dp83901a National Semiconductor Corporation, dp83901a Datasheet - Page 13

• Manufacturer Part Number: dp83901a
• Description: Serial Network Interface Controller
• Manufacturer: National Semiconductor Corporation

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 SNIC Thus the packets previously
received and still contained in the Ring will not be de-
stroyed
In heavily loaded network which cause overflows of the Re-
ceive Buffer Ring the SNIC may disable the local DMA and
suspend further receptions even if the Boundary register is
advanced beyond the Current register To guarantee this
will not happen a software reset must be issued during all
Receive Buffer Ring overflows (indicated by the OVW bit in
the Interrupt Status Register) The following procedure is
required to recover from a Receiver Buffer Ring Overflow
If this routine is not adhered to the SNIC may act in an
unpredictable manner It should also be noted that it is not
permissible 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 SNIC’s
overflow routine can be found on the next page
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 SNIC’s
2 Issue the STOP command to the SNIC This is accom-
3 Wait for at least 1 6 ms Since the SNIC will complete
4 Clear the SNIC’s Remote Byte Count registers (RBCR0
5 Read the stored value of the TXP bit from step 1 above
Command Register
plished by setting the STP bit in the SNIC’s Command
Register Writing 21H to the Command Register will stop
the SNIC
any transmission or reception that is in progress it is
necessary to time out for the maximum possible dura-
tion 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)
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 SNIC’s Interrupt Status Reg-
ister If either the Packet Transmitted bit (PTX) or Trans-
‘‘Resend’’
(Continued)
Received Packet Aborted If It Hits Boundary
13
10 Take the SNIC out of loopback This is done by writing
11 If the ‘‘Resend’’ variable is set to a 1 reset the ‘‘Re-
Note 1 If Remote DMA is not being used the SNIC does not need to be
started before packets can be removed from the receive buffer ring Hence
step 8 could be done before step 7
Note 2 When the SNIC is in STOP mode the Missed Talley Counter is
disabled
6 Place the SNIC in either mode 1 or mode 2 loopback
7 Issue the START command to the SNIC This can be
8 Remove one or more packets from the receive buffer
9 Reset the overwrite warning (OVW overflow) bit in the
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 SNIC’s ISR is
read to determine whether or not the packet was recog-
nized by the SNIC If neither the PTX nor TXE bit was
set then the packet will essentially be lost and re-trans-
mitted only after a time-out takes place in the upper lev-
el software By determining that the packet was lost at
the driver level a transmit command can be reissued to
the SNIC once the overflow routine is completed (as in
step 11) Also it is possible for the SNIC to defer indefi-
nitely when it is stopped on a busy network Step 5 also
alleviates this problem Step 5 is essential and should
not be omitted from the overflow routine in order for the
SNIC to operate correctly
the Transmit Configuration Register with the value it
contains during normal operation (Bits D2 and D1
should both be programmed to 0 )
send’’ variable and reissue the transmit command This
is done by writing a value of 26H to the Command Reg-
ister If the ‘‘Resend’’ variable is 0 nothing needs to be
done
This can be accomplished by setting bits D2 and D1 of
the Transmit Configuration Register to ‘‘0 1’’ or ‘‘1 0’’
respectively
accomplished by writing 22H to the Command Register
This is necessary to activate the SNIC’s Remote DMA
channel
ring
Interrupt Status Register
TL F 10469 – 11