util_cic: Allow partial gating of CIC comb and int stages

Allow to split a CIC int or comb block into multiple stages and be able to
dynamically gate some of the stages. Also prevent carry propagation in
gated stages to keep the adder output constant.

This is useful for multi-rate filter where not all bits are needed all the
time.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>

library/util_cic/cic_comb.v

@@ -37,10 +37,13 @@
 
 module cic_comb #(
   parameter DATA_WIDTH = 32,
-  parameter SEQ = 1
+  parameter SEQ = 1,
+  parameter STAGE_WIDTH = 1,
+  parameter NUM_STAGES = 1
 ) (
   input clk,
   input ce,
+  input [NUM_STAGES-1:0] enable,
   input [DATA_WIDTH-1:0] data_in,
   output [DATA_WIDTH-1:0] data_out
 );
@@ -76,8 +79,10 @@ end
 end
 endgenerate
 
+wire [DATA_WIDTH-1:0] mask;
 reg [DATA_WIDTH-1:0] data_in_seq;
 wire [DATA_WIDTH-1:0] storage_out;
+wire [DATA_WIDTH-1:0] diff = (data_in_seq | ~mask) - (storage_out & mask);
 
 always @(*) begin
   if (ce == 1'b1) begin
@@ -87,18 +92,19 @@ always @(*) begin
   end
 end
 
-always @(posedge clk) begin
-  if (ce == 1'b1 || active == 1'b1) begin
-    state <= data_in_seq - storage_out;
-  end
-end
-
 generate
-  genvar i;
+genvar i, j;
 
-  for (i = 0; i < DATA_WIDTH; i = i + 1) begin: shift_r
+for (j = 0; j < NUM_STAGES; j = j + 1) begin
+  localparam k = NUM_STAGES - j - 1;
+  localparam H = DATA_WIDTH - STAGE_WIDTH * j - 1;
+  localparam L = k == 0 ? 0 : DATA_WIDTH - STAGE_WIDTH * (j+1);
+
+  assign mask[H:L] = {{H-L{1'b1}},k != 0 ? enable[k] : 1'b1};
+
+  for (i = L; i <= H; i = i + 1) begin: shift_r
     always @(posedge clk) begin
-      if (ce == 1'b1 || active == 1'b1) begin
+      if (enable[k] == 1'b1 && (ce == 1'b1 || active == 1'b1)) begin
         if (SEQ > 1) begin
           storage[i] <= {storage[i][SEQ-2:0],data_in_seq[i]};
         end else begin
@@ -108,7 +114,16 @@ generate
     end
 
     assign storage_out[i] = storage[i][SEQ-1];
+
   end
+
+  always @(posedge clk) begin
+    if (enable[k] == 1'b1 && (ce == 1'b1 || active == 1'b1)) begin
+      state[H:L] <= diff[H:L];
+    end
+  end
+end
+
 endgenerate
 
 assign data_out = state;

library/util_cic/cic_int.v

@@ -36,24 +36,39 @@
 // ***************************************************************************
 
 module cic_int #(
-  parameter DATA_WIDTH = 12
+  parameter DATA_WIDTH = 12,
+  parameter STAGE_WIDTH = 1,
+  parameter NUM_STAGES = 1
 ) (
   input clk,
-  input ce,
+  input [NUM_STAGES-1:0] ce,
   input [DATA_WIDTH-1:0] data_in,
   output [DATA_WIDTH-1:0] data_out
 );
 
 reg [DATA_WIDTH-1:0] state = 'h00;
 wire [DATA_WIDTH-1:0] sum;
+wire [DATA_WIDTH-1:0] mask;
 
 assign data_out = state;
-assign sum = data_in + state;
+assign sum = (data_in & mask) + (state & mask);
+generate
+genvar i;
 
-always @(posedge clk) begin
-  if (ce == 1'b1) begin
-    state <= sum;
+for (i = 0; i < NUM_STAGES; i = i + 1) begin
+  localparam j = NUM_STAGES - i - 1;
+  localparam H = DATA_WIDTH - STAGE_WIDTH * i - 1;
+  localparam L = j == 0 ? 0 : DATA_WIDTH - STAGE_WIDTH * (i+1);
+
+  assign mask[H:L] = {{H-L{1'b1}},j != 0 ? ce[j] : 1'b1};
+
+  always @(posedge clk) begin
+    if (ce[j] == 1'b1) begin
+      state[H:L] <= sum[H:L];
+    end
   end
 end
 
+endgenerate
+
 endmodule