Nuts and Volts - March 2015

I chose the PIC16F886 (see Figure 6) because it is a cheap popular unit that does the job without overkill. Since there are no critical timing requirements, the internal 8 MHz oscillator works fine. This PIC also has just enough I/O pins ( 22 available). If you want to add RS-232 or USB, then try the PICF887. It has plenty of I/O and has USB support.

The display for this project is a 2x16 backlit alphanumeric version. This type of display usually connects with four-bit data bits, but sometimes eight bits are used (these bits are shared with the 74F579 bus) and two control signals (R/S’ and E’). Fortunately, most compiler products include display drivers so you don’t have to understand the inner workings of the display. (Look ahead in Figure 8 for the front panel components and connectivity.)

The top line is used to display the input frequency in 7/8 digits, plus up to two commas or a decimal point. This is followed by ‘Hz’ or ‘MHz’ and the letter ‘F’ when the gate is on and making a frequency measurement. The second line simultaneously displays the period of the input signal in scientific notation (like 1.2534E- 6), followed by ‘Sec’ and a ‘P’ when the gate is on and making a period measurement.

The final item in Figure 1 is the internal frequency reference. The measurement accuracy is totally dependent on the accuracy of the gate time, and thus on the accuracy of the internal frequency reference. The design goal of seven digits is within an oven-controlled crystal oscillator range. I used a piezo model 2920136 10 MHz oscillator that I bought on eBay for $25. It outputs a TTL square wave and warms up to seven-digit accuracy in about five to 10 minutes. Unlike cesium, rubidium, or GPS references, oven-controlled crystal oscillators have to be calibrated initially. If you don’t have access to a calibration facility, then here is an alternate technique that works.

The National Institute of Standards and Technology‘s (NIST) radio station WWV broadcasts high accuracy signals at several shortwave frequencies, including 10 and 20 MHz. These transmissions contain exact one second time markers, but the carriers themselves are also exact frequencies set by their cesium standards. If you tune a shortwave radio to one of the WWV frequencies at 10 or 20 MHz, you will hear the one second time ticks and