DP8390 NSC [National Semiconductor], DP8390 Datasheet - Page 9

• Manufacturer Part Number: DP8390
• Description: NIC Network Interface Controller
• Manufacturer: NSC [National Semiconductor]

7 0 Packet Reception
Buffer Ring Overflow
If the Buffer Ring has been filled and the DMA reaches the
Boundary Pointer Address reception of the incoming pack-
et will be aborted by the NIC Thus the packets previously
received and still contained in the Ring will not be de-
stroyed
In a heavily loaded network environment the local DMA may
be disabled preventing the NIC from buffering packets from
the network To guarantee this will not happen a software
reset must be issued during all Receive Buffer Ring over-
flows (indicated by the OVW bit in the Interrupt Status Reg-
ister) The following procedure is required to recover
from a Receiver Buffer Ring Overflow
If this routine is not adhered to the NIC may act in an unpre-
dictable manner It should also be noted that it is not per-
missible to service an overflow interrupt by continuing to
empty packets from the receive buffer without implementing
the prescribed overflow routine A flow chart of the NIC’s
overflow routine can be found at the right
Note It is necessary to define a variable in the driver which will be called
1 Read and store the value of the TXP bit in the NIC’s
2 Issue the STOP command to the NIC This is accom-
Note If the STP is set when a transmission is in progress the RST bit may
3 Wait for at least 1 6 ms Since the NIC will complete any
4 Clear the NIC’s Remote Byte Count registers (RBCR0
Received Packet Aborted if It Hits Boundary Pointer
Command Register
plished be setting the STP bit in the NIC’s Command
Register Writing 21H to the Command Register will stop
the NIC
transmission or reception that is in progress it is neces-
sary to time out for the maximum possible duration of an
Ethernet transmission or reception By waiting 1 6 ms this
is achieved with some guard band added Previously it
was recommended that the RST bit of the Interrupt
Status Register be polled to insure that the pending
transmission or reception is completed This bit is not a
reliable indicator and subsequently should be ignored
and RBCR1)
‘‘Resend’’
not be set In this case the NIC is guaranteed to be reset after the
longest packet time (1500 bytes
for the DP8390B) the NIC will be reset within 2 microseconds after
the STP bit is set and Loopback mode 1 is programmed
e
1 2 ms) For the DP8390D (but not
(Continued)
TL F 8582 – 8
9
5 Read the stored value of the TXP bit from step 1 above
If this value is a 0 set the ‘‘Resend’’ variable to a 0 and
jump to step 6
If this value is a 1 read the NIC’s Interrupt Status Regis-
ter If either the Packet Transmitted bit (PTX) or Trans-
mit Error bit (TXE) is set to a 1 set the ‘‘Resend’’ vari-
able to a 0 and jump to step 6 If neither of these bits is
set place a 1 in the ‘‘Resend’’ variable and jump to step
6
This step determines if there was a transmission in prog-
ress when the stop command was issued in step 2 If
there was a transmission in progress the NIC’s ISR is
read to determine whether or not the packet was recog-
nized by the NIC If neither the PTX nor TXE bit was set
Overflow Routine Flow Chart
TL F 8582 – 95