Doubt on how to interface a 5V circuit to a 3V microcontroller (STM32) - Input Pins

[Killstreet30]

 

Hello,

The input 1 and input2 signals will be fed from an STM32 but the problem is the signal sent from STM32 is 3.3V but the input to the IC is 5V. How do I convert the same? Please help.

[KnarfB · Accepted solution]

Well, that good olde L293D.

This is a no brainer because even 3.3V supplied GPIO output generates 5V TTL compatible signal levels.

See data sheet of your chip.

Application note AN4899 "STM32 microcontroller GPIO hardware settings..." is worth reading.

[Killstreet30]

 

I thought I'll use this circuit but how will I calculate the R and R' values? Any idea?

[LCE]

That way you build a voltage divider so that the input will never see neither 0 V nor 5 V.

So better check the datasheet what "5V input" means (high / low thresholds, min / max over complete temperature range), then you might find that what KnarfB said is correct.

The most simple level shifter would be putting a npn transistor (or N-mosfet) "between" your 2 resistors, although then you would have an inverted signal.

[KnarfB]

Well, the 2 R's work as intended if GPIO is configured as open-drain. Then R' pulls up to 5V when GPIO is open and if R is considerably lower, then it pulls down to a low voltage when the GPIO is pulling to gnd.

Dimensioning: make R' small enough that the 5V input gets enough current, but not much smaller. Then, make R such that the low voltage will be low enough when the GPIO is low, but not much smaller.

In case of doubt: use LTspice simulator.

hth

KnarfB

[LCE]

Oops, I never used STM32 GPIOs as open drain, so I actually forgot about that feature...