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By DAVID L. JONES
This low-cost
6-digit frequency
counter module uses
readily available components
and can measure up to 40MHz with
a resolution of 1kHz. It is ideal for adding
a digital frequency display to a function
generator or to some other project.
A 6-Digit Frequency
Counter Module
M
OST SIMPLE frequency
counter designs are based
on the 74C926 integrated
counter chip. Unfortunately, this device is now quite expensive and can
only handle four digits which means
the resolution isn’t all that great. By
contrast, this alternative design is
much lower in cost and has six digits
for improved display resolution.
As presented, the circuit features
two signal inputs. The first of these is
fed directly to the main counter circuit
and this gives a direct readout in Hertz
(Hz) up to a maximum of 999999Hz
(ie, just under 1MHz).
The second input is fed to the
counter circuit via an onboard divider circuit or prescaler. This prescaler
divides the input frequency by 1000,
which means that the display now
76 Silicon Chip
shows the measured frequency in kHz.
As a result, the circuit can now measure frequencies up to about 40MHz.
Because the display reads directly
in Hz or kHz, there is no need for
decimal point switching and the associated complexity that this involves.
There are no switches or controls –
you simply feed the signal into the
appropriate input for a direct readout
in Hz or kHz.
BCD output multiplexer with an internal oscillator. That’s quite a lot in
one package and makes the 4553 ideal
for a frequency counter application.
The basic principle behind the
frequency counter is to count the
number of input pulses that occur
within a precise one-second window.
The value stored in the counter chip
will then equal the exact frequency in
Hertz and this is then displayed on the
How it works
Let’s now take a look at the circuit
details – see Fig.1. At the heart of the
design are two 4553 triple BCD (binary
coded decimal) counters (IC4 & IC5),
which are wired in cascade fashion.
Each of these ICs contains three synchronously cascaded BCD counters,
three output latches and a 3-channel
Fig.1 (right): the circuit is based on
two 4553 triple BCD counters, which
are wired in cascade. The drive
the displays via 7-segment decoder
drivers IC6 & IC7 and transistors
Q1-Q3. IC2 provides the clock signal,
while IC1 & IC8 form a divide-by-1000
prescaler.
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August 2001 77
Specifications
Frequency Readout: 6-digit 7-segment LED display.
Frequency Range: 1Hz-1MHz (1Hz resolution); 1kHz-40MHz (1kHz resolution). Can be easily modified for other ranges and resolutions.
Input Signal Level: 5V TTL/CMOS compatible.
Decimal Point Switching: Optional.
Size: 116mm x 35mm x 25mm x 30mm.
Supply Requirements: 5V DC <at> 90mA.
6-digit display. Lets take a look at how
this works in detail.
IC2 is a 24-stage binary divider and
oscillator. It divides the 4.194304MHz
crystal (X1) frequency by 8,388,608
(223) at its Q23 output, to give a precise
0.5Hz clock signal. This signal then
drives the DISABLE input (pin 11) of
the first 4553 (IC5).
Let’s initially assume that IC4 and
IC5 have both been reset and that the
DISABLE line has just gone high. This
stops input pulses from clocking the
counter and is the Latch/Display phase
of the operation.
At the same time, the DISABLE
signal is differentiated by a .001µF
capacitor and 10kΩ resistor (to form a
short positive-going pulse) and fed to
IC3a. IC3a inverts this signal and the
resulting negative-going pulse on its
pin 3 output drives the latch enable
(LE) pins of ICs 4 & 5. This latches the
current counter value into the output
registers and this is the value that’s
displayed on the 7-segment LED readouts.
Pin 3 of IC3a also drives a second
differentiating circuit, again consisting of a .001µF capacitor and 10kΩ
resistor. The resulting signal is then
squared and buffered by IC3b and
IC3c to provide a master reset (MR)
pulse. This resets the counters in ICs
4 & 5, so that they are ready to start
counting again.
After that, the DISABLE signal line
goes low and the counters count the
input pulses for precisely one second
until the DISABLE line returns high
again. This is the counting phase of
the operation.
This Counting & Latch/Display sequence continues indefinitely while
ever power is applied. As a result, the
display is updated every two seconds
with the current input frequency.
The OVF output (pin 14) of IC5
provides the means to cas
cade the
second 4553 counter (IC4). It connects
to the CLK input of IC4, so that we
effectively have a 6-digit BCD counter.
IC5 has an internal free-running
oscillator between pins 3 & 4. Its frequency is set by the .001µF capacitor
between these two pins. Pin 3 also
directly drives pin 4 of IC4 to keep it in
sync and this also sets the multiplexing rate for the 7-segment displays.
As shown in Fig.1, the BCD outputs
from ICs 4 & 5 (ie, at pins 5, 6, 7 & 9)
Fig.3: this scope
shot shows the
latch enable
(LE) signal (top),
the master reset
(MR) signal centre and the 0.5Hz
clock signal
(bottom).
78 Silicon Chip
each drive 4511 BCD-to-7-segment
decoder/driver ICs (IC6 & IC7). These
in turn each drive the a-g display
segments of three displays. IC7 drives
the segments of displays DISP1-DISP3,
while IC6 drives the segments of
DISP4-DISP6.
Note that the corresponding segments of DISP1-DISP3 are all connected in parallel. The same goes for the
segments of DISP4-DISP6.
The DS1-bar, DS2-bar and DS3bar lines from counter IC4 control
the display multiplexing – ie, they
control which displays are turned on
at any given instant. When DS3-bar
goes low, PNP transistor Q3 turns on
and “sinks” the current for DISP1 and
DISP4. Similarly, when DS2-bar goes
low, transistor Q2 turns on and enables
DISP2 & DISP5. And when DS1-bar is
low, transistor Q1 switches on DISP3
& DISP6.
Prescaler circuit
IC1 and IC8, both dual 4-digit decade counters, form the divide-by-1000
prescaler circuit.
The first thing to note here is that
the MR line also drives the CP1-bar
input (pin 12) of IC1b. This may look
puzzling at first glance because IC1b
doesn’t appear to do anything. And
that’s exactly what it does – nothing!
This was just a convenient way to tie
the CLK input on the PC board layout.
After all, there’s no point adding extra
links or tracks if you don’t have to.
It’s the three remaining 4-bit decade
counters (IC1a, IC8a & IC8b) that do
all the prescaling work, with each
stage dividing by 10 to give an overall
division of 1000. As shown in Fig.1,
the input signal is fed to the CP0-bar
(pin 1) input of IC1a and the divided
output appears at the Q3 output (pin
7). This then drives the CP0-bar input
of IC8a which in turn drives decade
counter IC8b. Finally, IC8b drives the
clock input (pin 12) of IC5.
The maximum frequency that can be
displayed depends on the upper limit
of the 74HC390 chips, which is around
40MHz. Note that this circuit will only
accept TTL-level signals although
signal-conditioning circuitry could be
added to cater for other signal levels if
required. The 4553 BCD counter chips
will handle input frequencies up to
about 1MHz.
The 7-segment LED displays are
LT313 0.3-inch common-cathode
types. Compatible types are the
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Parts List
1 PC board, code 04108011, 35
x 116mm
1 PC board, code 04108012, 35
x 116mm
4 15mm M3 tapped spacers
8 M3 screws
5 PC board stakes
Fig.2: install the parts on PC boards as shown here, taking care to ensure that
all polarised parts are correctly orientated. The two boards are stacked together
on spacers and joined at both ends using wire links (see photo).
Agilent 5082-7613 and the smaller
HDSP7303/HDSP7803 units. These
were used in favour of the more popular FND500 displays (which have a
different pinout) to simplify the PC
board layout and to minimise the size.
The module requires a single +5VDC
supply rail and this could come from
any well-regulated source. The current
consumption is around 90mA.
output of IC8a to the clock input (pin
12) of IC5. The decimal point is just as
easy. All you have to do is connect a
470Ω resistor between pin 7 of DISP5
and the +5V rail.
So it’s quite easy to “customise” the
display to your own requirements.
If necessary, you could even add a
switching ar
rangement so that you
could easily select different ranges.
Possible modifications
Construction
The circuit can be easily modified
to display any resolution and direct
readout you require within its 40MHz
capability. For example, let’s say that
you wanted a direct readout in MHz
with a display resolution of 100Hz
– ie, the display must be capable of
showing 99.9999MHz (if the circuit
could handle frequencies that high).
This requires two things: a divide-by-100 prescaler and a decimal
point between DISP4 and DISP5. The
divide-by100 prescaler is easy – just
bypass IC8b by connecting the pin 7
The design is built on two PC boards
measuring 116 x 35mm. These are
stacked together on 20mm spacers to
form a single compact module.
The two boards are joined at both
ends using straight wire links running
directly from one to the other. Alternatively, flexible ribbon cable could be
used to join the two boards and this
could be left long enough so that the
two boards could be easily separated
at a later stage (eg, for modifications).
Straight wire links are much neater,
however.
Semiconductors
2 74HC390 dual 4-bit decade
counters (IC1,IC8)
1 4521 24-stage divider/oscillator
(IC2)
1 74HC132 quad NAND gate
(IC3)
2 4553 3-digit BCD counters
(IC4,IC5)
2 4511 BCD to 7-segment
decoder/driver (IC6,IC7)
3 BC557 PNP transistors (Q1-Q3)
6 LT313 or equivalent 8mm
7-segment LED displays
(DISP1-DISP6)
1 4.194304MHz crystal (X1)
Capacitors
1 10µF 10VW tantalum
3 .001µF ceramic
1 82pF ceramic
1 22pF ceramic
Resistors (0.25W, 1%)
1 3.3MΩ
2 10kΩ
1 4.7kΩ
3 1kΩ
2 470Ω 7-way DIL resistor packs
or 14 x 470Ω resistors
Fig.2 shows the assembly details
for the two PC boards. Begin the
assembly by installing the ICs and
7-segment LED displays on the display
board, taking care to ensure that the
displays are correctly orientated (ie,
decimal point to bottom right). Don’t
The prototype used sockets
for the ICs on the logic board
but we suggest that you
solder the ICs straight in.
Note that two resistors
adjacent to IC2 are mounted
“end-on”.
www.siliconchip.com.au
August 2001 79
A short length of insulated wire can be used to join the “Divider Output” to the
“Freq Input” if the counter is to be used in divide-by-1000 mode.
use sockets for the display board ICs
– this would raise the ICs above the
level of the displays and could lead
to mounting problems later on.
The displays can be mounted in
sockets to increase their height above
the PC board if desired (eg, you might
want them to protrude through the
front panel of an instrument case).
Normal
l y, however, the displays
would be mounted directly on the
PC board and would sit behind a Perspex window. Additional holes are
provided in each PC board to make
mounting easy.
Note that the prototype used two
470Ω DIL resistor packs which look
just like (gold-coloured) ICs. Alternatively, you can fit individual 470Ω
resistors here, as shown in Fig.2
You are now ready to build the logic
board. Begin by fitting wire links to the
locations shown, then fit PC stakes at
the five points marked with a “star”.
This done, you can fit the resistors
and capacitors, crystal (X1) and the
three transistors (Q1-Q3). The crystal
should be a low-profile HC49/4H type
to reduce its height.
You can now complete the logic
board assembly by installing the
six ICs. Make sure that each IC is in
Resistor Colour Codes
No. Value
1 3.3MΩ
2 10kΩ
1 4.7kΩ
3 1kΩ
14 470Ω
4-Band Code (1%)
orange orange green brown
brown black orange brown
yellow violet red brown
brown black red brown
yellow violet brown brown
5-Band Code (1%)
orange orange black yellow brown
brown black black red brown
yellow violet black brown brown
brown black black brown brown
yellow violet black black brown
Fig.3: here are the full-size etching patterns for the two PC boards. The extra
corner holes make it easy to mount the module on a panel or bracket.
80 Silicon Chip
its correct location and is correctly
orientated. The use of sockets is optional here – they were used on the
prototype but you can save money by
directly mounting the ICs on the PC
board.
Once the board assemblies are complete, they can be stacked together on
the 20mm spacers and secured using
machine screws, nuts and washers.
Finally, complete the assembly by installing the connecting links between
the two boards. There are 13 links in
all, seven at one end and six at the
other.
Note that once the two boards are
joined together, it can be difficult to
access the top of the logic board for
troubleshooting. For this reason, you
might like to temporarily “patch” the
two boards together using rainbow
cable, so that they can be powered up
and tested before the final assembly.
Using the module
The module can be mounted on the
front panel of your project by using
another set of spacers or by gluing it
in position. All external connections
are made via the PC stakes on the logic
board. Be careful when you connect
the 5V DC power supply – there’s no
reverse polarity protection, so if you
get the leads the wrong way around,
something’s bound to fry.
If you want the display to read in
“Hz”, connect your input signal directly to the “Freq Input” terminal. This
configuration can be used for signal
frequencies up to 999,999Hz.
Alternatively, if you want the display to read in kHz, connect the input
signal to the “Divider Input” terminal
instead. You will also have to connect
the “Divider Output” terminal to the
“Freq Input” terminal using an insulated lead (see photo). As previously
stated, this configuration allows measurements up to about 40MHz.
The frequency accuracy of the
module will be dependant on the
crystal used and the values of the two
associated capacitors (22pF and 82pF).
If necessary, these can be tweaked to
“calibrate” the module. An accurate
function generator or another frequency meter of known accuracy will be
required for this task.
That’s all there is to it – your module
should accurately indicate the input
frequency and update the display
every two seconds. Its application is
limited only by your imagination. SC
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