Tuesday, November 3, 2009

FCS Bin Size - Board #1

The FCS software provides four different user-selectable bin sizes. Nominally, the available bin sizes are 0.8 μs, 1 μs, 2 μs, and 5 μs. The actual bin sizes will be slightly different because of how they are calculated in the firmware. (I will go into more detail on this below.) In reality, the bin sizes used by the software are 0.818 μs, 0.998 μs, 1.995 μs, and 5.009 μs.

The FCS hardware is controlled with an SX48 microprocessor, and the different bin sizes are generated by adding or removing instruction cycles to the SX code. The SX uses a 50MHz crystal, so each instruction cycle is 20 ns. These actual bin sizes are the closest we could get to the ideal values given the 20 ns resolution. The code for the 0.818 μs bin size has no delay cycles added, so this is the smallest possible bin size.

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