VHDL 抛出“Type of aggregate cannot be determined without context”问题

Question

我是 VHDL 的新手，在实习期间我想实现一个数据序列化程序；因此该块将一个字节长的数据作为输入，将其存储在内部信号中，然后在每个时钟上升沿他输出其中的一位。 我希望块的功能是明确的。

如果他不输出数据，我希望块始终输出高阻抗，所以我写了这一行：

data_out <= s_stock(s_count) when(Start='1' and nCS='0') else (data_out => 'Z');

我得到的问题是：

“没有上下文就无法确定聚合类型；这里有 0 个可见类型匹配”

你能帮我解决这个问题吗？

我的完整 VHDL 代码是这样的：

library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.NUMERIC_STD.ALL;
  
entity SERIAL is
  generic(
    constant out_width : natural := 1; -- 1 bit en sortie
    constant in_width : natural := 8 -- 1 octet en entree
    );
        
  Port ( 
  data_in : in STD_LOGIC_VECTOR (in_width-1 DOWNTO 0); -- byte en entree
  Start : in STD_LOGIC; -- Start/Stop control
  clk : in STD_LOGIC; -- Clock 100MHz
  data_out : out STD_LOGIC; -- bits en sortie
  nCS : in STD_LOGIC; -- not chipselect
  READY : out STD_LOGIC -- status bit (READY)
  );
end SERIAL;

architecture Behavioral of SERIAL is
    signal s_stock : STD_LOGIC_VECTOR (in_width-1 DOWNTO 0) := (others => '0'); -- vector to store the input data from ram
    signal s_READY : STD_LOGIC := '1';
    signal s_count : integer range 0 to in_width-1 := 0;
    signal s_data_out : STD_LOGIC;
begin
   SERIALIZING : process(clk)
    variable count : integer range 0 to in_width-1; -- counter 0 --> 7
    begin
        if (s_READY = '1' and nCS ='0') then -- check if loading is ON and serializer is ready for reception
            s_stock <= data_in;
            count := 0;
            s_count <= count;
            s_READY <= '0'; -- toggle off the readiness of the serializer
        elsif rising_edge(clk) then
            if (Start = '1' and nCS ='0') then --check if starting to initialize is ON
                if (count<in_width) then --while the counter is minor than 8
                    data_out <= s_stock(count); --take every single bit of the byte and output it
                    s_data_out <= s_stock(count); --take every single bit of the byte and output it
                    count := count+1; -- counter incrementation
                    s_count <= count;
                elsif (count>=in_width) then
                    s_READY <= '1'; -- toggle on the readiness of the serializer
                end if;
            end if;
        end if;
   end process;

   READY <= s_READY;
   data_out <= s_stock(s_count) when(Start='1' and nCS='0') else (data_out => 'Z');

end Behavioral;