RSFL - Section full
(Core FIFO status)
MAC receive
FIFO read control
RAFL - Almost full
(FIFO write protection)
RSEM - Section empty
RAEM - Almost empty
(FIFO read control)
(Pause frame
generation)
Figure 46-61. Receive FIFO Overview
46.4.11.2 Transmit FIFO
Four programmable thresholds are available which control the core operation as
described below.
Table 46-80. Transmit FIFO Thresholds Definition
Register
Description
ENET
n
_TAEM
[TX_ALMOST
_EMPTY]
When the FIFO level reaches the ENET
n
_TAEM value and no end-of-frame is available for the frame, the
MAC transmit logic avoids a FIFO underflow by stopping FIFO reads and transmitting the Ethernet frame
with an MII error indication.
Set ENET
n
_TAEM to a minimum of 4.
ENET
n
_TAFL
[TX_ALMOST
_FULL]
When the FIFO level approaches the maximum, so that there is no more space for at least ENET
n
_TAFL
number of words, the MAC deasserts its control signal to the application.
If the application does not react on this signal, the FIFO write control logic avoids FIFO overflow by
truncating the current frame and setting the error status. As a result, the frame is transmitted with an MII
error indication.
Set ENET
n
_TAFL to a minimum of 4. Larger values allow more latency for the application to react on the
MAC control signal deassertion, before the frame is truncated. A typical setting is 8, which offers 3–4
clock cycles of latency to the application to react on the MAC control signal deassertion.
ENET
n
_TSEM
[TX_SECTION
_EMPTY]
When the FIFO level reaches the ENET
n
_TSEM value, a MAC status signal is deasserted to indicate that
the transmit FIFO is getting full.
This gives the application an indication to slow or stop its write transaction to avoid a buffer overflow.
This is a pure indication function to the application. It has no effect within the MAC.
When ENET
n
_TSEM is 0, the signal is never deasserted.
Table continues on the next page...
Chapter 46 10/100-Mbps Ethernet MAC (ENET)
K53 Sub-Family Reference Manual, Rev. 6, Nov 2011
Freescale Semiconductor, Inc.
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