ZL2PD LED Frequency Counter
A 5-digit microprocessor-based frequency
operates to over 35 MHz. Expansion of this basic design to cover higher
frequencies is easy.
5-digit frequency counter project is one of a series of 8051 based
frequency counters which I designed over a period of about two years
for a variety of applications. In this particular case, I wanted a
compact counter for use in a receiver project, and this was one of the
early versions I developed.
In an attempt to use the smallest possible LED display, this counter
uses a very small National Semiconductors 8 digit LED display which
came from an old calculator. (For those interested, the display part
number is NSA1488) I figured out the connections for the display with a
bit of patient probing with my multimeter, identifying the display
common and each of the segment pin connections for the display.
Any modern common cathode LED display can be used instead of this
display. It doesn't need to be as small as this display that I used. I
also ended up using larger common-cathode LED displays in a later
similar versions of this same counter design. I originally thought that
this ex-calculator display would be ideal for the compact receiver I
was building, but, as events transpired, the display ended up being a
few millimeters too large for the final receiver chassis, so I had to
keep looking for a set of even smaller LED displays.
counter uses a Philips 87C751 microprocessor. This is an EPROM member
of the 8051 family. It was, for a number of years, the smallest 8051
chip equipped with an I2C interface, although that feature was not used
in this project. It also came with 2k of EPROM and 64 bytes of RAM.
This may appear limited, but it's actually plenty of memory for this
sort of application.
Just a warning: The 87C751 chip is quite hard to find now, as is a
suitable programmer. However, I've described this project here because
I imagine that some probably exist in junkboxes here and there. I still
have a couple floating about in my junkbox, left over from earlier
projects. I'll get around to using them on something else one day, and
this may be just the project for someone to resurrect their 87C751 from
preamplifier circuit is a copy of a circuit commonly found in
simple frequency counters, and also operates from the 5V supply rail.
It uses a commonly available FET (I used an MPF102 but almost anything
will do here, such as a J310 etc) and a pair of RF transistors. I used
MPHS11 devices, but in other versions I built, I used the more commonly
available 2N2222 transistors.
preamplifier feeds a 74HC4040 divider, which divides the input
frequency by 64 to ensure the maximum frequency limitations of the 8051
family are not exceeded. The maximum input frequency is defined as
1/24th of the oscillator frequency. So, with a clock of 14.3 MHz, the
maximum input frequency is 600 kHz. Since the 4040 stage divides the
input by 64, the maximum frequency which can be counted is 38.4 MHz.
The 87C751 clock crystal is the reference for the counter, and the code
allows fine tuning to ensure reasonable accuracy. Comments in the
source code (below) describe all of the details. I used a 14.3 MHz
crystal taken from an old 8086 PC motherboard, but the code can be modified
for use with other crystals.
feature of the chip to watch out for is its unusual timer/counter
arrangement - Well, it's a bit unusual for the 8051 family. The 87C751
has two timer/counters; one is a standard 16 bit timer/counter, the
other is a 10 bit timer. The 16 bit timer/counter is configured in an
auto-reload mode. Here, the 16-bit counter is used in the main
frequency counting routine.
The 10-bit timer is normally dedicated to the I2C interface, but here
it is used as a fixed rate background timer. Combined with interrupts,
it drives the display digit multiplexing. One digit is displayed each
time the 10 bit background counter rolls over. This occurs once every
860 uS, and the same routine triggers the capture of new counter data
every 128 mS.
The supply current for the counter is
minimised by multiplexing the displays. That
means a pair of ports are used to drive each of the eight displays one
by one. One port outputs the LED segment data for each display while
the other port selects the required display. This requires a little
more care when writing the code.
This multiplexed display method does generate some clock noise, and the
counter may need to be mounted in a shielded box for some applications.
I ended up using another version of this counter in the receiver so
this one was put to use around the bench for some time, and the
multiplexer noise was rarely noted.
the counter on prototype board using point to point wiring. This design
was never intended for mass production and so a PCB was not developed.
Besides, most prototypes and one-off designs are faster and easier to
build this way. I used a mixture of regular hookup wire, wire-wrap wire
and solder-through wire. The latter wire is very thin and quite hard to
see in the photos (It's only used on the underside of the board). Made
a number of years ago by Vero in England, it is coated with a thin but
surprisingly tough layer of insulation which acts as a solder flux when
the wire is soldered into place. Fantastic stuff!
display is wired in at one end of the board and supported by a pair of
triangular shaped scraps of PCB. These LED displays need a fairly high
drive current to light up the LEDs brightly. Unfortunately, the 87C751
cannot drive these directly. As a result, there are a number of
discrete transistors which are required for display drivers. However,
they are cheap, so it's not all bad.
display driver transistors are all general purpose NPN and PNP devices.
Examples include BC547 (NPN) and BC557 (PNP) devices.
Note that two pullup resistors are required on the I2C port lines on
are a couple of test points which might just be able to be seen on the
counter board. These
were used during code debugging to show when sections of the
code were exited. I connected an oscilloscope to these points and this
allowed me to monitor how far the code was getting. I don't have an
in-circuit emulator for the 87C751,
so the development was all done using the well proven (but tedious)
"crash and burn" method.
those readers unfamiliar with this process, the term refers to the
experience encountered during the often highly repetitive code
development cycle. The designer writes some code, programs (burns) the
code into the EPROM in the chip, then watches the new code crash in
some spectacularly new and novel manner when yet another error is
encountered in the program. Fortunately, this project wasn't too bad in
When the counter is powered up, the display will briefly show
"HF2000A". This was the model number I gave to my new receiver. Feel
free to change the source code so your version of the counter either
displays something more appropriate. The data for this sign-on display
is located, along with other display related data, towards the very end
of the source code. And Although
the display has 8 digits available, the software I wrote was limited to displaying 5 digits
because I only wanted to display the receiver's frequency to the
Aside from the numeric frequency information, the display may
also show "BadCount". BadCount indicates the counter is attempting to
display a frequency in excess of 65,535 MHz. It is held briefly to show
the over-range count before the normal frequency counting routine
The counter is powered directly from a 5V
supply, and needs less than 100mA. It covers frequencies from 500 Hz to
above 30 MHz.
Intel HEX file - Clicking on this
link will allow you to download a zipped HEX format file (1 kB) for
programming an 87C751 chip for use in this counter
Source File - Clicking on this
link will allow you to download a zipped Metalink compatible ASCII TEXT
format source file (5 kB) for this counter. This will allow the
asembler code to be modified by experienced 8051 programmers who may
wish to use the code with another type of 8051 chip or to add more
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