Achronix Speedster22i 10G/40G/100G Ethernet - Manual (Page 32)

32 UG029, September 6, 2013
Credit Handling
The following figure provides an overview of the credit based application interface.
Figure 10: Credit based application interface
Receive Direction: FIFO to Application
Internally, the receive datapath FIFO implements a credit counter which defines how many
words the FIFO is allowed to deliver to the application. The credit counter is initialized by
the application at startup, to the maximum burst acceptable by the application (see registers
INIT_CREDIT and CREDIT_TRIGGER).
Once the credit counter has been initialized, the FIFO will provide data to the application as
long as it has credits available and the ff_rx_rdy signal is asserted. The user asserts ff_rx_rdy
to signal to the receive interface that it has consumed the data presently at the ff_rx_data
output. The user should monitor the ff_rx_afull flag if it is not able to continuously read the
received data. Alternatively, if the application can guarantee it is always operating faster
than the MAC will receive data, it may choose to permanently keep the ff_rx_rdy signal high.