architecture detailed_timing of counter is

  signal reset_ipd,                   -- data input port delayed
         clk_ipd : bit;               -- clock input port delayed
  signal q_zd : bit_vector(q'range);  -- q output with zero delay

begin

  input_port_delay : block is
  begin
    reset_ipd <= reset after tipd_reset;
    clk_ipd <= clk after tipd_clk;
  end block input_port_delay;

  functionality : block is

    function increment ( bv : bit_vector ) return bit_vector is
      variable result : bit_vector(bv'range) := bv;
      variable carry : bit := '1';
    begin
      for index in result'reverse_range loop
        result(index) := bv(index) xor carry;
        carry :=  bv(index) and carry;
        exit when carry = '0';
      end loop;
      return result;
    end function increment;

    signal next_count : bit_vector(q'range);

  begin
    next_count <= increment(q_zd) when reset_ipd = '0' else
                  (others => '0');
    q_zd <= next_count when clk_ipd = '1' and clk_ipd'event;
  end block functionality;

  output_port_delay : block is
  begin
    q <= q_zd after topd_q;
  end block output_port_delay;

  timing_checks : block is
  begin
    -- check setup time: reset before clk
    -- . . .
    -- check hold time: reset after clk
    -- . . .
  end block timing_checks;

end architecture detailed_timing;


-- not in book

entity fg_16_13 is
end entity fg_16_13;


architecture test of fg_16_13 is

  signal reset, clk : bit := '0';
  signal q : bit_vector(3 downto 0);

begin

  dut : entity work.counter(detailed_timing)
    generic map ( tipd_reset => 2 ns,
                  tipd_clk => 3 ns,
                  topd_q => 4 ns,
                  tsetup_reset => 3 ns,
                  thold_reset => 1 ns )
    port map ( reset => reset, clk => clk, q => q );

  clk_gen : clk <= '1' after 10 ns, '0' after 20 ns when clk = '0';

  reset <= '1' after 62 ns, '0' after 106 ns;

end architecture test;

-- end not in book

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