# voltage matching between two ICs

I am using a processor which operates at 5 V and a MUX with 3.3 V. If I want to give selection lines from the processor, I have step down the voltage level. how can i do this by simply placing a resistor across it. How can I find the value of the resistor?

• A single resistor won't do, a divider maybe, but who knows what signals this is about and if they won't break. Why don't you just use a mux with the voltage level of a processor (or the other way round)? Jul 24, 2017 at 9:10
• What part numbers? Jul 24, 2017 at 9:14
• Sounds like you want a logic level converter (LLC). Jul 24, 2017 at 9:42
• Wasn't something like this asked literally a few hours ago about using resistors to connect a 5V device to s 3V3 device? electronics.stackexchange.com/questions/320099/…
– MCG
Jul 24, 2017 at 9:53
• @HarrySvensson, re-read the other question. It's about a 5 V device connecting to inputs of a 3.3 V processor. So essentially the same problem. Jul 24, 2017 at 14:52

## 4 Answers

There are several options, e.g:

1. Use a voltage divider
2. Use a series resistor. This is used to limit the current into the protection diode of the input pin. This has to be done with care, the input characteristics and ratings have to be considered. The resistor must be bigger than (5.0V-3.3V)/MaxInputCurrent. The maximum input current is normally listed in the datasheet of the mux.
3. Use a level shifter or a logic buffer/inverter with 5V compatible inputs like the 74LVC series.
4. If you are lucky, the selection lines of the mux may be 5V-compatible, check the datasheet.
5. Depending on your circuit and type of the mux, you can supply the mux with 5V. No level shifting needed.

Please keep in mind that the resistor-based solutions are generally relatively slow due to the time constants involved. If you want to change you selection lines very fast, it may not be the appropriate solution.

For an almost complete guide on 3.3V to 5V (and vice-versa) interfacing, see http://ww1.microchip.com/downloads/en/DeviceDoc/chapter%208.pdf

Use a voltage level translator if they are only digital lines. Something like this might work: TXB0104

• The TXB0104 may work, but it's overkill for this task. Aug 25, 2017 at 8:18
• Additional note: The TXB0104 is a bidirectional, direction sensing level shifter. Those have normally a very weak drive strength and care must be taken with pullup resistors. I wouldn't use this type unless it's already in the design or it's specifically needed to fulfill the requirements, i.e. a bidirectional connection. Aug 25, 2017 at 8:25

I would prefer using the level translators i.e buck converter for your application. A simple 5V to 3.3V should be easily available in the market.

• You said: "I would prefer using the level translators i.e buck converter [...]" Note that level translators and buck converters are different things, have different characteristics and would be used to meet different requirements (signals vs. power). It doesn't make sense to link them in one sentence like that. :-( Jul 24, 2017 at 21:28
• I agree that my comment was very general and will make sure to be more elaborative in future. Jul 25, 2017 at 4:17