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 Gbps and is 25 MHz when operating at
100 Mbps. 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 Gbps and toggles every other clock cycle for 100 Mbps.
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 Gbps 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 Mbps.
