dsp_engine: don't use SD rounding in 8 bit mode, so we can have a flat noise floor.

2 files changed, 5 insertions, 4 deletions

diff --git a/usrp2/sdr_lib/dspengine_16to8.v b/usrp2/sdr_lib/dspengine_16to8.v
index 5f2a5e1a3..53c5d29da 100644
--- a/usrp2/sdr_lib/dspengine_16to8.v
+++ b/usrp2/sdr_lib/dspengine_16to8.v
@@ -208,14 +208,14 @@ module dspengine_16to8
      (.P(prod_i), .A(scale_factor), .B({i16,2'b00}), .C(clk), .CE(stb_mult), .R(reset) ); 
    clip_reg #(.bits_in(24),.bits_out(16),.STROBED(1)) clip_i 
      (.clk(clk), .in(prod_i[35:12]), .out(scaled_i), .strobe_in(stb_clip), .strobe_out());
-   round_sd #(.WIDTH_IN(16),.WIDTH_OUT(8)) round_i
+   round_sd #(.WIDTH_IN(16),.WIDTH_OUT(8),.DISABLE_SD(1)) round_i
      (.clk(clk), .reset(reset), .in(scaled_i), .strobe_in(stb_round), .out(i8), .strobe_out());
 
    MULT18X18S mult_q 
      (.P(prod_q), .A(scale_factor), .B({q16,2'b00}), .C(clk), .CE(stb_mult), .R(reset) ); 
    clip_reg #(.bits_in(24),.bits_out(16),.STROBED(1)) clip_q 
      (.clk(clk), .in(prod_q[35:12]), .out(scaled_q), .strobe_in(stb_clip), .strobe_out());
-   round_sd #(.WIDTH_IN(16),.WIDTH_OUT(8)) round_q
+   round_sd #(.WIDTH_IN(16),.WIDTH_OUT(8),.DISABLE_SD(1)) round_q
      (.clk(clk), .reset(reset), .in(scaled_q), .strobe_in(stb_round), .out(q8), .strobe_out());
 
 endmodule // dspengine_16to8
diff --git a/usrp2/sdr_lib/round_sd.v b/usrp2/sdr_lib/round_sd.v
index aeeb3502f..94584f6ef 100644
--- a/usrp2/sdr_lib/round_sd.v
+++ b/usrp2/sdr_lib/round_sd.v
@@ -2,7 +2,8 @@
 
 module round_sd
   #(parameter WIDTH_IN=18,
-    parameter WIDTH_OUT=16)
+    parameter WIDTH_OUT=16,
+    parameter DISABLE_SD=0)
    (input clk, input reset,
     input [WIDTH_IN-1:0] in, input strobe_in,
     output [WIDTH_OUT-1:0] out, output strobe_out);
@@ -15,7 +16,7 @@ module round_sd
    sign_extend #(.bits_in(ERR_WIDTH),.bits_out(WIDTH_IN)) ext_err (.in(err), .out(err_ext));
    
    add2_and_clip_reg #(.WIDTH(WIDTH_IN)) add2_and_clip_reg
-     (.clk(clk), .rst(reset), .in1(in), .in2(err_ext), .strobe_in(strobe_in), .sum(sum), .strobe_out(strobe_out));
+     (.clk(clk), .rst(reset), .in1(in), .in2((DISABLE_SD == 0) ? err_ext : 0), .strobe_in(strobe_in), .sum(sum), .strobe_out(strobe_out));
    
    round #(.bits_in(WIDTH_IN),.bits_out(WIDTH_OUT)) round_sum (.in(sum), .out(out), .err(err));