interfacing 3.3v and 5v logic circuits

It depends. Some 3V3 microcontrollers have 5V tolerant inputs, and they can be stuck on directly to HCT threshold logic, and HC logic but watch the worst- case high threshold, as it's typically about 3V but is specified at 3.5V max. For the rest, you'll need to read the data sheet of the device you want to connect to.

Pure outputs aren't much of a problem, either into HC type logic or FETs, watch the threshold again.

When, like the MSP430, the controller isn't 5V tolerant, there are matching logic families like LVCH that can match a few inputs. If you have a lot of IO to be matched, you may be as well off to interface via something like a small Coolrunner which is 5V tolerant.

Or you can use resistor matching networks, or resistors and zeners, which are cheaper in parts but might be more expensive in space and construction costs.

Paul Burke

Is the 3.3V microprocessor 5 volt tolerant ?

If not, have you considered using something like zener diodes ?

Which microprocessor is it anyway ?

Simon.

Pericom make some nice interfacing IC's - very quick on samples too...

If your micro isn't 5V tolerant, you must put something in your circuit between processor and 5V powered circuit. Something LVC (powered from 3V3 and are 5V tolerant) would do the job, e.g

Regards,

Paul.

Try a level translator from dallas semicondutor or some other company. You can plugin different voltages at different ports and the signals work for your application without damaging your chips. For details go to the website of maxim-ic . Check the level translator which suits your bandwidth / voltage requirements.

Regards, Amit.