Re: interfacing 3.3v and 5v logic circuits

Susan Perkova wrote:
Can 3.3v microprocessor directly be interfaced with a 5v TTL circuit or how to interface them?

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 *** 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
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