Minimum FIR length is set to 4.
Single Rate FIRs have no restriction placed on the FIR length selection.
However, rate-changing FIRs require each of the individual kernels to operate on a FIR length divisible by Decimation or Interpolation factor, imposing limits on the FIR length, e.g.: TP_FIR_LEN % TP_DECIMATE_FACTOR == 0 or TP_FIR_LEN % TP_INTERPOLATE_FACTOR == 0.
In SSR mode, see Super Sample Rate - Operation Modes, coefficients are distributed equally across all output paths. As a result, total number of FIRs coefficients must be divisible by number of paths, i.e. TP_FIR_LEN % TP_SSR == 0.
For rate-changing FIRs this raises additional limits on the FIR length, where FIR length can be described by e.g.: TP_FIR_LEN % (TP_DECIMATE_FACTOR * TP_SSR) == 0 or TP_FIR_LEN % (TP_INTERPOLATE_FACTOR * TP_SSR) == 0.
Invalid selection will report a message to the User in a form of Metadata error or a static_assert() with rule violation details and suggestion how to fix it.