Internal signals in vhdl

Lothar M. Moderator (Company: Titel) #5119761
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Hareesh M. wrote:
> How do i access the internal signals in the top module to testbench
> program.
Wht do you mean with "access"? Just to view the signals? Or to force 
them to a desired state?

And most important: What simulator do you use?

> The change in internal signals causes main signals to change.
Indeed thats usually the desired behaviour of almost every HDL design.
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Hareesh M. OP (Company: Mindteck) #5119762
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Lothar M. wrote:
> Hareesh M. wrote:
>> How do i access the internal signals in the top module to testbench
>> program.
> Wht do you mean with "access"? Just to view the signals? Or to force
> them to a desired state?
>
> And most important: What simulator do you use?
>
>> The change in internal signals causes main signals to change.
> Indeed thats usually the desired behaviour of almost every HDL design.

i want to force them to a desired state, and forcing them causes the 
changes in other signals too. Currently i'm using modelsim simulator.
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Hareesh M. OP (Company: Mindteck) #5119769
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Hareesh M. wrote:
> Lothar M. wrote:
>> Hareesh M. wrote:
>>> How do i access the internal signals in the top module to testbench
>>> program.
>> Wht do you mean with "access"? Just to view the signals? Or to force
>> them to a desired state?
>>
>> And most important: What simulator do you use?
>>
>>> The change in internal signals causes main signals to change.
>> Indeed thats usually the desired behaviour of almost every HDL design.
>
> i want to force them to a desired state, and forcing them causes the
> changes in other signals too. Currently i'm using modelsim simulator.

i'm writing the main code and testbench for the same
////////////////////////////////////////////////////////////////////
1
  library ieee;

2
  use ieee.std_logic_1164.all;

3
    package vhdl_pkg is

4
      constant HOTS_WAP:   std_logic_vector(2 downto 0):= "111";

5
    end vhdl_pkg;

6
  library ieee;

7
  use ieee.std_logic_1164.all;

8
  use ieee.numeric_std.all;

9
  use ieee.std_logic_unsigned.all;

10
  library work;

11
  use work.vhdl_pkg.all;

12
    

13
  Entity hotswap is

14
    port(ifc_clk: in std_logic;

15
        BD_SEL_PLD: in std_logic:= '1';

16
        HS_PG00D_PLD:in std_logic:= '1';

17
        HS_FLT_PLD: in std_logic:= '1';

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        ifc_ad0_7: inout std_logic_vector(2 downto 0);

19
        ifc_cs3_n: in std_logic;

20
        ifc_oe_n: in std_logic;

21
        ifc_we_n: in std_logic;

22
        ifc_a27_31: in std_logic_vector(2 downto 0)

23
         );

24
  end hotswap;

25
  

26
  Architecture mpu of hotswap is

27
    

28
    signal ifc_dout:   std_logic_vector(2 downto 0);

29
    signal cpld_cs:   std_logic;

30
    signal cpld_oe:   std_logic;

31
    signal cpld_we:  std_logic;

32
    signal flt_reg:   std_logic;

33
    signal hotswap_pgood : std_logic;

34
    signal bd_sel_pld_reg : std_logic;

35
    signal pon_rst_n:  std_logic;

36
  begin

37
    process(ifc_clk) 

38
    begin

39
      if(rising_edge(ifc_clk)) then

40
        if(pon_rst_n = '1') then

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          cpld_cs <= '0';

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          cpld_oe <= '0';      

43
        else

44
          cpld_cs <= ifc_cs3_n;

45
          cpld_oe <= ifc_cs3_n and ifc_oe_n;

46
          cpld_we <= ifc_cs3_n and ifc_we_n;

47
        end if;

48
      end if;

49
    end process;

50
    

51
    process(ifc_clk)

52
      begin      

53
      if((cpld_cs and cpld_oe)= '1') then

54
        case ifc_a27_31 is

55
          when HOTS_WAP =>

56
            ifc_dout(0) <= HS_FLT_PLD;

57
            ifc_dout(1) <= BD_SEL_PLD; 

58
            ifc_dout(2) <= HS_PG00D_PLD;

59
          when others =>

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            ifc_dout <= "000";

61
        end case;

62
      end if;

63
     end process;

64
     ifc_ad0_7 (0) <= ifc_dout(0);

65
     ifc_ad0_7 (1) <= ifc_dout(1);

66
     ifc_ad0_7 (2) <= ifc_dout(2);

67
  end mpu;

/////////////////////////////////////////////////////////////////////
and the testbench is
1
library ieee;

2
use ieee.std_logic_1164.all;

3
entity hot_swap is

4
end entity ;

5
architecture hotswapping of hot_swap is

6
 component hotswap 

7
  port( ifc_clk:   in std_logic;

8
        BD_SEL_PLD: in std_logic:= '1';

9
        HS_PG00D_PLD: in std_logic:= '1';

10
              HS_FLT_PLD: in std_logic:= '1';

11
              ifc_ad0_7: inout std_logic_vector(2 downto 0);

12
              ifc_cs3_n: in std_logic;

13
              ifc_oe_n: in std_logic;

14
              ifc_we_n: in std_logic;

15
              ifc_a27_31: in std_logic_vector(2 downto 0)

16
      );

17
 end component;

18
  signal  ifc_clk:    std_logic;

19
  signal  BD_SEL_PLD:  std_logic:= '1';

20
  signal  HS_PG00D_PLD: std_logic:= '1';

21
  signal  HS_FLT_PLD:  std_logic:= '1';

22
  signal  ifc_ad0_7:   std_logic_vector(2 downto 0);

23
  signal  ifc_cs3_n:   std_logic;

24
  signal  ifc_oe_n:    std_logic;

25
  signal  ifc_we_n:    std_logic;

26
  signal  ifc_a27_31:  std_logic_vector(2 downto 0);

27
  constant time_tb:   time:= 10 ms;

28
signal pon_rst_n : std_logic;

29
begin

30
  uut: hotswap port map(

31


32
  ifc_clk => ifc_clk,

33
  BD_SEL_PLD =>  BD_SEL_PLD,

34
  HS_PG00D_PLD => HS_PG00D_PLD,

35
  HS_FLT_PLD  => HS_PG00D_PLD,

36
  ifc_cs3_n =>  ifc_cs3_n,

37
  ifc_oe_n  =>  ifc_oe_n,

38
  ifc_we_n  => ifc_oe_n,

39
  ifc_a27_31 =>  ifc_a27_31);

40
clk: process

41
  begin

42
    ifc_clk <= '1';

43
    wait for time_tb/2;

44
    ifc_clk <= '0';

45
    wait for time_tb/2;  

46
     end process;

47
stimlus: process

48
  begin

49
  wait for 5 ms;

50
  pon_rst_n <= '0';

51
  wait for 5 ms;

52
  pon_rst_n <= '1';

53
  wait for 5 ms;

54
  ifc_a27_31 <= "111";

55
  wait;

56
       end process;

57
end;
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Lothar M. Moderator (Company: Titel) #5119780
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Usually you do NOT force an internal signal because you will not be able 
to do that in a real design. So apply the stimuli data (inputs) in a way 
you get the desired internal signals.

A few words to that design:
At all the simulation of this code will not match the reality due to a 
wrong and incomplete sensitivity list. Here in simulation it will look 
like the process depends on the clock, but in real life it is only a 
completely combinatorial process:
1
    process(ifc_clk)

2
      begin      

3
      if((cpld_cs and cpld_oe)= '1') then

4
        case ifc_a27_31 is

5
          when HOTS_WAP =>

6
            ifc_dout(0) <= HS_FLT_PLD;

7
            ifc_dout(1) <= BD_SEL_PLD; 

8
            ifc_dout(2) <= HS_PG00D_PLD;

9
          when others =>

10
            ifc_dout <= "000";

11
        end case;

12
      end if;

13
     end process;

And much more worse: you implement a latch here.
That is 1. not necessary, 2. not desired and 3. no good design practice.

I suggest to write it without any process (using the concatenate 
operator '&') this way:
1
      ifc_dout <= HS_PG00D_PLD & BD_SEL_PLD & HS_FLT_PLD when 

2
                        cpld_cs='1' and cpld_oe='1' and ifc_a27_31=HOTS_WAP

3
                  else "000";


And one word to that here:
1
if((cpld_cs and cpld_oe)= '1') then

Thats not a fine trick. Write it this way, it is "more usual" and better 
readable:
1
if (cpld_cs='1' and cpld_oe='1') then

If you want you can skip the parenthesis also:
1
if cpld_cs='1' and cpld_oe='1' then



BTW: Pls use the [vhdl] tokens around VHDL code (as described above 
every text edit box).
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Hareesh M. OP (Company: Mindteck) #5119789
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thanks for your suggestion. Actually the values of HS_PG00D_PLD, 
BD_SEL_PLD and HS_FLT_PLD might change while execution.The width of 
ifc_dout is 7, for simulation i made it 3. That's why i wrote like that. 
And i'll change the line as per your suggestion if((cpld_cs and 
cpld_oe)= '1') then.
and what about the testbench?
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Lothar M. Moderator (Company: Titel) #5119806
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Hareesh M. wrote:
> Actually the values of HS_PG00D_PLD, BD_SEL_PLD and HS_FLT_PLD might
> change while execution.
Then those signals must be in the sensitivity list!

If you want to march on with the process, then this is the correct 
sensitivity list:
1
    process(HS_PG00D_PLD, BD_SEL_PLD, HS_FLT_PLD, cpld_cs, cpld_oe, ifc_a27_31)

2
      begin      

3
      if((cpld_cs and cpld_oe)= '1') then

4
        case ifc_a27_31 is

5
          when HOTS_WAP =>

6
            ifc_dout(0) <= HS_FLT_PLD;

7
            ifc_dout(1) <= BD_SEL_PLD; 

8
            ifc_dout(2) <= HS_PG00D_PLD;

9
          when others =>

10
            ifc_dout <= "000";

11
        end case;

12
      end if;

13
     end process;


Hareesh M. wrote:
> and what about the testbench?
You already startet stimuli generation. Just go on with all the possible 
states:
1
stimlus: process

2
  begin

3
  wait for 5 ms;

4
  pon_rst_n <= '0';

5
  wait for 5 ms;

6
  pon_rst_n <= '1';

7
  wait for 5 ms;

8


9
  ifc_a27_31   <= "111";

10
  ifc_ad0_7    <= "ZZZ";

11
  BD_SEL_PLD   <=  '1';

12
  HS_PG00D_PLD <=  '0';

13
  HS_FLT_PLD   <=  '0';

14
  ifc_cs3_n    <=  '0'; -- low active

15
  ifc_oe_n     <=  '0'; -- low active

16
  ifc_we_n     <=  '0'; -- low active

17
  wait for 1 us;

18
  assert ifc_ad0_7="100" report "Read not correct!" severity failure;

19
  wait for 1 us;

20
  

21
  ifc_a27_31   <= "111";

22
  ifc_ad0_7    <= "ZZZ";

23
  BD_SEL_PLD   <=  '1';

24
  HS_PG00D_PLD <=  '1';

25
  HS_FLT_PLD   <=  '0';

26
  ifc_cs3_n    <=  '0'; -- low active

27
  ifc_oe_n     <=  '0'; -- low active

28
  ifc_we_n     <=  '0'; -- low active

29
  wait for 1 us;

30
  assert ifc_ad0_7="110" report "Read not correct!" severity failure;

31
  wait for 1 us;

32


33
  -- and so on

34


35
  wait;

36
end process;


BTW1:
obviously all of your select signals are low active due to the _n at the 
end of their names. Therefore this is rubbish:
1
          cpld_cs <= ifc_cs3_n;              -- low active --> high active??

2
          cpld_oe <= ifc_cs3_n and ifc_oe_n; -- cpld_oe is low active!

3
          cpld_we <= ifc_cs3_n and ifc_we_n; -- cpld_we also!

But at all this complete "control signal process" is not necessary at 
all. Instead it is leading to a kind of clocked bus handling. But that 
you will find out in simulation further on.
A hint: you MUST set the inout bus to HiZ when you don't use it for 
output!

BTW2:
That looks very, very confusing to me:
ifc_ad0_7: inout std_logic_vector(2 downto 0);
Must it be ifc_ad2_0 instead?
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