24.6.3.4 Transmit AHB Buffers

Frames to transmit are stored in one or more transmit AHB buffers. Transmit frames can be between 1 and 16384 Bytes long, so it is possible to transmit frames longer than the maximum length specified in the IEEE 802.3 standard. It should be noted that zero length AHB buffers are allowed and that the maximum number of buffers permitted for each transmit frame is 128.

The start location for each transmit AHB buffer is stored in 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 is set in software using the Transmit Buffer Queue Base Address register. Each list entry consists of two words. The first is the Byte address of the transmit buffer and the second containing the transmit control and status. For the packet buffer DMA, the start location for each AHB 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 Cyclic Redundancy Checksum (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.

An entry in the transmit buffer descriptor list is described in this table:

Table 24-3. Transmit Buffer Descriptor Entry
Bit	Function
Word 0
31:0	Byte address of buffer
Word 1
31	Used—must be zero for the GMAC to read data to the transmit buffer. The GMAC sets this to one 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	Reserved.
27	Transmit frame corruption due to AHB error—set if an error occurs while midway through reading transmit frame from the AHB, including HRESP errors and buffers exhausted mid frame (if the buffers run out during transmission of a frame then transmission stops, FCS shall be bad and GTXER asserted). Also set if single frame is too large for configured packet buffer memory size.
26	Late collision, transmit error detected.
25:23	Reserved
22:20	Transmit IP/TCP/UDP checksum generation offload errors: 000: No Error. 001: The Packet was identified as a VLAN type, but the header was not fully complete, or had an error in it. 010: The Packet was identified as a SNAP type, but the header was not fully complete, or had an error in it. 011: The Packet was not of an IP type, or the IP packet was invalidly short, or the IP was not of type IPv4/IPv6. 100: The Packet was not identified as VLAN, SNAP or IP. 101: Non supported packet fragmentation occurred. For IPv4 packets, the IP checksum was generated and inserted. 110: Packet type detected was not TCP or UDP. TCP/UDP checksum was therefore not generated. For IPv4 packets, the IP checksum was generated and inserted. 111: A premature end of packet was detected and the TCP/UDP checksum could not be generated.
19:17	Reserved
16	No CRC to be appended by MAC. When set, this implies that the data in the buffers already contains a valid CRC, hence no CRC or padding is to be appended to the current frame by the MAC. This control bit must be set for the first buffer in a frame and will be ignored for the subsequent buffers of a frame. Note that this bit must be clear when using the transmit IP/TCP/UDP checksum generation offload, otherwise checksum generation and substitution will not occur.
15	Last buffer, when set this bit will indicate the last buffer in the current frame has been reached.
14	Reserved
13:0	Length of buffer

To transmit frames, the buffer descriptors must be initialized by writing an appropriate Byte address to bits [31:0] of the first word of each descriptor list entry.

The second word of the transmit buffer descriptor is initialized with control information that indicates the length of the frame, whether or not the MAC is to append CRC and whether the buffer is the last buffer in the frame.

After transmission the status bits are written back to the second word of the first buffer along with the used bit. Bit 31 is the used bit which must be zero when the control word is read if transmission is to take place. It is written to '1' once the frame has been transmitted. Bits[29:20] indicate various transmit error conditions. Bit 30 is the wrap bit which can be set for any buffer within a frame. If no wrap bit is encountered the queue pointer continues to increment.

The Transmit Buffer Queue Base Address register can only be updated while transmission is disabled or halted; otherwise any attempted write will be ignored. When transmission is halted the transmit buffer queue pointer will maintain its value. Therefore when transmission is restarted the next descriptor read from the queue will be from immediately after the last successfully transmitted frame. As long as transmit is disabled by writing a '0' to the Transmit Enable bit in the Network Control register (NCR.TXEN), the transmit buffer queue pointer resets to point to the address indicated by the Transmit Buffer Queue Base Address register (TBQB).

Note: Disabling receive does not have the same effect on the receive buffer queue pointer.

Once the transmit queue is initialized, transmit is activated by writing a '1' to the Start Transmission bit of the Network Control register (NCR.TSTART). Transmit is halted when a buffer descriptor with its used bit set is read, a transmit error occurs, or by writing to the Transmit Halt bit of the Network Control register (NCR.THALT). Transmission is suspended if a pause frame is received while the Transmit Pause Frame bit is '1' in the Network Configuration register (NCR.TXPF). Rewriting the Start bit (NCR.TSTART) while transmission is active is allowed. This is implemented by the Transmit Go variable which is readable in the Transmit Status register (TSR.TXGO). The TXGO variable is reset when:

Transmit is disabled
A buffer descriptor with its used bit set, is read
Bit 10, THALT, of the Network Control register is written
There is a transmit error such as too many retries or a transmit underrun

To set TXGO, write a '1' to NCR.TSTART. Transmit halt does not take effect until any ongoing transmit finishes.

If the DMA is configured for packet buffer Partial Store and Forward mode and a collision occurs during transmission of a multi-buffer frame, transmission will automatically restart from the first buffer of the frame. For packet buffer mode, the entire contents of the frame are read into the transmit packet buffer memory, so the retry attempt will be replayed directly from the packet buffer memory rather than having to re-fetch through the AHB.

If a used bit is read midway through transmission of a multi-buffer frame, this is treated as a transmit error. Transmission stops, GTXER is asserted and the FCS will be bad.

If transmission stops due to a transmit error or a used bit being read, transmission restarts from the first buffer descriptor of the frame being transmitted when the transmit start bit is rewritten.