AM79C976 datasheet(192/309 Pages) AMD

192 / 309 page

192

Am79C976

8/01/00

P R E L I M INARY

Table 86. Receive Watermark Programming

Read/Write

accessible.

RCVFW[1:0] is set to a value of

01b (64 bytes) after H_RESET or

S_RESET and is unaffected by

STOP.

11-10

XMTSP[1:0] Transmit Start Point. XMTSP

controls the point at which pre-

amble transmission attempts to

commence in relation to the num-

ber of bytes written to the MAC

Transmit FIFO for the current

transmit frame. When the entire

frame is in the MAC Transmit

FIFO, transmission will start re-

gardless of the value in XMTSP.

If the network interface is operat-

ing in half-duplex mode, regard-

less of XMTSP, the FIFO will not

internally overwrite its data until

at least 64 bytes (or the entire

frame if shorter than 64 bytes)

have been transmitted onto the

network. This ensures that for

collisions within the slot time win-

dow, transmit data need not be

rewritten to the Transmit FIFO,

and retries will be handled auton-

omously by the MAC. If the Dis-

able Retry feature is enabled, or if

the network is operating in full-

duplex mode, the Am79C976

controller can overwrite the be-

ginning of the frame as soon as

the data is transmitted, because

no collision handling is required

in these modes.

Note that when the No Underflow

(NOUFLO) bit (BCR18, bit 11) is

set to 1, there is the additional re-

striction that the complete trans-

mit frame must be DMA’d into the

Am79C976 controller and reside

within the MAC Transmit FIFO.

This mode is useful in a system

where high latencies cannot be

avoided. See Table 87.

Read/Write accessible. XMTSP

is set to a value of 01b (64 bytes)

after H_RESET or S_RESET and

is unaffected by STOP.

Table 87. Transmit Start Point Programming

9-8

XMTFW[1:0] Transmit FIFO Watermark. XMT-

FW specifies the point at which

transmit

DMA

requested,

based upon the number of bytes

that could be written to the Trans-

mit FIFO without FIFO overflow.

Transmit DMA is requested at

any time when the number of

bytes specified by XMTFW could

be written to the FIFO without

causing Transmit FIFO overflow,

and the internal state machine

has reached a point where the

Transmit FIFO is checked to de-

termine if DMA servicing is re-

quired. See Table 88.

Table 88. Transmit Watermark Programming

Read/Write accessible. XMTFW

is set to a value of 00b (16 bytes)

after H_RESET or S_RESET and

is unaffected by STOP.

7-0

DMATC[7:0]Obsolete function. Writing has no

effect. Read as undefined.

RCVFW[1:0]

Bytes Received

128

256

XMTSP[1:0]

NOUFLO

Bytes Written

128

Full Frame

XMTFW[1:0]

Bytes Available

128

256

AM79C976 датащи(PDF) 192 Page - Advanced Micro Devices

AM79C976 датащи(HTML) 192 Page - Advanced Micro Devices

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 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 ...More

датащи скачать

ссылки URL