Design Ideas: October 26,, 1995

You can plug the circuit in Fig 1 into a PC's printer port to measure frequency. The circuit needs no external power supply, measures frequencies from 1 Hz to 5 MHz, and displays the results on screen. IC₁ (CD4536) is a 16- or 24-bit programmable counter. In this application, you set the counter to 16 bits. IC₂ is a 12-bit counter; you use only the first 4 bits. The combination of IC₁ and IC₂ yields a counter with a total length of 20 bits.

The PC's printer port has five input lines. You use four lines to read IC₂'s 4-bit parallel data. The last line serially reads IC₁'s 16-bit data. The PC sends a 4-bit counter-selection signal to IC₁'s A to D inputs to select one of the IC's 16-bit counters. The printer port powers the circuit through R₇ and C₁. Pin 8 resets IC₁ and IC₂.

For software, a C program uses the PC's internal timer to measure input frequency. (You can download the C source code from EDN BBS /DI_SIG #1771.) The timer generates a 0.989-sec interruption interval using the following steps:

1. The timer installs the timer and resets IC₁ and IC₂.
2. The timer then enables the counter by setting IC₃'s pin 9 high for 0.989 sec.
3. The software reads back the count number, calculates the corresponding frequency, updates the screen data, and awaits the next time-out interrupt. (DI #1771)