6MHz ARDUINO FREQUENCY COUNTER

Arduino Uno or Nano generates an accurate 1 second time base for the counter by cascading timer0 and timer2. The link between digital inputs 3 and 4 connects the output of timer2, 250 Hz, to input of timer0. The software waits for the output of timer0 to go positive to start the count of frequency input to timer1. Timer1 is a 16 bits timer, it overflows at the count of 2 power of 16, that in turn advances overFlow register. At the end of the 1 second the 16 bit register is recorded. The Arduino then sends to the PC via the USB cable and the Com Port 6 bytes of data.
The Arduino has to be connected to the PC first and then the VB6 app started. The app searches for the Com Port by sending a byte and checking if the byte is echoed. It takes a few seconds. The app has to be switched off when the Arduino IDE programs the Arduino.
The frequency input at pin 5 is TTL signal levels, for low signal a video amp has to be added. The specs recommend input up to 6 MHz, I tested it at 2 MHz. Included a Visual Basic (VB6) code and .EXE file and Arduino sketch.

SERIAL PORT 100KHz FREQUENCY COUNTER

Self powered frequency counter plugged into the serial port. Included a Visual Basic (VB6) code, PIC code, and a circuit diagram. The PIC16F628A microcontroller is programmed to count the input frequency and transmit it serially to the serial port. The microcontroller is powered by the signals of the serial port, so no external power supply is needed. The counter is 17 bit (16 bits + overflow). The crystal can be changed to a ceramic resonator if accuracy isn't essential.
The signal input is connected via the chip comparator to allow sensitivity down to 100mV.
You can read more about serial interface in http://www.beyondlogic.org/spp/serial.htm
You are free to use the circuit diagram and the software with no limitations.
Software includes PIC code and VB6 code. You are free to use the circuit diagram and the software with no limitations.

SERIAL PORT 1MHz FREQUENCY COUNTER

Self powered frequency counter plugged into the serial port. Included a Visual Basic (VB6 and VS2005) code and .EXE file, PIC assembly, PIC C code, and a circuit diagram. The PIC16F628A microcontroller is programmed to count the input frequency and transmit it serially to the serial port. The micro controller is powered by the signals of the serial port, so no external power supply is needed. The counter is 20 bit transmitted by the UART in 3 bytes. The crystal can be changed to a ceramic resonator if accuracy isn't essential.

You can read more about serial interface in http://www.beyondlogic.org/spp/serial.htm
Code is for MPLAB, MPLABX and compilers HI TECH or XC8.

The VB application

Circuit Description
See also Technical Tips

Pin 4 at the serial port is turned to +12V by the software enabling RTS. It is regulated to 5V by 78L05. The 5V is the supply to the microcontroller.
Pin 3 is at -12V and pin 7 at +12V constantly and they are used to drive RX input between -9V (input HIGH) and +8v (input LOW) . These levels are sufficient to drive the RS242 input.

Pin 12 is the frequency input, input signal has to be TTL type.
Pin 8 is the UART output, it transmits the 20 bit count in three bytes in RS232 format. When pin 8 is high both transistors conduct and voltage at pin 2 of serial port is at -9V.
The other pins are not in use but must be left open circuit.

Circuit Description
See also Technical Tips

Pin 4 at the serial port is turned to +12V by the software enabling RTS. It is regulated to 5V by 78L05. The 5V is the supply to the microcontroller.
Pin 3 is at -12V and pin 7 at +12V constantly and they are used to drive RX input between -9V (input HIGH) and +8v (input LOW) . These levels are sufficient to drive the RS232 input.

PIC16F628A contains a UART for serial comm and a 16 bit counter.
Input signal can be 0.1 to 5 vollts peak to peak.
Pin 17 is the frequency input, it's the PIC comparator input. Referance voltage to the comparator is 2.5V at pin 1. Output of the comparator at pin 2 is linked to the PIC counter input at pin 12.
Pin 8 is the UART output, it transmits the 16 bit count and overflow in three 8 bit words in RS232 format.
The other pins are not in use but must be left open circuit.