I see the following error when I choose IOSTANDARD as LVPECL for output IOs on Spartan-3E, Spartan-3A, Spartan-3AN, and Spartan-3A DSP families:
"ERROR:Pack:2364 - The I/O component <IO name> has output logic with an I/O standard LVPECL_25 (or LVPECL_33) that is input only. Please correct the IOSTANDARD property value."
The LVPECL IO Standard is input only on Spartan-3E, Spartan-3A, Spartan-3AN, Spartan-3A DSP platforms. The error message is valid when the LVPECL IO Standard is applied to output IOBs.
For more information on LVPECL restriction of Spartan-3E/-3A/-3AN-/3A DSP, see Table 10-20 (pg.356) and Table 10-21 (pg.357) of the Spartan-3 Generation FPGA User Guide located at:
However, for the Spartan-3 family, LVPECL is available for output if Vcco is 2.5V. Also see the Table 10-22 (pg.357) of the Spartan-3 Generation FPGA User Guide located at: