Fix saturation block to synthesize

This is the strangest instance of something that would simulate but not
synthesize that I've seen.  Make a note to write it up for the website
later.  Unless I'm going crazy and it didn't actually simulate earlier.

diff --git a/libraries/dsp/saturate.vhd b/libraries/dsp/saturate.vhd
index 58744fc6cba2b73510a98da2b71239c8a3fda45e..61689e872ecf14641213c7913a0fb789def6f56d 100644 (file)
--- a/libraries/dsp/saturate.vhd
+++ b/libraries/dsp/saturate.vhd
@@ -16,21 +16,19 @@ use unisim.vcomponents.all;
 
 
 entity saturate is
+       generic (
+               WIDTH_IN:  positive := 18;
+               WIDTH_OUT: positive := 16
+       );
        port (
-               dat_i: in  std_logic_vector;
-               dat_o: out std_logic_vector
+               dat_i: in  std_logic_vector(WIDTH_IN-1  downto 0);
+               dat_o: out std_logic_vector(WIDTH_OUT-1 downto 0)
        );
 end saturate;
 
 
 architecture behavioral of saturate is
 
-       constant WIDTH_IN:   positive := dat_i'length;
-       constant WIDTH_OUT:  positive := dat_o'length;
-
-       constant MAX: std_logic_vector(WIDTH_OUT-1 downto 0) := (WIDTH_OUT-1 => '0', others => '1');
-       constant MIN: std_logic_vector(WIDTH_OUT-1 downto 0) := (WIDTH_OUT-1 => '1', others => '0');
-
        signal upper: std_logic_vector(WIDTH_IN downto WIDTH_OUT);
 
        signal is_negative:  std_logic;
@@ -44,8 +42,12 @@ begin
        is_saturated <= '0' when and_reduce(upper) = '1' or
                                 nor_reduce(upper) = '1' else '1';
 
-       dat_o <= dat_i(WIDTH_OUT-1 downto 0)         when is_saturated = '0' else
-                MAX when is_negative = '0' else
-                MIN;
+       dat_o(WIDTH_OUT-2 downto 0) <= dat_i(WIDTH_OUT-2 downto 0) when is_saturated = '0' else
+                                      (others => '1')             when is_negative = '0' else
+                                      (others => '0');
+
+       dat_o(WIDTH_OUT-1) <= dat_i(WIDTH_OUT-1) when is_saturated = '0' else
+                             '0'                when is_negative = '0' else
+                             '1';
 
 end behavioral;