I had been receiving parts that had circuit revisions of C and, recently, D. I just got parts with a circuit revision of A. How can this happen?
The three letter code that precedes the datecode contains the following information:
circuit revision, fab source, process size
For more detail on part marking definitions (see Xilinx Answer 1067).
The circuit revision from the newer He Jian fab reset to A.
Circuit revisions between the He Jian and UMC fabs will not be in synch. The latest devices from UMC (fab code M) have a circuit revision of D, which is equivalent to the devices from He-Jian (fab code W) with circuit revision A.