5V system and 3.3V system level conversion

    When designing a system circuit with MCU or arm, we often meet that the VCC of MCU is 3.3V, but the peripheral circuit needs 5V. Sometimes it‘s the other way around. Although the IO of MCU claims to support TTL level, none of us wants to connect the IO port of MCU directly to 5V. Even if the IO port is connected in series with a resistor and then connected to 5V, we are always worried about burning the MCU. Besides, MCU claims that the IO port supports TTL level, but not all io.

    There are hidden dangers anyway.

    The solution is level conversion.

    Specifically, I only talk about the methods I use in the actual project, and the ones that are more convenient to use in the project. I won‘t say anything else.  

    The first one: using OC or od gate circuit, so that the collector or drain can be pulled up to a new VCC through a resistor, and its base or gate can be connected to another VCC. In this way, 3.3V can control the level signal output of 5V and 12V. Note that it is necessary to select the upper resistance value and consider the driving ability of mcuio. Most of these circuits are used in level conversion circuits on output circuits.

    The second is to use a specific level conversion chip to convert 3.3V and 5V. For example, 74lvc4245a and 74alvc164245 are used more.



    The figure above shows the pin diagram and specific electrical meaning of 74lvc4245a chip. It has 5vvcca and 3.3vvccb, two power pins, so that the conversion between 5V and 3.3V can be realized. At the same time, dir controls the data direction, so that the conversion between 3.3V to 5V and 5V to 3.3V can also be realized. In addition, 74lvc4245a can also increase the current driving capacity of mcuio. I will choose this chip if necessary in my design. 74alvc164245 is 16bit, and its function is similar to 74lvc4245a.

    Of course, there are other level conversion schemes, but by comparison, these two methods are the most practical.