All FPGA manufactures are continually improving their products to increase performance and reduce price. Often this means changing the design of an FPGA or moving a part from one process generation to the next without changing the part number (and often without changing the specifications).
FPGA companies usually explain their part history in their data books. 1 The following history of Actel FPGA ACT 1 part numbers illustrates changes typical throughout the IC industry as products develop and mature:
- The Actel ACT 1 A1010/A1020 used a 2 m m process.
- The Actel A1010A/A1020A used a 1.2 m m process.
- The Actel A1020B was a die revision (including a shrink to a 1.0 m m process). At this time the A1020, A1020A, and A1020B all had different speeds.
- Actel graded parts into three speed bins as they phased in new processes, dropping the distinction between the different die suffixes.
- At the same time as the transition to die rev. 'B', Actel began specifying timing at worst-case commercial conditions rather than at typical conditions.
From this history we can see that it is often possible to have parts from the same family that use different circuit designs, processes, and die sizes, are manufactured in different locations, and operate at very different speeds. FPGA companies ensure that their products always meet the current published worst-case specifications, but there is no guarantee that the average performance follows the typical specifications, and there are usually no best-case specifications.
There are also situations in which two parts with identical part numbers can have different performance—when different ASIC foundries produce the same parts. Since FPGA companies are fabless, second sourcing is very common. For example, TI began making the TPC1010A/1020A to be equivalent to the original Actel ACT 1 parts produced elsewhere. The TI timing information for the TPC1010A/1020A was the same as the 2 m m Actel specifications, but TI used a faster 1.2 m m process. This meant that “equivalent” parts with the same part numbers were much faster than a designer expected. Often this type of information can only be obtained by large customers in the form of a qualification kit from FPGA vendors.
A similar situation arises when the FPGA manufacturer adjusts its product mix by selling fast parts under a slower part number in a procedure known as down-binning . This is not a problem for synchronous designs that always work when parts are faster than expected, but is another reason to avoid asynchronous designs that may not always work when parts are much faster than expected.