DP83907 datasheet(44/70 Pages) NSC

44 / 70 page

60 Operation of DP83907 (Continued)

4) Clear the DP83907’s Remote Byte Count registers

(RBCR0 and RBCR1)

5) Read the stored value of the TXP bit from step 1

above

If this value is a 0 set the ‘‘Resend’’ variable toa0and

jump to step 6

If this value is a 1 read the DP83907’s Interrupt Status

Transmit Error bit (TXE) is set to a 1 set the ‘‘Resend’’

variable to a 0 and jump to step 6 If neither of these

bits is set placea1inthe ‘‘Resend’’ variable and jump

to step 6

This step determines if there was a transmission in

progress when the stop command was issued in step 2

If there was a transmission in progress the DP83907’s

ISR is read to determine whether or not the packet was

recognized by the DP83907 If neither the PTX nor TXE

bit was set then the packet will essentially be lost and

retransmitted only after a time-out takes place in the

upper level software By determining that the packet

was lost at the driver level a transmit command can be

reissued to the DP83907 once the overflow routine is

completed (as in step 11) Also it is possible for the

DP83907 to defer indefinitely 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 over-

flow routine in order for the DP83907 to operate cor-

rectly

6) Place the DP83907 in either mode 1 or mode 2 loop-

back This can be accomplished by setting bits D2 and

D1 of the Transmit Configuration Register to ‘‘01’’ or

‘‘10’’ respectively

7) Issue the START command to the DP83907 This can

be accomplished by writing 22H to the Command Reg-

ister This is necessary to activate the DP83907’s Re-

mote DMA channel

8) Remove one or more packets from the receive buffer

ring

9) Reset the overwrite warning (OVW overflow) bit in the

Interrupt Status Register

10) Take the DP83907 out of loopback This is done by

writing the Transmit Configuration Register with the val-

ue it contains during normal operation (Bits D2 and D1

should both be programmed to 0)

11) If the ‘‘Resend’’ variable is set to a 1 reset the ‘‘Re-

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

Note 1

If Remote DMA is not being used the DP83907 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 eliminating or

reducing the time spent polling in step 5

Note 2

When the DP83907 is in ST0P mode the Missed Packet Tally

counter is disabled

TLF12082 – 27

FIGURE 32 Overflow Routine

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