So my Radio Shack frequency counter died. I decided that since I had a few 7-segment LED’s I would just make a bench top frequency counter.
I started with the frequency counter designed by Wolf Buscher, DL4YHF.
He used a Microchip PIC, which I no longer have debugging tools for. A while ago I got rid of all my debuggers except for my J-Link ARM debugger. Having worked with ST, Microchip, Motorola processors, I decided that I would commonize on the ARM architectures. Specifically, on the Cortex-M series.
In my frequency counter I am using seven 7-segment displays, I also wanted a display that was MCU independent. So I used a 74LS47 BCD to 7-Segment driver and a 74HCT239 3 to 8 addressable bus. This means that I need 3 bits to select a segment and 4 bits to select the character to display. The 7-segment displays have their segment lines all tied together and their supply lines are controlled via the 74HCT239.
This works by turning power off to all 7-segment displays, writing the segment values to the 74LS47 chip, then enabling power to the segment using the 74HCT239. This is done for each digit in sequence at a rate faster than 60Hz. To the human eye this gives the impression that all digits are on simultaneously.
This is very similar to the construction method shown here
Front End Circuit
Because I wanted a fast input that converted low voltage sine waves into a square wave at the voltage of the mcu, I am using the following circuit.
For the processor I chose to you a NXP Kinetix Freedom K22F. This processor has a ARM Cortex-M4 processor with an internal floating point unit, DMA, USB, and standard MCU peripherals. The FRDM-K22F costs $30USD and includes an onboard debugger.
I used Kinetis Design Studio (KDS) and the Kinetis Software Development Kit (KSDK) to write the software for the frequency counter. The KSDK provides drivers for the onboard peripherals. For the Real-Time Operating System, I used FreeRTOS. It is my favorite embedded RTOS due to its ease of use and because it is very light weight. Giving it a very small memory footprint.