I want to connect an STM32 based system to an AppleII, 74LS bus. I'm thinking of using logic converters, but since the STM32 is tolerant to 5V I/O, I wonder if connecting its outputs directly to the TTL bus wouldn't be problematic. While the STM32 inputs should work well with TTL outputs, what problems might occur when driving TTL inputs? Or would it be better to use logic converters to drive TTL inputs straight away?

Be cautious and read the data sheet carefully - not _all_ of the STM32s I/O is 5v tolerant (annoyingly, you might find one out of the sixteen pins on a port isn't). I have an idea also that it varies by family, too.

In general, driving from 5v TTL (0 = < 0.8v; 1 = > 2.4v) will give you logic levels without worry for a 3v3 (5v tolerant) CMOS input: 0 < 1.65v; 1 > 1.65v. From a 5v CMOS, again you should have no worries since it wants to drive rail-to-rail so should be clear of the cross-over point.

Going the other way, the 3v3 CMOS of the STM32 will drive rail-to-rail so either 5v CMOS or 5v TTL parts can be driven without issue. However, you may need to be aware of the output's drive speed selection - the lower speeds can be affected by temperature so they rise and fall more slowly, and therefore later, than you expect. Probably not an issue, but bit me in a synchronous sampling system.

Neil

Thank you for your answer. I will try to design a printed plate without buffering and check it.