VHDL - Writing to FPGA Register - vhdl

Question

I have an FPGA with four push buttons - the two left most ones should cycle up and down the 16 registers, while the two right most ones should increment and decrement the value stored in this register. Here is my attempt at the code to do this:
entity raminfr is --inferring the RAM here
port (
clk : in std_logic;
we : in std_logic;
a : in unsigned(3 downto 0);
di : in unsigned(7 downto 0);
do : out unsigned(7 downto 0)
);
end raminfr;
architecture rtl of raminfr is
type ram_type is array (0 to 15) of unsigned(7 downto 0);
signal RAM : ram_type;
signal read_a : unsigned(3 downto 0);
begin
U1: entity work.lab1 port map ( --ERROR ON THIS LINE
register_counter => a,
value_counter => di
);
process (clk)
begin
if rising_edge(clk) then
if we = '1' then
RAM(to_integer(a)) <= di;
end if;
read_a <= a;
end if;
end process;
do <= RAM(to_integer(read_a));
end rtl;
--lab1 starts here
library ieee;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;
entity lab1 is
port(
clock : in std_logic;
key : in std_logic_vector(3 downto 0);
value_counter : out unsigned(7 downto 0) ; --value to be written to register
register_counter : out unsigned(3 downto 0) --register to write value to
);
end lab1;
architecture up_and_down of lab1 is --actual button logic here
begin
process(clock)
begin
if rising_edge(clock) then
if (key(3)='0' and key(2)='0' and key(1)='1' and key(0)='0') then
value_counter <= value_counter + "1";
elsif (key(3)='0' and key(2)='0' and key(1)='0' and key(0)='1') then
value_counter <= value_counter - "1";
elsif (key(3)='1' and key(2)='0' and key(1)='0' and key(0)='0') then
register_counter<= register_counter + "1";
elsif (key(3)='0' and key(2)='1' and key(1)='0' and key(0)='0') then
register_counter<= register_counter - "1";
end if;
end if;
end process;
end architecture up_and_down;
I get the error Error (10577): VHDL error at DE2_TOP.vhd(312): actual port "a" of mode "in" cannot be associated with formal port "register_counter" of mode "out"on the line indicated above. It is obvious this is not how I would go about doing what I want to do. Can someone shed some light on this?

Answer 1

Change your point of view: Put the RAM under the Pushbutton-FSM. Not vice versa.
This RAM description should be synthesiable. If not take a look in the Synthesis Guide of your tool vendor.
entity raminfr is --inferring the RAM here
port (
clk : in std_logic;
we : in std_logic;
a : in unsigned(3 downto 0);
di : in unsigned(7 downto 0);
do : out unsigned(7 downto 0)
);
end entity raminfr;
architecture rtl of raminfr is
type ram_type is array (0 to 15) of unsigned(7 downto 0);
signal RAM : ram_type;
begin
process (clk)
begin
if rising_edge(clk) then
if we = '1' then
RAM(to_integer(a)) <= di;
end if;
do <= RAM(to_integer(a));
end if;
end process;
end architecture rtl;
You also forgot to activate the write enable for the RAM.
Maybe you try this code (Always do a simulation first!):
--lab1 starts here
library ieee;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;
entity lab1 is
port(
clock : in std_logic;
key : in std_logic_vector(3 downto 0);
value_counter : out unsigned(7 downto 0); --value to be written to register
ram_data : out unsigned(7 downto 0); --value from 'RAM-register'
register_counter : out unsigned(3 downto 0) --register to write value to
);
end lab1;
architecture up_and_down of lab1 is --actual button logic here
signal value : unsigned(7 downto 0) := (others => '0');
signal ram_a : unsigned(3 downto 0) := (others => '0');
signal ram_we : std_logic;
begin
-- infer your RAM
your_ram: entity work.raminfr
port map (
clk => clock, --: in std_logic;
we => ram_we, --: in std_logic;
a => ram_a, --: in unsigned(3 downto 0);
di => value, --: in unsigned(7 downto 0);
do => ram_data --: out unsigned(7 downto 0)
);
process(clock)
begin
if rising_edge(clock) then
-- default
ram_we <= '0';
-- change value
if key(1) = '1' then
value <= value + 1;
end if;
-- change value
if key(0) = '1' then
value <= value - 1;
end if;
-- change 'register'
if key(3) = '1' then
ram_a <= ram_a + 1;
end if;
-- write value to register
if key(2) = '1' then
ram_we <= '1';
end if;
end if;
end process;
value_counter <= value;
register_counter <= ram_a;
end architecture up_and_down;