Calculating Voltage Dividers And Pull-Ups For Reliable Microcontroller Interfacing

Voltage dividers are simple yet powerful circuits that use resistors to divide a voltage source into two lower voltages. They are heavily utilized when interfacing microcontrollers and other integrated circuits to scale signals, reduce voltage levels for compatibility, and limit current.

This article will cover the key concepts around properly calculating and selecting voltage divider resistors for reliably interfacing 5V and 3.3V microcontrollers and devices. Proper resistor selection ensures the divided voltages remain within specified limits for the connected chips during operation.

Calculating Resistance Values for Desired Voltage Drops

The standard voltage divider equation can be used to select resistor values that produce the exact voltage drops needed:

Table of Contents

Where:

Vout is the desired output voltage
Vin is the supply voltage
R1 is the first resistor
R2 is the second resistor

For example, to create a 3.3V output from a 5V supply, R1 and R2 can be calculated:

Therefore 330Ω and 680Ω resistors would form an ideal voltage divider for this application.

Selecting Resistors Based on Power Ratings

While the resistor values calculate out mathematically, real-world parts have power limitations that must be considered. Power dissipation on resistors can lead to overheating or failure over time.

The power dissipated by each resistor is calculated based on the voltage dropped across it and current through both resistors using Joule’s first law:

Resistors should be selected with a power rating well above the expected dissipation. Commonly used ratings are 1/4 watt, 1/2 watt, 1 watt, and upwards from there.

Using Voltage Dividers to Scale Down Voltages

Voltage dividers can scale down higher supply voltages to the lower logic-level voltages used by modern semiconductors. For example, dividing 5V down to 3.3V for a microcontroller’s inputs.

The output impedance should be kept low for clean signal integrity. Lower resistor values allow more current and thus lower impedance. But higher resistances dissipate less power.

As a rule of thumb, keep the equivalent resistance of the divider less than 10k ohms for clean signals. Calculate the equivalent resistance as:

Optimal performance comes with equivalent resistances from 1k to 5k ohms. Going much below 1k ohms increases power dissipation without noticeable signal benefits.

Interfacing 5V Microcontrollers with 3.3V Devices

Bidirectional voltage dividers can interface 5V and 3.3V devices like microcontrollers. The resistors scale down the 5V I/O to 3.3V for the 3.3V chip. And scale up the 3.3V signal to 5V for the 5V chip’s inputs.

Carefully calculate resistor values to keep voltages seen by each chip within spec while minimizing current flow. Pay close attention to the power dissipation, larger resistances are needed to prevent overheating.

Preventing Excess Current Flow with Pull-up Resistors

Pull-up resistors are used to prevent excess current flow through an input pin. They “pull up” the signal to the positive rail voltage when nothing else is driving the line low. Common values are 1k to 100k ohms.

Microcontroller GPIO pins are often in a high impedance input state by default. This makes them susceptible to floating or picking up stray voltages from circuit noise that can trigger false digital input signals or draw excess current.

Adding the pull-up resistor guarantees the pin gets pulled to the rail supply voltage when floating. And limits the current that can flow during an erroneous short circuit.

Choosing Appropriate Pull-up Resistor Values

To select the ideal pull-up resistor value for a microcontroller application, consider:

Input impedance of the GPIO pin – Lower values overwhelm the impedance
Desired rise time – Larger resistors increase rise time
Current sink capacity – Must be able to sink current through the resistor
Power dissipation – Higher resistances minimize wasted power

10k to 47k ohms serves well for the majority of 5V and 3.3V microcontroller applications. But consult your device datasheet to identify the recommended impedance range and sink currents.

Example Circuits Using Voltage Dividers and Pull-ups

Here are two example microcontroller interfacing circuits using the voltage divider and pull-up concepts:

1. 5V Microcontroller Monitoring a 3.3V Sensor

Key points:

Two 10k resistors form the voltage divider
Divides the 3.3V sensor signal to a 2.2V logic-high signal for the 5V micro’s GPIO
1k resistor pulls the GPIO input high when the sensor output is disconnected

2. Bidirectional Interface Between 5V and 3.3V Microcontrollers