TX Descriptor Processing

Versal ACAP Technical Reference Manual (AM011)

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1.4 English

Each transmit frame is stored in one or more memory buffers. Zero length memory buffers are allowed. The maximum number of buffers permitted for each TX frame is 128. The size of the descriptor entry is described in the Descriptor Length section. To transmit frames, the buffer descriptors must be initialized by writing an appropriate byte address to bits [31:0] in the first word of each descriptor list entry.

The second word of the TX descriptor is initialized with control information that indicates the length of the frame, whether 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 that, if transmission is to take place, must be zero when the control word is read. It is written to one once the frame is 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. When 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 is ignored. When transmission is halted, the transmit-buffer queue pointer maintains its value. Consequently, when transmission is restarted, the next descriptor read from the queue is from immediately after the last successfully transmitted frame. While transmit is disabled, the Network_Control [enable_transmit] register bit is set = 0, the transmit-buffer queue pointer resets to point to the address indicated by the Tx_Q_Ptr start address register. Disabling receive does not have the same effect on the receive-buffer queue pointer.

When the transmit queue is initialized, transmit is activated by writing a 1 to the Network_Control [transmit_start] register bit. Transmit is halted when the used bit of the buffer descriptor is read, a transmit error occurs, or by writing to the transmit halt bit of the network control register.

Transmission is suspended if a pause frame is received while the Network_Config [pause_enable] register bit is set = 1. Rewriting the start bit while transmission is active is allowed. The [enable_transmit] bit is reset when the following occurs:

  • Transmit is disabled.
  • A buffer descriptor’s ownership bit set is read.
  • The Network_Control [transmit_halt] register bit is written.
  • There is a transmit error due to too many retries, late collision (gigabit mode only), or a transmit under-run.

To start transmitting, write a 1 to the Network_Control [transmit_start] register bit.

Transmit halt does not take effect until any ongoing transmit finishes. The entire contents of the frame are read into the transmit packet buffer memory, any retry attempt is replayed directly from the packet buffer memory rather than re-fetching it through the AXI. If a used bit is read mid-way through transmission of a multi-buffer frame, the bit is treated as a transmit error. Transmission stops, tx_er is asserted, and the FCS is bad. If transmission stops due to a transmit error or a used bit being read, transmission is restarted from the first buffer descriptor of the frame being transmitted when the transmit start bit is rewritten.