Logic Levels

by hacker

So, I am pretty sure you’ve come across various products (if you’re a hardware hacker) that have different logic levels. Like the ARM processors/controllers work on 3v3 or 3.3V and chips like those of ATMEL and MICROCHIP or even 8051 which work on 5V. They mention in their data sheets the compatibility factors for shields and stuff like 3v3 TTL or 5v TTL. So, lets go a bit deeper and figure it out, shall we?


Okay first things first, I am sure you’ve heard about logic gates. The AND, OR, NOT gates. When we create a digital circuit, everything proceeds through logical gates. The universal gates, the NOR and NAND are the gates that are used to get stuff done. But having these gates on a piece of paper and having it on the integrated circuit base is a different thing entirely. In order to realize the logic that the designers create, we use components like resistances or transistors among other things. So when you have, say an AND gate created using transistors and another one created using diodes, you need to be sure that they communicate at the same voltage.

Because, maybe your diode circuit output gives an output at say 3v3 and the transistor expects a 0v or a 5v signal. So, if you connect these two, the transistor might just take all of the input signals as a 0v signal due to threshold values.

When I say threshold values I mean that every device has some threshold above or below it recognizes it to be a logic level. So, if say a transistor threshold for a HIGH is 4.3v to 5.2v that will count as a HIGH and 0v – 3.9v as LOW.


And Gate - Example

So there you have it, all the devices need a common platform to work on and that is exactly what a logic level converter does. It converts the incoming 5v to 3v3 (or any to any).

Now that we know a bit about Logic Levels, we can effectively categorize the most heard of Logic Level Families:

We have many voltage level converters available. Here’s one from SparkFun


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