求大神指教，这个错误怎么改？
0
[待解决问题]
Error (10821): HDL error at test.vhd(206): can't infer register for "year4[0]" because its behavior does not match any supported register model
代码是这样的：
if (clkout'event and clkout='1') then
if(load='1') then
if(sw='1')then
if(add1='0') then
if (tma="1001" )then tma<="0000";
if (tmb="101" )then tmb<="000";
else tmb<=tmb+1;
end if;
else tma<=tma+1;
end if;
end if;
是在上升沿那出错的，为什么这里不能用上升沿，怎么改
