When implementing a filter with symmetric coefficients using the Multiply-Accumulate architecture, you must be aware that the core reorganizes the filter coefficients if required to exploit symmetry, and this might alter the filter response. This is only necessary if the core is configured such that all processing cycles are not used. For example, when the core has four cycles to process each sample for a 30-tap symmetric response filter, the core pads the coefficient storage out as shown in the following figure.
The appended zeroes after the non-zero coefficients do not affect the filter response, but the prepended zero coefficients do alter the phase response of the filter implementation when compared to the ideal coefficients. There are two ways to avoid this issue: First, and simplest, you can force the Coefficient Structure to be Non-Symmetric. This avoids the issue of prepending zero coefficients to the coefficient vector, and only appended zeroes are used to pad out the filter response to the required number of cycles. Second, and more efficient, you can increase the number of taps implemented by the filter at little or no cost in resource usage. In the previous example, the filter could process 32 taps in the same time, with the same hardware resources, and with the same cycle latency as the 30-tap implementation, and the phase response of the 32-tap filter would be unaltered.
The Vivado IDE displays the actual number of coefficients calculated on the Implementation Details tab. You can use this information to determine if you can increase the number of coefficients used by your filter definition.