Buffering Configuration to Guarantee Three Packets Per Uframe ISO Operation for the FX2LP.

Question: What minimum buffering configuration is needed to guarantee three packets per micro frame isochronous operation on the FX2LP

 

Answer:

At a minimum, 4x buffering is necessary on an isochronous endpoint, i.e., EP2, 1024, 4x buffering. This will ensure that there will be enough room to keep the datastream continuous until the host can pick up three more packets in the next microframe.


Typically, for a design that needs to achieve three packets per uframe performance (~24MB/s on the USB side), the interface side will be filling the FIFO buffers greater than the 24 MHz rate. Therefore, by providing 4x buffering, two over-arching requirements are met: Enough buffering is provided so that the interface side is not backed up and; 2) The isochronous pipe is never starved of data since the interface is running greater than 24 MHz.