The AXI Ethernet Subsystem provides the
signal axi_str_avbtx_aclk, which is derived from the TEMAC
transmit MAC interface clock. This clock operates at 125 MHz when operating at 1 Gb/s and is
25 MHz when operating at 100 Mb/s. During a transfer, the AXI Ethernet Subsystem uses the
TEMAC transmit MAC interface clock enable to toggle axi_str_avbtx_tready. The clock enable is High for every clock cycle when
operating at 1 Gb/s and toggles every other clock cycle for 100 Mb/s. When the AXI Ethernet
Subsystem is ready to transmit an AVB frame, it drives the axi_str_avbtx_tready signal High. When the external logic is ready to transmit a
frame, it drives the axi_str_avbtx_tvalid signal High and
provides the first byte of data on the axi_str_avbtx_tdata
bus. Now the external logic must provide a new byte of data on every clock cycle that the
axi_str_avbtx_tready signal is High while tvalid is active until the end of the frame is reached.
The external logic cannot throttle the AVB transmit
interface. The AXI Ethernet Subsystem accepts the first byte and then drives
axi_str_avbtx_tready Low until the TEMAC has started the transmit, then it
drives it back to High and continues to use it as a clock enable for the remainder of that
frame.
On the last byte of the frame, the external logic drives
the axi_str_avbtx_tlast signal High for one clock cycle with
axi_str_avbtx_tready. If it does not have any additional frames to
transmit, it removes the tvalid signal when it takes the
tlast signal Low. However, if another frame is ready, the external logic
leaves the tvalid signal High.
The tuser
signal is intended to allow the external logic to indicate that the current frame in progress
has an error such as an underflow and the frame should be aborted. It is intended that this be
connected to the underflow input of the AVB to force the current frame to be aborted, but the
current AVB core does not provide an AVB underflow input. The following first figure shows a
transmit AXI4-Stream waveform for 1 Gb/s mode where there are
additional AVB frames available after the completion of the current frame. The second figure
shows the TX client interface operating at 100 Mb/s.