AM79C970 datasheet(104/168 Pages) AMD

104 / 168 page

P R E L I M I N A R Y

AMD

1-971

Am79C970

13–12 RCVFW[1:0] Receive

FIFO

Watermark.

RCVFW controls the point at

which receive DMA is requested

in relation to the number of re-

ceived bytes in the receive FIFO.

RCVFW specifies the number of

bytes which must be present

(once the frame has been veri-

fied as a non-runt) before receive

DMA is requested. Note however

that in order for receive DMA to

be performed for a new frame, at

least 64 bytes must have been

received. This effectively avoids

having to react to receive frames

which are runts or suffer a colli-

sion during the slot time (512 bit

times). If the Runt Packet Accept

feature is enabled, receive DMA

will be requested as soon as

either the RCVFW threshold is

reached, or a complete valid re-

ceive frame is detected (regard-

less of length). RCVFW is set to

a value of 10b (64 bytes) after

H_RESET or S_RESET and is

unaffected by STOP.

RCVFW[1:0]

Bytes Received

Reserved

Read/write accessible only when

STOP bit is set.

Certain combinations of water-

mark programming and LINBC

(BCR18, bits 2–0) programming

may create situations where no

linear bursting is possible, or

where the FIFO may be exces-

sively read or excessively writ-

ten.

Such

combinations

are

declared as illegal.

Combinations of watermark set-

tings and LINBC (BCR 18, bits

2–0) settings must obey the fol-

lowing relationship:

watermark (in bytes)

≥

LINBC (in bytes)

Combinations of watermark and

LINBC settings that violate this

rule

may

cause

unexpected

behavior.

11–10 XMTSP[1:0]

Transmit Start Point. XMTSP

controls the point at which pre-

amble

transmission

attempts

commence in relation to the num-

ber of bytes written to the

transmit FIFO for the current

transmit frame. When the entire

frame is in the FIFO, transmis-

sion will start regardless of the

value in XMTSP.

XMTSP is

given a value of 10b (64 bytes)

after H_RESET or

S_RESET and is unaffected by

STOP. Regardless of XMTSP,

the FIFO will not internally over

write its data until at least 64

bytes (or the entire frame if < 64

bytes) have been transmitted

onto the network. This ensures

that for collisions within the slot

time window, transmit data need

not be re-written to the transmit

FIFO, and re-tries will be handled

autonomously by the MAC. This

bit is read/write accessible only

when the STOP bit is set.

XMTSP[1:0]

Bytes Written

112

9–8 XMTFW[1:0]

Transmit

FIFO

Watermark.

XMTFW specifies the point at

which

transmit

DMA

stops,

based upon the number of write

cycles that could be performed to

the transmit FIFO without FIFO

overflow. Transmit DMA is al-

lowed at any time when the num-

ber of write cycles specified by

XMTFW could be executed with-

out causing transmit FIFO over-

flow. XMTFW is set to a value of

00b (8 cycles) after H_RESET or

S_RESET and is unaffected by

STOP. Read/write accessible

only when STOP bit is set.

XMTFW[1:0]