Altera Triple Speed Ethernet MegaCore Function - Manual (Page 52)

Transmit FIFO Buffer Underflow
If the transmit FIFO buffer hits the almost-empty threshold during transmission and the FIFO buffer does
not contain the end-of-packet indication, the MAC function stops reading data from the FIFO buffer and
initiates the following actions:
1. The MAC function asserts the RGMII/GMII/MII error signals (
tx_control
/
gm_tx_err
/
m_tx_err
) to
indicate that the fragment transferred is not valid.
2. The MAC function deasserts the RGMII/GMII/MII transmit enable signals
(
tx_control
/
gm_tx_en
/
m_tx_en
) to terminate the frame transmission.
3. After the underflow, the user application completes the frame transmission.
4. The transmitter control discards any new data in the FIFO buffer until the end of frame is reached.
5. The MAC function starts to transfer data on the RGMII/GMII/MII when the user application sends a
new frame with an SOP.
Figure 4-7: Transmit FIFO Buffer Underflow
Figure illustrates the FIFO buffer underflow protection algorithm for gigabit Ethernet system.
[1] [2] [5]
[3] [4]
ff_tx_data
ff_tx_sop
ff_tx_eop
ff_tx_rdy
ff_tx_wren
ff_tx_crc_fwd
ff_tx_err
ff_tx_septy
ff_tx_uflow
ff_tx_a_full
ff_tx_a_empty
gm_tx_err
gm_tx_en
gm_tx_d
GMII Transmit
Transmit FIFO
valid valid
valid valid
Congestion and Flow Control
In full-duplex mode, the MAC function implements flow control to manage the following types of congestion:
• Remote device congestion—the receiving device experiences congestion and requests the MAC function
to stop sending data.
• Receive FIFO buffer congestion—when the receive FIFO buffer is almost full, the MAC function sends
a pause frame to the remote device requesting the remote device to stop sending data.
• Local device congestion—any device connected to the MAC function, such as a processor, can request
the remote device to stop data transmission.
Related Information
MAC Configuration Register Space on page 6-1
Functional Description Altera Corporation
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Transmit FIFO Buffer Underflow 4-16
2014.06.30