f_nom_period_low_bond<=12.50 ns;            N:=N_high;        END IF;         tu<=(f_nom_period/N);    END PROCESS;----------------------------------------------------------------------------------  Search Mode Process---------------------------------------------------------------------------------- This process is to detect which of the 8 output pin is connected -- to the feedback pin. This process is not repeatable. Because of-- this process, REF_ipd can't be inserted until 10 ns after.-- If feedback is connected to either 1Qx or 2Qx then M=1, if 3Qx then M=4,-- 4Qx then M=2.--------------------------------------------------------------------------------    search_process:PROCESS    BEGIN        wait for 250 us;  -- half PLL lock time        a <= '1';        b <= '1';        wait for 250 us;  -- half PLL lock time        IF (FB_ipd='1') THEN            M <=1;        ELSE            c <= '1';            wait for 2 ns;            IF (FB_ipd='1') THEN                M <= 4;            ELSE                d <='1';                wait for 2 ns;                IF (FB_ipd='1') THEN                    M <=2;                END IF;            END IF;        END IF;        wait;     END PROCESS;----------------------------------------------------------------------------------  Ouput Generate Process (Operation Mode output)---------------------------------------------------------------------------------- This process delay, divide, or multiply the Nominal Frequency (f_nom)-- according to the input of Fx0 and Fx1. The time unit delay (tu)-- is calcualted at tu process.-- Note that these are the outputs connect to the output pins of the-- model during operation mode. --------------------------------------------------------------------------------    output_generate_process:PROCESS        VARIABLE temp3: std_logic:='0';    BEGIN        WAIT ON f_nom;                  IF (F10='0' and F11='0') THEN            qa1<=transport f_nom after (f_nom_period- 4 * tu);        ELSIF (F10='Z' and F11='0') THEN            qa1<=transport f_nom after (f_nom_period-3 * tu);        ELSIF (F10='1' and F11='0') THEN            qa1<=transport f_nom after (f_nom_period-2 * tu);        ELSIF (F10='0' and F11='Z') THEN            qa1<=transport f_nom after (f_nom_period-1 * tu);        ELSIF (F10='Z' and F11='Z') THEN            qa1<=transport f_nom ;        ELSIF (F10='1' and F11='Z') THEN            qa1<=transport f_nom after (1 * tu);        ELSIF (F10='0' and F11='1') THEN            qa1<=transport f_nom after (2 * tu);        ELSIF (F10='Z' and F11='1') THEN            qa1<=transport f_nom after (3 * tu);        ELSIF (F10='1' and F11='1') THEN            qa1<=transport REF_ipd after (4* tu);        END IF;                   IF (F20='0' and F21='0') THEN            qa2<=transport f_nom after (f_nom_period - 4 * tu);        ELSIF (F20='Z' and F21='0') THEN            qa2<=transport  f_nom after (f_nom_period-3 * tu);        ELSIF (F20='1' and F21='0') THEN            qa2<=transport  f_nom after (f_nom_period-2 * tu);        ELSIF (F20='0' and F21='Z') THEN            qa2<=transport  f_nom after (f_nom_period-1 * tu);        ELSIF (F20='Z' and F21='Z') THEN            qa2<=transport  f_nom ;        ELSIF (F20='1' and F21='Z') THEN            qa2<=transport  f_nom after (1 * tu);        ELSIF (F20='0' and F21='1') THEN            qa2<=transport  f_nom after (2 * tu);        ELSIF (F20='Z' and F21='1') THEN            qa2<=transport  f_nom after (3 * tu);        ELSIF (F20='1' and F21='1') THEN            qa2<=transport  f_nom after (4* tu);        END IF;        IF (F30='0' and F31='0') THEN            IF (f_nom='1') THEN                qa3<= not qa3;            END IF;        ELSIF (F30='Z' and F31='0') THEN            qa3<=transport f_nom after (f_nom_period-6 * tu);        ELSIF (F30='1' and F31='0') THEN            qa3<=transport f_nom after (f_nom_period-4 * tu);        ELSIF (F30='0' and F31='Z') THEN            qa3<=transport f_nom after (f_nom_period-2 * tu);        ELSIF (F30='Z' and F31='Z') THEN            qa3<=transport f_nom ;        ELSIF (F30='1' and F31='Z') THEN            qa3<=transport f_nom after (2 * tu);        ELSIF (F30='0' and F31='1') THEN            qa3<=transport f_nom after (4 * tu);        ELSIF (F30='Z' and F31='1') THEN            qa3<=transport f_nom after (6 * tu);        ELSIF (F30='1' and F31='1') THEN             IF (f_nom='1') THEN                temp3:= not temp3;                IF (temp3='1') THEN                    qa3<= not qa3;                END IF;             END IF;        END IF;        IF (F40='0' and F41='0') THEN            IF (f_nom='1') THEN                qa4<=not qa4;            END IF;        ELSIF (F40='Z' and F41='0') THEN            qa4<=transport f_nom after (f_nom_period-6 * tu);        ELSIF (F40='1' and F41='0') THEN            qa4<=transport f_nom after (f_nom_period-4 * tu);        ELSIF (F40='0' and F41='Z') THEN            qa4<=transport f_nom after (f_nom_period-2 * tu);        ELSIF (F40='Z' and F41='Z') THEN            qa4<=transport f_nom ;        ELSIF (F40='1' and F41='Z') THEN            qa4<=transport f_nom after (2 * tu);        ELSIF (F40='0' and F41='1') THEN            qa4<=transport f_nom after (4 * tu);        ELSIF (F40='Z' and F41='1') THEN            qa4<=transport f_nom after (6 * tu);        ELSIF (F40='1' and F41='1') THEN            qa4<=transport  not f_nom;        END IF;    END PROCESS;----------------------------------------------------------------------------------  Generate grid siganl---------------------------------------------------------------------------------- This process generate grid signal which is used to sample the reference-- and feedback signal.  The period of the reference signal (ref_period)-- is calculated then divided by 64 to obtain the grid signal period-- (grid_period). Grid signal is generated after the second reference-- signal.-- Variable loop1 is used to keep track with the numbe of the loop.-- Note that the last "WAIT FOR grid_period" was void to prevent aliasing.--------------------------------------------------------------------------------    ref_grid_process:PROCESS        VARIABLE redge1 : time:= 0 ns;        VARIABLE redge2 : time:= 0 ns;        VARIABLE temp   : time;        VARIABLE loop1  : integer:=0;    BEGIN        wait on REF_ipd;        IF (TEST_ipd='0') THEN           IF (REF_ipd='1') THEN             ref_count<=ref_count+1;             redge2 :=redge1;             redge1 := NOW;                 ref_period<= (redge1-redge2);                 grid_period <= (ref_period/64);             IF (ref_count>1) then                 FOR i in 0 to 30 loop                     grid<='1';                     WAIT FOR grid_period;                     grid<='0';                     WAIT FOR grid_period;                      loop1:=loop1+1;                 END LOOP;                     grid<='1';                     WAIT FOR grid_period;                     grid<='0';                     loop1:=0;             END IF;           END IF;        END IF;    END PROCESS;----------------------------------------------------------------------------------  Weight Count Process---------------------------------------------------------------------------------- This process samples reference signal and feedback signal with grid signal.-- Weight Count (weight_count) signal is used to adjust the nominal-- period ( to be speed up or slow down). -- The adjusted nominal period is f_nom_period_fin.-- f_nom_period_fin is then to compare with the lower and upper bond-- of the frequency range. If it exceeds the range, it will be clipped.--------------------------------------------------------------------------------    weight_count_process:PROCESS        VARIABLE loop2  : integer:=0;        VARIABLE loop3  : integer:=0;    BEGIN        WAIT ON grid;        IF (TEST_ipd='0') THEN            IF (FB_ipd='1') THEN                FOR i in 0 TO 31 LOOP                    IF( grid='1') THEN                        IF ( FB_ipd='1' and REF_ipd='0') THEN                            weight_count<=weight_count-1;                        ELSIF (REF_ipd='1' and FB_ipd='Z') THEN                            weight_count<=weight_count+1;                        ELSE                            NULL;                        END IF;                        loop2:=loop2+1;                        WAIT FOR grid_period*2;                    END IF;                END LOOP;                loop2:=0;                f_nom_period_fin<= (f_nom_period+weight_count*grid_period);                loop3:=loop3+1;                IF (f_nom_period_fin<f_nom_period_low_bond) THEN                    f_nom_period_fin<=f_nom_period_low_bond;                ELSIF (f_nom_period_fin>f_nom_period_up_bond) THEN                    f_nom_period_fin<=f_nom_period_up_bond;                ELSE                    NULL;                END IF;            END IF;        END IF;            END PROCESS;---------------------------------------------------------------------------------- Feedback Process---------------------------------------------------------------------------------- This process is to calculated feedback period.--------------------------------------------------------------------------------    fb_process:PROCESS        VARIABLE fedge1 : time:= 0 ns;        VARIABLE fedge2 : time:= 0 ns;        VARIABLE temp   : time:= 0 ns;    BEGIN        wait on FB_ipd;        IF (FB_ipd='1') THEN            fb_count<=fb_count+1;            fedge2 := fedge1;            fedge1 := NOW;            IF (fb_count>1) then                fb_period<= (fedge1-fedge2);            END IF;         END IF;    END PROCESS;---------------------------------------------------------------------------------- Nominal Period Mux Process---------------------------------------------------------------------------------- This process decides when nominal period gets updated by the adjusted-- nominal period.--------------------------------------------------------------------------------    f_nom_period_mux_process:PROCESS(f_nom_period_ini,                                     f_nom_period_fin,ref_count)    BEGIN        IF (ref_count<4) THEN            f_nom_period<=f_nom_period_ini;        ELSE            f_nom_period<=f_nom_period_fin;        END IF;    END PROCESS;END vhdl_behavioral;