On Thursday 30 December 2004 03:50 pm, Russell McMahon wrote:
> I know this was already discussed here but I still cannot get any
> clear understanding.
> Could somebody explain it to me, please.
>
> 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.
> Question1: Can I safely connect directly PIN1 to PIN2?

This depends on IC2.
Old CMOS chips would preferably have the inputs lifted as high as possible, 
otherwise the chip has the PNP and NPN junctions both conducting.
If the input is a schottky type input, then you have a better chance of 
saying yes. If IC2 specifications seem to be okay with it, then you are okay 
with IC2.
If IC2 has an internal pull-up, you may have trouble with IC1.

> Question2: Is the information sufficient or does the answer depend on
> what is behind PIN1 and PIN2 (are these TTL, CMOS and such)? (I
> suspect that the answer on question1 will be YES if PIN2 is CMOS, and
> NO if TTL (like LS series))

If Pin2 is TTL, I would think it generally more forgiving of the input being 
only 3.3v.  LS even better.  If you have a breadboard, why not test IC2 with 
pin2 at 5v and at 3.3v and at 3.3v-0.7v, take a look at the current draw on 
the power lines, and also take a look at which way the current flows on the 
input pin.

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

You got a problem then.
You clearly do not have a way for PIN1 to sink the voltage down to 3.3v 
because it is open collector and you are going to use some sort of pull-up.
You may want to use a pull-up to the 3.3v instead of the 5v rail but then you 
have a problem with the brownout circuit.
Go back to answer of question2. Breadboard test.

What happens to IC2 if the 3.3v drops to 0v?

You may want to look at capacitor coupling.

> 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').

Sure.
>From the questions above, it appears it is some sort of monitor, so speed is 
not a concern.
If you want to be hardware safe, but curent of little concern, I'd probably 
put a pulldown resistor on PIN4, say 330k to GND, and a resistor between 
PIN3 and PIN4 of 180k.

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