How to Combine 8 N/8 FFT's into one N FFT I need to make in FPGA (using Verilog) an FFT. Input data is N=8192 points at 1 GSPS. However, the FPGA operates at 125 MHz, therefore the data is split into 8 channels (each one at 125 MHz). This splitting of data and computing the N/8 FFT is not problematic for me, it is already done. 

What I don't understand is how to combine the outputs of the N/8 FFTs to create one N FFT. I have created a schematic showing the flow of data, where blue blocks represent what is done and green blocks represent what I don't understand. 

I understand generally the Cooley-Tukey FFT algorithm and Butterfly diagrams as they relate to 8 point or 16 point data, but I don't understand how these can be expanded to a 8192 input sequence. 

Any help regarding the theory, math, or FPGA implementation behind how the green blocks are implemented is greatly appreciated!
After giving it some thought I think I've come up with the answer. I was confused since most butterfly diagrams begin with 4 separate n/4 DFT's, like in the image below (annotations added).

Whereas in my problem I begin with 8 separate N/8 DFTs. I was confused about how to connect the outputs of each of the N/8 DFTs to recreate an N DFT. 

Now, however, I believe that nothing really changes between the butterfly diagram attached and my problem. I think that each output of my N/8 FFTs is directly fed into each input of on the diagram attached. And then the twiddle factors increase by a factor of 8 after each iteration. 

Does that sound correct?