Voltage Translation Interfacing

Many sensor modules use digital interfacing, but with different voltage levels. For example, a typical microcontroller like STM32 might use $3.3\text{ V}$ while an Arduino uses $5\text{ V}$.

Device digital inputs and outputs conform to a standardized set of acceptable values called a “logic family”.

• On the driver side, $V_{\text{OH}}$ is the output high voltage and $V_{\text{OL}}$ is the output low voltage expected from the digital circuit.
• On the receiver side, $V_{\text{IH}}$ is the input high voltage that indicates a digital 1. $V_{\text{IL}}$ is the input low voltage, and is the maximum allowable voltage that indicates a digital 0.

Step-Down Voltage Interfacing

A simple voltage divider is sufficient if your signal is one-directional and the driver outputs a higher voltage than the receiver can handle.

• Recall that GPIO inputs appear as capacitors, and will form RC time constant with divider resistors.

Step-Up Voltage Interfacing

We use a N-channel MOSFET with a pull-up resistor.The example below steps-up from 3.3 V to 5.0 V.

• There will be a rise time set by the $R_{\text{PU}}$ and $5\text{ V}$ input capacitance. The $100\text{ k}\Omega$ value can be lowered to increase the maximum frequency of the digital signal at the cost of increased static power when the MOSFET is conducting.

Bidirectional Voltage Interfacing

A simple circuit that translates digital signals from one level to another is shown. It uses an N-channel MOSFET. The MOSFET drain (D) connects to the higher voltage signal (e.g. 5V signal from a device pin), and the source (S) connects to the lower voltage signal (3.3 MCU pin). This circuit operates bidirectionally but can only be used for digital signals, not analog signals.