Generate HSPICE input signal
Without seeing the schematic and some values it's impossible to tell. You could try to get around using either a behavioural source with int(), round(), floor(), or ceil() -- if a continuous voltage is applied at the input -- or a PWL() source, something like: PWL(1 0 +1m 1 2 1 +1m 3 ...) -- which will not need any input signal. There may be other, more complicated ways, by creating circuits that generate this kind of staircase signal, but since you didn't say anything else, there's not much else to talk about.