ok to power level-flexible peripherals with 5V and control with 3.3V?

Hi all,

I have a project with multiple peripherals that work at 3.3-5V. To protect my controller (3.3V only), I’d like to power the peripherals with a separate supply, but only have a 5V on hand.

My question is whether it is a problem to power the peripherals with 5V at Vin pins, but communicate with them via 3.3V logic from the controller?

While I’m sure this could be hardware specific, I was wondering if there were any general rules regarding this.

In case this is hardware specific, my level-flexible peripherals I’m particularly curious about are a 3.5" display (3.5 TFT 320x480 + Touchscreen Breakout Board w/MicroSD Socket [HXD8357D] : ID 2050 : $39.95 : Adafruit Industries, Unique & fun DIY electronics and kits) and 12-bit DAC (MCP4725 Breakout Board - 12-Bit DAC w/I2C Interface : ID 935 : $4.95 : Adafruit Industries, Unique & fun DIY electronics and kits).

I’ve tested this for short periods of time with no issues detected. However, I felt it was best to ask.

My apologies if this has been addressed before, I did a quick search but wasn’t able to find anything straightforward.

Thanks in advance for the help!

Yes, generally you can't safely connect 5V and 3.3V logic directly. Some 3.3V processors have a few "5V tolerant" pins, which means they can tolerate 5V on the input, but of course can't produce more than 3.3V when configured as an output.

Otherwise, when you do it, excess currents flow through protection diodes on the input pins. This can have destructive results under some but not all circumstances. It can be perceived as working, but it is really functioning in zones that haven't been vetted and tested in the design and manufacturing process. Hence it may work today, but nobody knows about tomorrow and the day after that.

As always read the datasheet for the particular hardware involved, guessing will simply lead to mistakes.