With TTL, you're looking into the emitter of an NPN transistor clamped to the rails. Since it sources some amount, ~600uA for LS and ~1.2mA for standard TTL, of current, it shouldn't require a pullup when driven from a CMOS device like these MCU's. However, if you have a CMOS device out there from the uncommitted port 0, you need pullups, else the CMOS inputs will float to whatever the environment lets it float, and it may change quite randomly as you wave your hands around it.

RE