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
26 March 2015
■ FIGURE 6. PIC assembly.
■ FIGURE 7. Counter bank and synchronizer assembly.