Altera Triple Speed Ethernet MegaCore Function Bedienungsanleitung PDF herunterladen (Seite 51)

Transmit Thresholds

Figure 4-6: Transmit FIFO Thresholds

Network

Switch Fabric

Frame Buffer n

Frame Buffer n - 1

Frame Buffer k

Frame Buffer 1

The remaining

unwritten entries in

the FIFO buffer

before it is full.

Almost full

The remaining

unread entries in

the FIFO buffer

before it is empty.

Almost empty

An early indication

that the FIFO

buffer is getting

full.

Section Empty

Sufficient unread

entries in the FIFO

buffer for the

transmitter to start

transmission.

Section full

Frame Buffer 2

Table 4-7: Transmit Thresholds

DescriptionRegister NameThreshold

The number of unread entries in the FIFO buffer before the buffer

is empty. When the level of the FIFO buffer reaches this threshold,

the MAC function asserts the ff_tx_a_empty signal. The MAC

function stops reading from the FIFO buffer and sends the Ethernet

frame with GMII / MII/ RGMII error to avoid FIFO underflow.

tx_almost_emptyAlmost

empty

The number of unwritten entries in the FIFO buffer before the buffer

is full. When the level of the FIFO buffer reaches this threshold, the

MAC function asserts the ff_tx_a_full signal. The MAC function

deasserts the ff_tx_rdy signal to backpressure the Avalon-ST

transmit interface.

tx_almost_fullAlmost full

An early indication that the FIFO buffer is getting full. When the

level of the FIFO buffer reaches this threshold, the MAC function

deasserts the ff_tx_septy signal. This threshold can serve as a

warning about potential FIFO buffer congestion.

tx_section_emptySection

empty

This threshold indicates that there are sufficient entries in the FIFO

buffer to start frame transmission.

Set this threshold to 0 to enable store and forward on the transmit

path. When you enable the store and forward mode, the MAC

function forwards each frame as soon as it is completely written to

the transmit FIFO buffer.

tx_section_fullSection full

Altera Corporation

Functional Description

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4-15

Transmit Thresholds

UG-01008

2014.06.30

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