The following diagram shows the interpolation filter stages in Gen 3/DFE.
Figure 1. Interpolation Filter Hierarchy
There are four stages of interpolation filters cascaded; each interpolation stage can be bypassed independently. The FIR1 stage contains three interpolation filters - FIR1a (2x), FIR1b (3x), and FIR1c (5x) - only one of them can be enabled for a specified configuration. The FIR2, FIR3, and FIR4 blocks all have an interpolation factor of 2. Using a combination of filters the following list shows all possible interpolation factors:
1x (bypass), 2x, 3x, 4x, 5x, 6x, 8x, 10x, 12x, 16x, 20x, 24x, 40x