Cypress S6J3200 Series, Manual

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CHAPTER 21:Ethernet MAC
586 S6J3200 Series Hardware Manual Document Number: 002-04852 Rev. *G
3.1.5.
Transmit DMA Buffers
Frames to transmit are stored in one or more transmit buffers. Transmit frames can be between 1 and
16.384 bytes long, so it is possible to transmit frames longer than the maximum length specified in the
IEEE Std 802.3 standard. It should be noted that zero length buffers are allowed and that the maximum
number of buffers permitted for each transmit frame is 128.
The start location for each transmit buffer is stored in system memory in a list of transmit buffer
descriptors at a location pointed to by the transmit buffer queue pointer. The base address for this queue
pointer must be configured by software using the TX Buffer Queue Base Address registers
(ETHERNETn_transmit_q_ptr, ETHERNETn_transmit_q1_ptr to ETHERNETn_transmit_q3_ptr).
The number of words in each buffer descriptor (BD) is dependent on the operating mode. Each BD word
is defined as 32 bits. The first two words (Word 0 and Word 1) are used for all BD modes.
In Extended Buffer Descriptor mode (ETHERNETn_dma_config[29]:tx_bd_extended_mode_en = 1), two
BD words (Word 2 and Word 3) are added for timestamp capture mode if timestamp capture mode is
enabled (ETHERNETn_tx_bd_control[5:4]:tx_bd_ts_mode > 0h). There are therefore either two or four
BD words in each BD entry depending on the operating mode, and every BD entry will have the same
number of words. To summarize:
−
Every descriptor will be 64 bits wide when descriptor time capture mode is disabled.
−
Every descriptor will be 128 bits wide when descriptor time capture mode is enabled.
The following description details the functionality of Word 0 and Word 1. Each list entry consists of the
same first two words. The first (Word 0) is the byte address of the transmit buffer and the second (Word 1)
contains the transmit control and status. For the packet buffer DMA, the start location for each AXI buffer
is a byte address, the bottom bits of the address being used to offset the start of the data from the
data-word boundary (i.e. bits 2, 1, and 0 are used to offset the address for 64-bit data paths).
Frames can be transmitted with or without automatic CRC generation. If CRC is automatically generated,
pad will also be automatically generated to take frames to a minimum length of 64 bytes. When CRC is
not automatically generated (as defined in Word 1 of the transmit buffer descriptor), the frame is assumed
to be at least 64 bytes long and pad is not generated.
Table 3-3: Transmit Buffer Descriptor Entry
Bit Function
Word 0
31:0 Byte address of buffer
Word 1
31
Used - must be "0" for the Ethernet MAC to read data to the transmit buffer. The Ethernet MAC
sets this 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.
30
Wrap - marks last descriptor in transmit buffer descriptor list. This can be set for any buffer within
the frame.
29 Retry limit exceeded, transmit error detected
28 Unused