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Frequency counter using AVR microcontroller




Universal Counter

The frequency counter is the most popular instrument in the home maid instruments. I think that the reason why it is built widely is: it can be built easily because it is digital circuit, it is generic measurement and many construction kits are available. Many electronics hobbyists will experienced to build any frequency counter.

This project is second trial for me. But building two ordinaly frequency counter is not smart so that some additional functions ware impremented to the frequency counter. The function is not that abundant compared to generic universal counter however it can measure pulse period/width besides frequency, I named this project “Universal Counter”.

The basic theory of Frequency Counter

What is the Frequency?

The Frequency means number of electrical or mechanical vibration cycles per unit time, to explain this might not be needed now… Hz (hertz) is used for a unit of the frequency and is defined as an SI unit, and it means number of cycles per second: e.g. when 100 cycles are counted per a second, it is 100 Hz. Formarly c/s was used for unit of the frequency.

A frequency measurement system which is especially realized in digital counting circuit seems being called “Frequency Counter”.

Direct Counting Method

When count number of cycles of the input signal for one second, the value of frequency can be got in the counter. This can be realized by simple counter circuit so that this was the conventional frequency measureing method. Many counter IC have been released and many frequency counters thak work in this method are being shipped now. This is the simplest method but the measureing resolution is limited at low frequency. To ensure more resolution, the gate time must be expanded: e.g. when measure at resolution of 1 mHz, 1000 seconds is needed to measure one time.

Frequency Counter

Reciprocal Method

This is one of the frequency measureing methods that was thought out to make up the defect of direct counting method, and it is prevailing now. This method measures the period (T) of the input signal instead of number of cycles, and calcurate its frequency in the formula f = 1 / T with a microprocessor. Its frequency resolution depends on only time resolution independent of the input frequency.

It can measure low frequency with high resolution quickly however any trigger gitter due to noise decrease measureing resolution. To maintain sufficient time resolution, it generally measures the period of several number of cycles. The reciprocal method seems to also be impmemented to cheap frequency counter because of price of microcontrollers falls.

 

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