In the Lattice IBIS files, the series resistance code of all IO type is 'a', which mean "off". I want to know in simlation, what resistance value should I set for output?
There are two ways to specify the output drivers. One is to specify series resistance, the other is to specify output current. Lattice IBIS models specify output current. They are equivalent. For example, 4mA is equivalent to 100ohms (0.4V(vol max)/4mA(iol max) = 100 ohms). So our IBIS model has the output impedance build in already, you should not add additional resistance on top of our model in simulation.