FSM compilation error: Can't resolve multiple constant drivers for net

[Opel_Corsa]

I have a code for an FSM:

library ieee;
use ieee.std_logic_1164.all;
use ieee.std_logic_arith.all;

entity FSM is
     port(
        key : in std_logic_vector(3 downto 0);   -- pushbutton switches
        sw : in std_logic_vector(8 downto 0);    -- slide switches
        ledg : out std_logic_vector(7 downto 0); -- green LED's (you might want to use
                                                 -- this to display your current state)
        lcd_rw : out std_logic;   -- R/W control signal for the LCD
        lcd_en : out std_logic;   -- Enable control signal for the LCD
        lcd_rs : out std_logic;   -- Whether or not you are sending an instruction or character
        lcd_on : out std_logic;   -- used to turn on the LCD
        lcd_blon : out std_logic; -- used to turn on the backlight
        lcd_data : out std_logic_vector(7 downto 0));  -- used to send instructions or characters
end FSM ;

architecture behavioural of FSM is
		type state_type is (M,o,s,t,a);
		signal y : state_type;
begin
        lcd_blon <= '1';   -- backlight is always on
        lcd_on <= '1';     -- LCD is always on
        lcd_en <= key(0);  -- connect the clock to the lcd_en input
		ledg(0) <= key(0); -- send the clock to a green light, to help you debug
        lcd_rw <= '0';     -- always writing to the LCD

        [color=red]process(key)[/color]
		begin
		if (key(3) = '0') then  -- reset LCD
			lcd_rs <= '0';  -- send instructions
			lcd_data <= "00111000";  -- prepare LCD
			lcd_data <= "00111000";
			lcd_data <= "00001100";
			lcd_data <= "00000001";
			lcd_data <= "00000110";
			lcd_data <= "10000000";
		elsif (key(3) = '1') then
			lcd_rs <= '1';  -- send characters
			if (key(0)'event and key(0) = '1') then
				y <= M;
				case y is
					when M => if (sw(0) = '0') then
							  	  y <= o;
							  elsif (sw(0) = '1') then
								  y <= a;
							  end if;
					when o => if (sw(0) = '0') then
							  	  y <= s;
							  elsif (sw(0) = '1') then
								  y <= M;
							  end if;
					when s => if (sw(0) = '0') then
							  	  y <= t;
							  elsif (sw(0) = '1') then
								  y <= o;
						      end if;
					when t => if (sw(0) = '0') then
							  	  y <= a;
							  elsif (sw(0) = '1') then
								  y <= s;
							  end if;
					when a => if (sw(0) = '0') then
							  	  y <= M;
							  elsif (sw(0) = '1') then
								  y <= t;
							  end if;
				end case;
			end if;
		end if;
		end process;

		with y select
			lcd_data <= "01001101" when M,  -- display M
				 		"01101111" when o,  -- display o
						"01110011" when s,  -- display s
						"01110100" when t,  -- display t
						"01100001" when a;  -- display a

end behavioural;

However it doesn't compile and gives me the following error:
Error (10028): Can't resolve multiple constant drivers for net "lcd_data[7]" at FSM.vhd(29)
Line 29 is highlighted on the code.

Any help is greatly appreciated.

 

[aji_vlsi]

Re: Compilation error

I believe you are doing RTL design, coding intended for synthesis and this error came from your synthesis tool, which tool do you use?

Your lcd_data is being driven by a process as well as a concurrent statement with select and hence the issue. Why do you drive it from 2 places?

Regards
Ajeetha, CVC
www.noveldv.com

 

[keano]

Re: Compilation error

		if (key(3) = '0') then  -- reset LCD
			lcd_rs <= '0';  -- send instructions
			lcd_data <= "00111000";  -- prepare LCD
			lcd_data <= "00111000";
			lcd_data <= "00001100";
			lcd_data <= "00000001";
			lcd_data <= "00000110";
			lcd_data <= "10000000";

What value should the signal lcd_data get when key(3)='0' ?
If you put it like that you'll get "10000000", so why the other assignments?

        process(key)
		begin
		if (key(3) = '0') then  -- reset LCD
			lcd_rs <= '0';  -- send instructions
			lcd_data <= "00111000";  -- prepare LCD
			lcd_data <= "00111000";
			lcd_data <= "00001100";
			lcd_data <= "00000001";
			lcd_data <= "00000110";
			lcd_data <= "10000000";
                     .   .   .
                     .   .   .
                     .   .   .

		end process;

		with y select
			lcd_data <= "01001101" when M,  -- display M
				 		"01101111" when o,  -- display o
						"01110011" when s,  -- display s
						"01110100" when t,  -- display t
						"01100001" when a;  -- display a

You can't drive the signal lcd_data in 2 diffrent processes.

 

[Opel_Corsa]

Re: Compilation error

yes, you were both right. i heavily modified my code and now it works. thanks!