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I have been using a papilio pro with a OV7670 camera feeding a frame buffer made of block memory which is then used as the image source for a VGA generator driving the VGA wing. A simple change in the code from; outRGBWrite <= byte1(6) & byte1(4) & byte1(7) & byte1(5) & "00"; --Works! to outRGBWrite <= byte1(6) & byte1(4) & byte1(7) & "000"; --Fails! where; outRGBWrite : out STD_LOGIC_VECTOR(5 DOWNTO 0); and SIGNAL byte0: STD_LOGIC_VECTOR(7 DOWNTO 0); Clearly there was little logical sense in this change making such a difference. I eventually discovered that the top level has; reset: in STD_LOGIC; the value of reset is '1' at startup and then should make the transition to '0', however in the failed case it stays at '1' leaving the system in the reset state. Some of my code set initial values when reset was 1 then doing stuff with those values when reset went to 0. If it never went to 0 the system never started. FACT: I have been working with VHDL for approx 3 weeks so forgive me for not knowing where the reset signal comes from? QUESTION: My problem is solved by ignoring the reset signal and initialising the variables in their declaration rather than letting reset initalise them but the question remains, how is the reset signal being corrupted by the simple change to the lines above that are totally not connected to reset in any way.