我编了个数字相位计，测两个同频输入方波的相位差+-180度，其中有个鉴相器，程序如下：
library ieee;
use ieee.std_logic_1164.all;
use ieee.std_logic_unsigned.all;
entity pd is
port( a,b : in std_logic;
      en,flag: out std_logic);
end pd;
architecture art of pd is
begin
en <= a xor b;
process(b)is
begin
if (b'event and b='1')then
flag <= a;
end if;
end process;
end art;
我综合过了，想用Spartan2 实现，结果在“Map”时出错了，显示如下：
Started process "Map".
Using target part "2s100pq208-6".
ERROR:MapLib:93 - Illegal LOC on IPAD symbol "b" or BUFGP symbol "b_BUFGP"
   (output signal=b_BUFGP), IPAD-IBUFG should only be LOCed to GCLKIOB site.
Error found in mapping process, exiting...
Errors found during the mapping phase.  lease see map report file for more
details.  Output files will not be written.
Design Summary
--------------
Number of errors   :   1
Number of warnings :   0
ERROR: MAP failed
Process "Map" did not complete.
Mapping Module phasemeter . . .
MAP command line:
map -intstyle ise -p xc2s100-pq208-6 -cm area -pr b -k 4 -c 100 -tx off -o phasemeter_map.ncd phasemeter.ngd phasemeter.pcf
Mapping Module phasemeter: failed


请各位看看吧，给小弟说明下原因，下周还没解决的话，我这个科目将不及格了。我们老师要求太高了。