A generalization that is not entirely correct.



Most micros are NOT TTL compatible. They require more Vih than the Voh that TTL provides. Further, the output current from most "TTL" devices was on the order of 12-16 mA, and Iih was not relevant, since the input is really an emitter. On LSTTL devices the Iih is lower, but still on the order of 0.6 mA. Ioh is still the same, but for LSTTL or TTL was irrelevant since the driven load was an emitter or, at best, a diode pair from a resistor to Vcc. CMOS, OTOH, required essentially no current as either sink or source to drive its inputs, but it does require that the voltage be adequate. Not all micros have a sufficiently high Voh to satisfy CMOS input Vih requirements. Most micros can source at least 0.6 mA, and will sink at least 1.2 mA, but beyond that, you can't make any assumptions. Many can do more, but you've got to read the datasheet to be sure.

A lot depends on the specifics of the devices you've chosen to apply. You've got to use data that applies to those particular devices. If you don't you'll be disappointed.

RE