8101 datasheet(40/172 Pages) LSI

40 / 172 page

2-22

Functional Description

2.9 Transmit FIFO

The transmit FIFO acts as a temporary buffer between the system

interface and transmit MAC section. The transmit FIFO size is 4 Kbytes.

Data is clocked into the transmit FIFO with the 33

−66 MHz system

interface clock, SCLK. Data is automatically clocked out of the transmit

FIFO with the 125 MHz 8B10B PCS clock whenever a full packet has

been loaded into the FIFO (an EOF is written into the FIFO on the

system interface), or the FIFO data exceeds the transmit FIFO AutoSend

threshold. There are two programmable watermark outputs, TXWM1n

and TXWM2n, which aid in managing the data ﬂow into the transmit

FIFO.

2.9.1 AutoSend

The AutoSend feature causes a packet in the transmit FIFO to be

automatically transmitted when data in the transmit FIFO exceeds a

certain threshold.

The transmit AutoSend threshold is programmable over the lower

2 Kbytes of the transmit FIFO. The AutoSend threshold can be

programmed with the six TASND[5:0] bits that reside in the Transmit

FIFO Threshold register. Whenever the data in the FIFO exceeds this

threshold, the packet is automatically transmitted to the 8B10B PCS

section and out the 10-bit PHY interface. A packet is also automatically

transmitted if an EOF is written into the transmit FIFO for that packet,

regardless of the AutoSend threshold setting.

All of the bit settings for the transmit AutoSend threshold are evenly

distributed over the lower half of the transmit FIFO range, except for the

0b000000 setting. The 0b000000 setting automatically starts

transmission when the transmit FIFO is full, thus facilitating the

transmission of oversize packets. Refer to “Register 17–Transmit FIFO

Threshold‚" Section 4.3.14, description for more details on the AutoSend

(TASND[5:0]) bit settings.

2.9.2 Watermarks

There are two transmit watermarks for the transmit FIFO which are

output on the TXWM1n and TXWM2n pins. These watermarks are

asserted when the transmit FIFO data exceeds the thresholds

associated with the watermarks.

8101 Datasheet(PDF) 40 Page - LSI Computer Systems