DP83907 datasheet(43/70 Pages) NSC

43 / 70 page

60 Operation of DP83907 (Continued)

Linking Receive Buffer Pages

If the length of the packet exhausts the first 256 byte buffer

the DMA performs a forward link to the next buffer to store

the remainder of the packet For a maximal length packet

the buffer logic will link six buffers to store the entire packet

Buffers cannot be skipped when linking a packet will always

be stored in contiguous buffers Before the next buffer can

be linked the Buffer Management Logic performs two com-

parisons The first comparison tests for equality between

the DMA address of the next buffer and the contents of the

Page Stop Register If the buffer address equals the Page

Stop Register the buffer management logic will restore the

DMA to the first buffer in the Receive Buffer Ring value

programmed in the Page Start Address Register The sec-

ond comparison tests for equality between the DMA ad-

dress of the next buffer address and the contents of the

Boundary Pointer Register If the two values are equal the

reception is aborted The Boundary Pointer Register can be

used to protect against overwrfting any area in the receive

buffer ring that has not yet been read When linking buffers

buffer management will never cross this pointer effectively

avoiding any overwrites If the buffer address does not

match either the Boundary Pointer or Page Stop Address

the link to the next buffer is performed

Linking Buffers

Before the DMA can enter the next contiguous 256 byte

buffer the address is checked for equality to PSTOP and to

the Boundary Pointer If neither are reached the DMA is

allowed to use the next buffer

TLF12082 – 26

FIGURE 31 Linking Receive Buffer Pages

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 DP83907 Thus the packets previ-

ously received and still contained in the Ring will not be

destroyed

In heavily loaded networks which cause overflows of the

Receive Buffer Ring the DP83907 may disable the local

DMA and suspend further receptions even if the Boundary

event that the DP83907 should encounter a receive buffer

overflow it is necessary to implement the following routine

A receive buffer overflow is indicated by the DP83907’s as-

sertion of the overflow bit (OVW) in the Interrupt Status

If this routine is not adhered to the DP83907 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

DP83907’s overflow routine can be found in

Figure 32

Note

It is necessary to define a variable in the driver which will be called

‘‘Resend’’

1) Read and store the value of the TXP bit in the

DP83907’s Command Register

2) Issue the STOP command to the DP83907 This is ac-

complished by setting the STP bit in the DP83907’s

Command Register Writing 21H to the Command Reg-

ister will stop the DP83907

3) Wait for at least 16 ms Since the DP83907 will com-

plete any transmission or reception that is in progress

it is necessary to time out for the maximum possible

duration of an Ethernet transmission or reception By

waiting 16 ms this is achieved with some guard band

added Previously it was recommended that the RST

bit of the nterrupt Status Register be polled to insure

that the pending transmission or reception is complet-

ed This bit is not a reliable indicator and subsequently

should be ignored

DP83907 датащи(PDF) 43 Page - National Semiconductor (TI)

DP83907 датащи(HTML) 43 Page - National Semiconductor (TI)

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70

датащи скачать

ссылки URL