DP8390D datasheet(14/56 Pages) NSC

14 / 56 page

90 Remote DMA (Continued)

2 Issue the ‘‘dummy’’ Remote Read command

3 Read the Current Remote DMA Address (CRDA) (both

bytes)

4 Compare to previous CRDA value if different go to 6

5 Delay and jump to 3

6 Set up for the Remote Write command by setting the

Remote Byte Count and the Remote Start Address (note

that if the Remote Byte count in step 1 can be set to the

tramsmit byte count plus one and the Remote Start Ad-

dress to one less these will now be incremented to the

correct values)

7 Issue the Remote Write command

FIFO AND BUS OPERATIONS

Overview

To accommodate the different rates at which data comes

from (or goes to) the network and goes to (or comes from)

the system memory the NIC contains a 16-byte FIFO for

buffering data between the bus and the media The FIFO

threshold is programmable allowing filling (or emptying) the

FIFO at different rates When the FIFO has filled to its pro-

grammed threshold the local DMA channel transfers these

bytes (or words) into local memory It is crucial that the local

DMA is given access to the bus within a minimum bus laten-

cy time otherwise a FIFO underrun (or overrun) occurs

To understand FIFO underruns or overruns there are two

causes which produce this condition

1) the bus latency is so long that the FIFO has filled (or

emptied) from the network before the local DMA has

serviced the FIFO

2) the bus latency or bus data rate has slowed the through-

put of the local DMA to point where it is slower than the

network data rate (10 Mbs) This second condition is

also dependent upon DMA clock and word width (byte

wide or word wide)

The worst case condition ultimately limits the overall bus

latency which the NIC can tolerate

FIFO Underrun and Transmit Enable

During transmission if a FIFO underrun occurs the Trans-

mit enable (TXE) output may remain high (active) Generally

this will cause a very large packet to be transmitted onto the

network The jabber feature of the transceiver will terminate

the transmission and reset TXE

To prevent this problem a properly designed system will not

allow FIFO underruns by giving the NIC a bus acknowledge

within time shown in the maximum bus latency curves

shown and described later

FIFO at the Beginning of Receive

At the beginning of reception the NIC stores entire Address

field of each incoming packet in the FIFO to determine

whether the packet matches its Physical Address Registers

or maps to one of its Multicast Registers This causes the

FIFO to accumulate 8 bytes Furthermore there are some

synchronization delays in the DMA PLA Thus the actual

time that BREQ is asserted from the time the Start of Frame

Delimiter (SFD) is detected is 78 ms This operation affects

the bus latencies at 2 and 4 byte thresholds during the first

receive BREQ since the FIFO must be filled to 8 bytes (4

words) before issuing a BREQ

FIFO Operation at the End of Receive

When Carrier Sense goes low the NIC enters its end of

packet processing sequence emptying its FIFO and writing

the status information at the beginning of the packet figure

below This NIC holds onto the bus for the entire sequence

The longest time BREQ may be extended occurs when a

packet ends just as the NIC performs its last FIFO burst

The NIC in this case performs a programmed burst transfer

followed by flushing the remaining bytes in the FIFO and

completes by writing the header information to memory The

following steps occur during this sequence

1) NIC issues BREQ because the FIFO threshold has been

reached

2) During the burst packet ends resulting in BREQ extend-

3) NIC flushes remaining bytes from FIFO

4) NIC performs internal processing to prepare for writing

the header

5) NIC writes 4-byte (2-word) header

6) NIC deasserts BREQ

TLF8582 – 97

End of Packet Processing

End of Packet Processing (EOPP) times for 10 MHz and

20 MHz have been tabulated in the table below

End of Packet Processing Times for Various FIFO

Thresholds Bus Clocks and Transfer Modes

Mode

Threshold

Bus Clock

EOPP

Byte

2 bytes

70 ms

4 bytes

10 MHz

86 ms

8 bytes

110 ms

Byte

2 bytes

36 ms

4 bytes

20 MHz

42 ms

8 bytes

50 ms

Word

2 bytes

54 ms

4 bytes

10 MHz

62 ms

8 bytes

74 ms

Word

2 bytes

30 ms

4 bytes

20 MHz

32 ms

8 bytes

36 ms

Threshold Detection (Bus Latency)

To assure that no overwriting of data in the FIFO the FIFO

logic flags a FIFO overrun as the 13th byte is written into the

FIFO effectively shortening the FIFO to 13 bytes The FIFO

logic also operates differently in Byte Mode and in Word

Mode In Byte Mode a threshold is indicated when the na1

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