188.8.131.52.2. Considerations for Small Designs
The raw FIT for the entire device is always correct. In contrast, the utilized FIT is very conservative, and only becomes accurate for designs that reasonably fill up the chosen device. FPGAs contain overhead, such as the configuration state machine, the clock network control logic, and the I/O calibration block. These infrastructure blocks contain flip flops, memories, and sometimes I/O configuration blocks.
The Projected SEU FIT by Component report includes the constant overhead for GPIO and HSSI calibration circuitry for first I/O block or transceiver the design uses. Because of this overhead, the FIT of a 1-transceiver design is much higher than 1/10 the FIT of a 10-transceiver design. However, a trivial design such as “a single AND gate plus flipflop” could use so few bits that its CRAM FIT rate is 0.01, which the report rounds to zero.
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