8101 Datasheet, PDF (40/172 Pages) LSI Computer Systems – Gigabit Ethernet Controller

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.
2-22
Functional Description
Copyright © 2000–2001 by LSI Logic Corporation. All rights reserved.