VHDL oddity

[Alex]

I would greatly appreciate if someone could explain the behavior I'm
seeing for me.

In the inner most if-state, where I write to bDATA_OUT ---- if I run
the program as written, it does nothing (my DATA_OUT lines remain in
the state they were previously). If I remove the "else, bDATA_OUT <=
"11000000"" segment, it properly outputs 00001010. I don't understand
why it would work w/o the else, but not w/.

This is a snippet of a larger VHDL, trimmed down for debugging.

Thank you.

Alex McHale




entity driver is
Port ( CLOCK : in STD_LOGIC;
ACTIVE : in STD_LOGIC;
CLOCK_IN : in STD_LOGIC;
LATCH_IN : in STD_LOGIC;
DATA_IN : in STD_LOGIC_VECTOR (7 downto 0);
ADDRESS_IN : in STD_LOGIC_VECTOR (4 downto 0);
DATA_CLOCK_OUT : out STD_LOGIC;
CLOCK_OUT : out STD_LOGIC;
LATCH_OUT : out STD_LOGIC;
DATA_OUT : out STD_LOGIC_VECTOR (7 downto 0) );
end driver;


architecture Behavioral of driver is
signal mode : STD_LOGIC := '0';
signal column_out : STD_LOGIC_VECTOR(9 downto 0) := "0000000000";
signal bDATA_OUT : STD_LOGIC_VECTOR(7 downto 0);
begin
process( CLOCK )
begin
if( rising_edge( CLOCK ) ) then
if mode='0' then
CLOCK_OUT <= '0';
LATCH_OUT <= '0';

mode <= '1';
elsif mode='1' then -- DATA INCOMING
if column_out(0)='0' then
bDATA_OUT <= "00001010";
else
bDATA_OUT <= "11000000";
end if;

CLOCK_OUT <= '1';
LATCH_OUT <= '1';
column_out <= column_out + 1;

mode <= '0';
end if;
end if;
end process;

DATA_OUT <= bDATA_OUT;
end Behavioral;

 

[smithodude]

I'm having a quick guess now as I won't have access to any simulation
tools until tomorrow...

Take your assignation of 'mode' outside of the process. You're trying
to assign 'mode' a value AND test its state with an IF statement.

Mode is just going 0..1..0..1..0..1 on each clock edge I think so
generate a separate process to do just that, something like....

--------------------------------
process(clock)
begin

if rising_edge(clock) then
mode <= NOT mode;
end if;
end process;
-------------------------------

And remove the lines in your VHDL process that read:
"mode <= '1';"
and
"mode <= '0';"

DISCLAIMER...this might be wrong but like I say I haven't got any VHDL
tools right now to simulate with

Good luck
Matt

 

[smithodude]

OK, compiling and simulating this on Modelsim 5.6a and the design seems
to work - DATA_OUT toggles between the two values.

I did however need to change the 'column_out+1' line because the code
is trying to add an integer to a std_Logic_vector

column_out <=
std_logic_vector(to_unsigned((to_integer(unsigned(column_out)) +
1),10));


This is a long winded way of adding 1 to a std_logic_vector and I'm
sure someone else may have a better more concise way.

Hope this works
Matt

 

[alessandro basili]

what about this:

column_out <= column_out + '1';

adding the ieee.std_logic_arith.all package?

 

[alessandro basili]

Sorry but I don't think there is a problem assigning mode and then
checking it, it is pretty ok.

 

[David Ashley]

OK, compiling and simulating this on Modelsim 5.6a and the design seems
to work - DATA_OUT toggles between the two values.

I did however need to change the 'column_out+1' line because the code
is trying to add an integer to a std_Logic_vector

column_out <=
std_logic_vector(to_unsigned((to_integer(unsigned(column_out)) +
1),10));


This is a long winded way of adding 1 to a std_logic_vector and I'm
sure someone else may have a better more concise way.

Hope this works
Matt

Use unsigned instead of std_logic_vector for internal logic
and include ieee.numeric_std.all
for new designs. Convert to std_logic_vector in
your port/interface for compatability.

-Dave