Why Does the Output Value of the Frequency Show a Minus When I Put 35 kHz and Above Using Arduino UNO?

Introduction

The Arduino is a popular and versatile platform for embedded systems and hobbyist electronics projects. However, when working with frequency measurements, especially with frequencies above 35 kHz, you may encounter a peculiar situation where the output value appears as a negative number. This article aims to demystify this issue and provide solutions to ensure accurate frequency measurement.

Understanding the Problem

If yoursquo;re experiencing negative values for frequencies above 35 kHz, itrsquo;s highly likely that you are using a 16-bit signed integer to store the frequency value. A 16-bit signed integer can only represent values from -32768 to 32767 due to the bit allocation (15 bits for the actual value with the most significant bit as the sign bit).

Root Cause of the Issue

1. Data Type Limitations

1.1. Signed Integer Range
A 16-bit signed integer can only store values from -32768 to 32767. When you attempt to store a value above 32767, it is represented as a negative number due to the overflow. For example, 35000 kHz would be stored as -7236 in a 16-bit signed integer.

1.2. Reason with Frequency in Hz
Regardless of whether you are measuring in kHz or Hz, the fundamental limitation remains. If you measure 35000 Hz in a 16-bit signed integer, the maximum value that can be stored is 32767 Hz.

1.3. Register Value vs. Frequency Counters
The 32767 limit is due to the register value, not the frequency scanning process or analog-to-digital conversion. This is a hardware limitation of the 16-bit integer data type used by the Arduino.

Resolving the Issue

2. Choosing the Correct Data Type

To avoid this issue, it is recommended to use a data type that can accommodate larger values. Here are a few options:

2.1. Unsigned 16-bit Integer
An unsigned 16-bit integer can store values from 0 to 65535. This will be adequate for frequencies up to 65535 Hz, but it is still not sufficient for measuring frequencies above 32 kHz.

2.2. Long Integer
A 32-bit long integer can store values from -2147483648 to 2147483647, which is more than enough for most practical applications. This will allow you to measure frequencies well above 32 kHz.

2.3. Floating-Point Numbers
If you need to store and manipulate decimal values as well, using a float or double data type is a viable option. However, this comes with a decrease in precision and increased memory usage.

Practical Example

Letrsquo;s consider a scenario where you are using an Arduino UNO to measure the frequency of a signal. You have selected a pin, say digital pin 2, to connect your frequency sensor. To ensure accurate measurements, you should modify your code to use a 32-bit long integer for storing the frequency value.

// Example code using a 32-bit long integer
unsigned long frequency  0;
void setup() {
  pinMode(2, INPUT);
}
void loop() {
  int pulseCount  pulseIn(2, HIGH); // Count the time between successive HIGH pulses
  // Assuming a stable clock source, calculate frequency
  frequency  (F_CPU / (2 * pulseCount));
  // Ensure frequency is stored in a 32-bit long integer
  long long int storedFrequency  frequency;
  // Do something with storedFrequency
}

Conclusion

The visibility of negative values for frequencies above 35 kHz is a result of the data type limitations in the Arduino UNO. By choosing the appropriate data type (such as a 32-bit long integer), you can ensure accurate and reliable frequency measurements. Understanding the underlying principles and choosing the right data types is crucial for effective embedded system design and development.