Ho! No FPGA-reset button on armadeus card.

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That is a constant comment about the armadeus APF9328 board. Most of the FPGA designers learn that it's mandatory to use a reset in each module with structure like it:

myprocess : process (clk,reset)
begin
  if reset = '1' then
    -- init values
  elsif rising_edge(clk) then
    -- processing code
  end if;
end process myprocess;

But by default there is no reset button on the board, then what to do ?

Plug a reset button

Of course it's possible to solder a reset button on card, there are a large amount of I/O on connector and one can be used for this usage.

Generate the reset internally

VHDL tricks

But it is also possible to use initial state of the FPGA after configuration with this code:

Warning: This code only work with FPGA, because states of flip-flop after configuration is defined to '0'. This can't work with CPLD for example because beginning state is unknown.

 ---------------------------------------------------------------------------
 Entity clock_and_reset_gen is 
 ---------------------------------------------------------------------------
 generic(
     invert_reset : std_logic := '0' -- 0 : not invert, 1 invert
 );
 port 
 (
 	-- external signals
 	ext_clk : in std_logic ;
 	--internal generated signals
 	gls_clk : out std_logic ;
 	gls_reset : out std_logic 
 );
 end entity;
 
 ---------------------------------------------------------------------------
 Architecture clock_and_reset_gen_1 of clock_and_reset_gen is
 ---------------------------------------------------------------------------

 	signal dly: std_logic := '0';
 	signal rst: std_logic := '0';
 	signal int_reset : std_logic;

 begin

 	int_reset <= '0';
    ----------------------------------------------------------------------------
    --  RESET signal generator.
    ----------------------------------------------------------------------------
    process(ext_clk)
    begin
      if(rising_edge(ext_clk)) then
        dly <= ( not(int_reset) and     dly  and not(rst) )
            or ( not(int_reset) and not(dly) and     rst  );
    
        rst <= ( not(int_reset) and not(dly) and not(rst) );
      end if;
    end process;
    
    gls_clk <= ext_clk;
    gls_reset <= rst xor invert_reset ;
 	
 end architecture clock_and_reset_gen_1;

Xilinx template

Xilinx provide a reset generator under its FPGA, it can be instanciated with this vhdl template :

TODO