Frequency measuring using Arduino (AC analog input)
This project includes a frequency counter.

https://www.instructables.com/id/Oscilloscope-in-a-Matchbox-Arduino/

From the project page:
This oscilloscope costs the price of an Arduino Nano (£2 and a display (£3) plus a few pence for resistors, etc. It's specification is: 


max 1M samples/second, min 1000sps
8-bits per sample
DC 0-5V; AC +/- 550mV, AC +/- 117mV, AC +/- 25mV
USB "PC scope" or built-in display
could be battery-powered
optional logic display
optional frequency meter
optional voltmeter
optional signal generator


My comment: I build it, it's a nice little project and the author is an expert programmer.