriptors in the RRA as described above The RXrsrc buff
wc0 1 fields of the RRA indicate the length of the RBA
When the SONIC-T gets an RBA from the RRA the
RXrsrc buff wc0 1 values are loaded into the Remaining
Buffer Word Count registers (RBWC0 1) These registers
keep track of how much space (in words) is left in the buffer
When a packet is buffered in a RBA it is buffered contigu-
ously (the SONIC-T will not scatter a packet into multiple
buffers or fragments) Therefore if there is not enough
space left in a RBA after buffering a packet to buffer at least
one more maximum sized packet (the maximum legal sized
packet expected to be received from the network) a new
buffer must be acquired The End of Buffer Count (EOBC)
register is used to tell the SONIC-T the maximum packet
size that the SONIC-T will need to buffer
5 4 2 1 End of Buffer Count (EOBC)
The EOBC is a boundary in the RBA based from the bottom
of the buffer The value written into the EOBC is the maxi-
mum expected size (in words) of the network packet that
the SONIC-T will have to buffer This word count creates a
line in the RBA that when crossed causes the SONIC-T to
fetch a new RBA resource from the RRA
Case
Case
Case
(RBWC0 1
1 SONIC-T buffers next packet in same RBA
2 SONIC-T detects an exhausted RBA and will buffer the next packet in another RBA
1
(Continued)
FIGURE 5-4 Receive Resource Area Format
t
EOBC)
FIGURE 5-5 Receive Buffer Area
27
Case
(RBWC0 1
Figure 5-5 illustrates the SONIC-T’s actions for (1)
Note The EOBC is a word count not a byte count Also the value pro-
5 4 2 2 Buffering the Last Packet in an RBA
At the start of reception the SONIC-T stores the packet
beginning at the Current Receive Buffer Address (CRBA0 1)
and continues until the reception is complete Concurrent
with reception the SONIC-T decrements the Remaining
Buffer Word Count (RBWC0 1) by one in 16-bit mode or by
two in 32-bit mode At the end of reception if the packet has
crossed the EOBC boundary the SONIC-T knows that the
next packet might not fit in the RBA This check is done by
comparing the RBWC0 1 registers with the EOBC If
RBWC0 1 is less than the EOBC (the last packet buffered
has crossed the EOBC boundary) the SONIC-T fetches the
next resource descriptor in the RRA If RBWC0 1 is greater
than or equal to the EOBC (the EOBC boundary has not
been crossed) the next packet reception continues at the
present location pointed to by CRBA0 1 in the same RBA
RBWC0 1
tion 5 4 4 4 for specific information about setting the EOBC
Note It is important that the EOBC boundary be ‘‘crossed ’’ In other words
grammed into EOBC must be a double word (32-bit) quantity when
the SONIC-T is in 32-bit mode (e g in 32-bit mode EOBC should be
set to 760 words not 759 words even though the maximum size of an
IEEE 802 3 packet is 759 words for double work boundary align-
ment)
case
occurs without case
buffer The result of this will be a buffer overflow (RBAE in the Inter-
rupt Status Register Section 6 3 6)
2
k
EOBC will not work properly and the SONIC-T will not fetch a new
k
EOBC)
t
1 in Figure 5-5 must exist before case
EOBC and (2) RBWC0 1
1 having occurred first the test for RBWC0 1
TL F 11719– 18
k
TL F 11719– 17
EOBC See Sec-
2 exists If case
2