ISE约束 文件

RAM的写和RAM之间的约束：
NET "ram_we" TNM_NET = FFS ram_we;
INST "u_ram" TNM = RAMS rams_1;
TIMSPEC TS_RAM = FROM "ram_we" TO "rams_1" 14ns;
参考UG612上的：RAM写信号和RAM之间的约束，人为扩大之间的延时；
报错：
ERROR:ConstraintSystem:300 - In file: clk_cross_test.ucf(60): Invalid parameter:
   'TS_RAM' for constraint type 'TIMSPEC':
   This constraint type cannot accept the 'NAME' value as a parameter.

请帮忙看一下。