编了一个29进制计数器，出现以下错误，怎么回事？

WARNING:Xst:528 - Multi-source in Unit <ershijiu> on signal <c> not replaced by logic
Signal is stuck at GND
ERROR:Xst:415 - Synthesis failed
CPU : 1.91 / 3.02 s | Elapsed : 1.00 / 3.00 s

源程序：library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.STD_LOGIC_ARITH.ALL;
use IEEE.STD_LOGIC_UNSIGNED.ALL;

entity ershijiu is
port(
cp:in std_logic;
q:out std_logic_vector(7 downto 0);
c:out std_logic);
end ershijiu;

architecture Behavioral of ershijiu is
signal qa:std_logic_vector(3 downto 0);
signal qb:std_logic_vector(3 downto 0);
signal cin:std_logic;
begin
q(7 downto 4)<=qb;
q(3 downto 0)<=qa;
process(cp)
begin
if rising_edge(cp) then
  if ((qa=9) or (qa=8 and qb=2)) then
   qa<="0000";
   c<='0';
  elsif qa=8 then
  cin<='1';
  qa<=qa+1;
  else
  qa<=qa+1;
  cin<='0';
  end if ;
  end if ;
  end process;
process (cin,cp)
  begin
if rising_edge(cp) then
   if (qa=8 and qb=2) then
   qb<="0000";
   c<='1';
   else
   c<='0';
   end if ;
   if cin='1' then
   qb<=qb+1;
   end if ;
end if ;
   end process;
end Behavioral;

能不能说的具体一点，这个程序是我从书上照抄下来的~~

怎么该啊？