VHDL code for register, to use in a binary multiplication circuit

Question

I wrote a piece a VHDL code for a register (to make a shift register circuit) in a binary multiplication circuit. Once I analyzed it in Quartus II several syntax errors were displayed.

This is my code:

ENTITY memory IS
PORT (can_load, can_shift, can_ad, sb_input, ab_input, UserInput : IN BIT;
        Out_Bit, Z : OUT BIT);
END memory; 

ARCHITECTURE logic OF memory IS
    SIGNAL State: BIT := '0';
    BEGIN
        IF (can_load = '1') THEN
            State <= UserInput;
        ELSE
            IF (can_ad = '1') THEN
                Z <= State; --Z is the output that goes to the 4 bit adder
                State <= ab_input;
            END IF;
            IF (can_shift = '1') THEN
                Out_Bit <= State;
                State <= sb_input;
            END IF;
        END IF;
END logic;

These are the error messages:

Info: *******************************************************************

Info: Running Quartus II 64-Bit Analysis & Synthesis Info: Version 14.0.0 Build 200 06/17/2014 SJ Web Edition Info: Processing started: Sun Oct 19 16:28:22 2014 Info: Version 14.0.0 Build 200 06/17/2014 SJ Web Edition Info: Processing started: Sun Oct 19 16:28:22 2014

Info: Command: quartus_map --read_settings_files=on --write_settings_files=off memory -c memory

Warning (20028): Parallel compilation is not licensed and has been disabled

Error (10500): VHDL syntax error at memory.vhd(9) near text "IF"; expecting "end", or "(", or an identifier ("if" is a reserved keyword), or a concurrent statement

Error (10500): VHDL syntax error at memory.vhd(9) near text "THEN"; expecting "<="

Error (10500): VHDL syntax error at memory.vhd(11) near text "ELSE"; expecting "end", or "(", or an identifier ("else" is a reserved keyword), or a concurrent statement

Error (10500): VHDL syntax error at memory.vhd(12) near text "THEN"; expecting "<="

Error (10500): VHDL syntax error at memory.vhd(15) near text "IF"; expecting ";", or an identifier ("if" is a reserved keyword), or "architecture"

Error (10500): VHDL syntax error at memory.vhd(16) near text "THEN"; expecting "<="

Error (10500): VHDL syntax error at memory.vhd(19) near text "IF"; expecting ";", or an identifier ("if" is a reserved keyword), or "architecture"

Info (12021): Found 0 design units, including 0 entities, in source file memory.vhd

I have already checked several books for the correct syntax, and code examples and yet I cannot find where's my mistake.

I also tried to take away the parentheses in sections like this:

IF (can_load = '1') THEN

having something like this:

IF can_load = '1' THEN

but I ended up with most of the same syntax errors.

I'd appreciate any help to solve this issue. Thank you.

IF ...THEN is a sequential statement. You are using it in the parallel processing region of an architecture. Use it in a process inside the architecture. — user_1818839
Thank you, I did it and it worked. I had no clue I couldn't use IF...THEN outside a process. — David

Unknown Unknown · Accepted Answer · 2014-10-20T00:34:19

I used a different tool to demonstrate the errors:

ghdl -a memory.vhdl
memory.vhdl:9:9: a generate statement must have a label
memory.vhdl:9:29: 'generate' is expected instead of 'then'
ghdl: compilation error

Note the analyzer is complaining about a generate statement. This is because an if statement is a sequential statement only found in a process or other concurrent statement or in a subprogram.

A generate statement with an conditional scheme (hence the if) is a concurrent process statement and requires a label.

Putting the if statement in a process:

entity memory is
port (can_load, can_shift, can_ad, sb_input, ab_input, userinput : in bit;
        out_bit, z : out bit);
end memory; 

architecture logic of memory is
    signal state: bit := '0';
begin
SOME_PROCESS:
    process (userinput, ab_input, state, sb_input)
    begin
        if can_load = '1' then
            state <= userinput;
        else
            if can_ad = '1' then
                z <= state; --z is the output that goes to the 4 bit adder
                state <= ab_input;
            end if;
            if can_shift = '1' then
                out_bit <= state;
                state <= sb_input;
            end if;
        end if;
    end process;
end logic;

Analyzes.

Note I added userinput, ab_input, state, and sb_input to the process sensitivity list (everything that showed up on the right hand side of an assignment).

The presence of state also brings up another point. The new value of state is not available in the current simulation cycle. In your example the value of out_bit would be the value of state found before the process is executed.

And in most cases the parentheses in an if statement condition are superfluous. Parentheses are required for cases when the left right evaluation order isn't sufficient to determine the correct meaning (e.g. mixing and and or operators), operators are functions, and functions are expressions.

VHDL code for register, to use in a binary multiplication circuit

1 Answers