Frequency doubler with xor and delay

[Altera_Forum]

hello, 

I need to double an input frequency (input frequency range 1KHz-100KHz) using an EPM1270 MAX II device. I know that it is possible using an XOR gate with a lot of delay on one of the inputs.I tried to do this by connecting an input of the XOR gate directly to the input frequency and the other input with a series of 12 NOT gates to achieve a delay of a few tens of nanoseconds, but the output of the xor does not switch. Can anyone help me? thank you massimo

[Altera_Forum]

Firsly look at the post snythesis netlist (technology map viewer). Most likely the inverters have been discarded during synthesis. Read about the "synthesis keep" attributes that have to be applied. There are several previous forum threads about generating logic cell delay. 

 Your expectations about the delay are probably wrong. I would calculate about 0.25 ns per LE with MAX II or Cyclone series.