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LH79524/LH79525 User's Guide
A Receive Overrun condition occurs when either the AHB bus was not granted in time or
because the response was 'Not OK'. In a Receive Overrun condition, the Receive Overrun
Interrupt is asserted and the buffer currently being written is recovered. The next received
frame whose address is recognized reuses the buffer
If NETCONFIG:DISCARDFCS is 1, the FCS bytes of received frames are not copied to
memory. The Length Of Frame field is reduced by four bytes in this case.
6.1.2.3 Transmit Buffer
One or more transmit buffers store frames pending transmission. Transmit buffers can be
between 0 and 2,047 bytes long, so it is possible to transmit frames longer than the max-
imum length specified in IEEE standard 802.3. Zero length buffers are also allowed. The
maximum number of buffers permitted for each transmit frame is 128.
The start location for each transmit buffer is stored in memory in a Transmit Buffer Descrip-
tor List at a location pointed to by the Transmit Buffer Queue Pointer register (TXBQP).
LIke the receive buffers, each list entry consists of two words. The first word contains only
the byte address of the transmit buffer; the second word contains the transmit control and
status. Frames can be transmitted with or without automatic CRC generation. If CRC is
automatically generated, a pad will also be automatically generated to make frames a min-
imum length of 64 bytes. Table 6-2 defines the transmit buffer descriptor list. For each sta-
tus bit, 1 = TRUE and 0 = FALSE.
BIT
31:0
31
30
29
28
27
26:17
16
15
14:11
10:0
Table 6-2. Transmit Buffer Descriptor List
Byte Address Of Buffer
Used Must be zero for the EMAC to send data to the transmit buffer. The EMAC
programs this bit to 1 for the first buffer of a frame once it has been successfully
transmitted. Software must clear this bit before the buffer can be used again.
Wrap Marks last descriptor in transmit buffer descriptor list.
Retry Limit Exceeded Too many retries were made without successful transmission.
Transmit Underrun This error occurs either when response is not OK or the transmit
data could not be fetched in time or when buffers are exhausted in mid-frame.
Buffers Exhausted In Mid-Frame The buffers ran out of data before the entire frame
could be transmitted.
Reserved Reading returns 0. Write the reset value.
No CRC When programmed to 1, no CRC will be appended to the current frame. This
bit only needs to be programmed for the last buffer of a frame.
Last Buffer Indicates the last buffer in the current frame has been reached.
Reserved Reading returns 0. Write the reset value.
Length Of Buffer The length of the buffer is programmed to this field.
Version 1.0
.
FUNCTION
WORD 0
WORD 1
Ethernet MAC Controller
6-7
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