Russell McMahon wrote:
> I have two chips IC1 with Vdd @ 3.3V and IC2 with Vdd @ 5V. I want to
> connect an OUT pin of IC1 (let call it PIN1) to an IN pin of IC2
> (PIN2). According to the datasheet for IC1 the maximal voltage for
> PIN1 cannot exceed 3.6V.

That doesn't matter, since PIN1 is an output.  It will be driven by IC1 from
0 to 3.3V.

> Question1: Can I safely connect directly PIN1 to PIN2?

Yes.  PIN2 can tolerate 0-5V, and PIN1 will only produce 0-3.3V.

> Question2: Is the information sufficient or does the answer depend on
> what is behind PIN1 and PIN2 (are these TTL, CMOS and such)?

Yes, no.

> Question3 (more specific one): Let PIN1 be an OPEN COLLECTOR and
> PIN2 - a MCLR of a PIC. What is the answer then (on question1)?

Still yes.  There is nothing inherently unsafe about connecting the pins.
However, the pullup voltage must not exceed the maximum allowable operating
voltage of PIN1.  Note that open collector outputs are often rated for
higher voltages than the chip supply voltage.

> Question4 (backward): Let PIN3 be some OUT pin of same IC2 and PIN4 an
> IN pin of IC1 (maximal voltage for PIN4 being still 3.6V of course).
> Let PIN3 be an OPEN COLLECTOR. Can I just connect the pins (adding a
> pull-up resistor to the 3.3V rail)? (I think - 'yes').

Yes, and this arrangement will even work too.


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