OnButtonLeft_Down:
  process(OnBtnLeft_DownEvent)
  begin
    if rising_edge(OnBtnLeft_DownEvent) then
        if (PulsePerPeriod_VAR > 0) then
              PulsePerPeriod_VAR := PulsePerPeriod_VAR - 1;           --<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
       end if;
     end if;
  end process OnButtonLeft_Down;

  OnButtonRight_Down:
  process(OnBtnRight_DownEvent)
  begin
    if rising_edge(OnBtnRight_DownEvent) then
        if (PulsePerPeriod_VAR < PulsePerPeriod_VAR_MAX) then
              PulsePerPeriod_VAR := PulsePerPeriod_VAR + 1;           --<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
       end if;
     end if;
  end process OnButtonRight_Down;

UCF
Net "Btn_Left" Loc = "D18" | IOStandard = LVTTL | PullDown;
Net "Btn_Right" Loc = "H13" | IOStandard = LVTTL | PullDown;

VHD
  shared variable PulsePerPeriod_VAR: integer range 0 to PulsePerPeriod_VAR_MAX := 624;

. . . . .
  
  ButtonsPolling:
  process(Clock_100Hz)
  begin
    if rising_edge(Clock_100Hz) then
        if Btn_Left = '1' then
            Key_01 := '1';
            if Key_01 = Key_02 then
                OnBtnLeft_DownEvent <= '1';
            end if;
        else
            OnBtnLeft_DownEvent <= '0';
        end if;
        if Btn_Right = '1' then
            Key_01 := '1';
            if Key_01 = Key_02 then
                OnBtnRight_DownEvent <= '1';
            end if;
        else
            OnBtnRight_DownEvent <= '0';
        end if;
    Key_02 := Key_01;
  end process ButtonsPolling;

  OnButtonLeft_Down:
  process(OnBtnLeft_DownEvent)
  begin
    if rising_edge(OnBtnLeft_DownEvent) then
        if (PulsePerPeriod_VAR > 0) then
            PulsePerPeriod_VAR := PulsePerPeriod_VAR - 1;
        end if;
    end if;
  end process OnButtonLeft_Down;

  OnButtonRight_Down:
  process(OnBtnRight_DownEvent)
  begin
    if rising_edge(OnBtnRight_DownEvent) then
        if (PulsePerPeriod_VAR < PulsePerPeriod_VAR_MAX) then
            PulsePerPeriod_VAR := PulsePerPeriod_VAR + 1;
        end if;
    end if;
  end process OnButtonRight_Down;

  OnButtonLeftRight_Down:
  process(OnBtnLeft_DownEvent,OnBtnRight_DownEvent) -- изменение длительности импульса
  begin
    if rising_edge(OnBtnLeft_DownEvent) then
       if (PulsePerPeriod_VAR > 0) then
           PulsePerPeriod_VAR := PulsePerPeriod_VAR - 1;
       end if;
    elsif rising_edge(OnBtnRight_DownEvent) then
       if (PulsePerPeriod_VAR < PulsePerPeriod_VAR_MAX) then
           PulsePerPeriod_VAR := PulsePerPeriod_VAR + 1;
       end if;
    end if;
  end process OnButtonLeftRight_Down;

  
   --================================================
    -- 0 разряд сигнала buttons соответствует кнопке 0
    -- 1 разряд сигнала buttons соответствует кнопке 1
    --================================================
    OnButtonLeftRight_Down : process(clk)
    begin
        if rising_edge(clk) then
            case buttons is
                when "10"   =>  PulsePerPeriod_VAR := PulsePerPeriod_VAR - 1;
                            
                when "01"   =>  PulsePerPeriod_VAR := PulsePerPeriod_VAR + 1;
                            
                when others => null;
           end case;
        end if;
    end process;