Error (10028): Can't resolve multiple constant drivers for net "sda" at I2C_com.vhd(185) - vhdl

Question

i'm trying to make my own I2C communication and i have a problem with multiply drivers, it's not like i don't understand them i just don't see them (i'm still fresh at vhdl), so please just take a look at my code and tell mi why is there such mistake.
i try to operate on flags to have multiple signal drivers on bus but there's just something not right. The multiple drivers are on scl, sda, start_clk and stop_clk. Is it because those flags are for example in two different processes?
library ieee;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;
entity I2C_com is
port (
reset_en: in std_logic;
clk: in std_logic;
sda: inout std_logic;
scl: out std_logic;
RD:in std_logic;
WR: in std_logic;
addr: buffer std_logic_vector(7 downto 0)
);
end I2C_com;
architecture MAIN of I2C_com is
signal data :std_logic_vector (12 downto 0):="0000000000010";
signal i2c_clk: std_logic ;
signal clk_count : unsigned(19 downto 0):="00000000000000000100";
type program_state is (start,init,error_rd_wr,slave,ack);
signal state: program_state;
signal write_data: std_logic_vector (7 downto 0):=(others => '0');
signal read_data: std_logic_vector (7 downto 0):=(others => '0');
signal clk_enable: std_logic;
signal reset: std_logic:='1';
signal start_clk: std_logic:= 'Z';
signal stop_clk: std_logic:= 'Z';
signal strech: std_logic := '0';
signal cnt_addr: integer := 0;
signal ack_error: std_logic;
signal sda_data: std_logic;
signal start_data: std_logic:= 'Z';
begin
i2c_clock: process(clk,reset_en,reset)
begin
if reset_en = '1' or reset = '1' then
elsif falling_edge(clk) then
if clk_count < unsigned(data) then
clk_count <= clk_count + 1;
clk_enable <= '1';
else
clk_count <= x"00000";
clk_enable <= '0';
end if;
i2c_clk <= clk_enable;
if start_clk = '1' then
sda <= '0';
scl <= '0';
start_clk <= '0';
end if;
if stop_clk = '1' then
sda <= '0';
scl <= '0';
stop_clk <= '0';
end if;
end if;
end process i2c_clock;
--
process(i2c_clk,reset_en,reset)
begin
if reset_en = '1' or reset = '1' then
reset <= '0';
cnt_addr <= 0;
state <= init;
elsif rising_edge(i2c_clk) then
case state is
when init =>
if RD = '1' or WR = '1' then
state <= start;
else
state <= error_rd_wr;
end if;
when start =>
start_clk <= '1';
state <= slave;
when slave =>
start_data <= '1';
if cnt_addr < 8 then
sda_data <= addr(cnt_addr);
cnt_addr <= cnt_addr + 1;
else
cnt_addr <= 0;
state <= ack;
end if;
when error_rd_wr =>
reset <= '1';
when ack =>
start_data <= '0';
ack_error <= sda;
if ack_error = '1' then
stop_clk <= '1';
reset <= '1';
else
end if;
if RD = '1' then
elsif WR = '1' then
else
stop_clk <= '1';
reset <= '1';
end if;
end case;
end if;
end process;
sda <= sda_data when start_data = '1' else 'Z';
scl <= i2c_clk when start_clk = '0' and stop_clk = '0' else 'Z';
end MAIN;

Answer 1

A signal for synthesis can be driven from only one process or one continuous assign; for simulation multiple drivers are possible using resolved signals like std_logic.
The scl and sda are driven both from the i2c_clock process and the continuous assign in the end of the file.
The start_clk and stop_clk are driven both from the i2c_clock process and the other unnamed process.
One possibility for scl and sda is to only drive these from the continuous assign, since synthesis tools often prefer tri-state output to be written like:
q <= value when en = '1' else 'Z';