constant drivers for net, vhdl shiftreg - vhdl

Question

I'm trying to make a shiftregister in vhdl.
My issue is when I try to store values in the regisgter. This is the code that's causing trouble:
architecture behave of chan_mod is
signal adc_shfreg : std_logic_vector(15 DOWNTO 0);
signal dac_shfreg : std_logic_vector(15 DOWNTO 0);
begin
Rcv_adc:
process(mclk, reset)
begin
if rising_edge(mclk) then
if (reset = '0') then
adc_out <= "0000000000000000";
elsif(chan_on = '1' AND subcycle_cntr = "01" AND chan_sel = '0' AND bit_cntr < 16) then
adc_shfreg <= adc_shfreg(14 DOWNTO 0) & adcdat;
end if;
end if;
end process;
adc_out <= adc_shfreg; --compilation error here
the error i get is this:
Error (10028): Can't resolve multiple constant drivers for net
"adc_out[13]" at chan_mod.vhd(40)
dont know if you need to see my ports, but here they are:
entity chan_mod is
Port ( mclk : in std_LOGIC;
reset : in std_logic;
chan_on : in std_logic;
chan_sel : in std_logic;
adcdat : in std_logic;
dacdat : out std_logic;
bit_cntr : in std_logic_vector(4 DOWNTO 0);
subcycle_cntr : in std_logic_vector(1 downto 0);
dac_in : in std_logic_vector(15 DOWNTO 0);
adc_out : out std_LOGIC_vector(15 DOWNTO 0);
rd : in std_logic;
wr : in std_logic);
end chan_mod;
(as you probably guessed a few of these are used later in the code and are therefore not in my code-sample)

Answer 1

Your problem is that you're driving adc_out in the process as well as using a concurrent assignment. You should replace the assignment to adc_out in the reset case with an assignment to adc_shfreg.
architecture behave of chan_mod is
signal adc_shfreg : std_logic_vector(15 DOWNTO 0);
signal dac_shfreg : std_logic_vector(15 DOWNTO 0);
begin
Rcv_adc:
process(mclk, reset)
begin
if rising_edge(mclk) then
if (reset = '0') then
adc_out <= "0000000000000000"; <--- BAD! Replace adc_out with adc_shfreg
elsif(chan_on = '1' AND subcycle_cntr = "01" AND chan_sel = '0' AND bit_cntr < 16) then
adc_shfreg <= adc_shfreg(14 DOWNTO 0) & adcdat;
end if;
end if;
end process;
adc_out <= adc_shfreg; --compilation error here