Circular Buffer: Selecting Range of Indices that Include the Wraparound Point

Question

I think this question is best understood with an example. So here we go:

Imagine the following are defined:

parameter number_of_points_before_point_of_interest = 4;

logic [15:0] test_data = 16'b0000111100001111;
logic [3: 0] point_of_interest;
logic [7: 0] output_data;

if the value assigned to point_of_interest is 1 and the value to number_of_points_before_point_of_interest is 4. I want my output_data to be {test_data[E: F], test_data[5:0]} or 8'b00111100.

So in essence, I want to take 8 bits starting from (point_of_interest - number_of_points_before_point_of_interest) and ending at (point_of_interest - number_of_points_before_point_of_interest + 7).

Since point_of_interest is a variable number, the following two indexing methods are invalid:

To make the code more concise: point_of_interest --> pot

number_of_points_before_point_of_interest --> num_pt_before_pot

buffer[pot - num_pt_before_pot: 4'hF]  // Invalid since pot not constant
buffer[pot -: num_pt_before_pot]       // Part-select doesn't work either 

Note: Variability of pot is not an issue in the second case since starting point can be variable. Regardless, part-select does not provide the desirable results in this example.

Your help is very much appreciated. Thanks in advance

Answer 1

A simple trick you can do is replicate your test_data, then take a slice of it

  output_data   = {2{test_data}}[16+pot-before_pot-:2*before_pot];