Electronic Modules
Using Electronic Modules with Jim Rowe
4-digit, 14-segment
LED module
Instead of seven segments, this LED display
module has 14 segments per character, so
it can display letters, digits and even a few
symbols. It has a built-in I2C serial interface,
allowing popular microcontrollers like the Arduino Uno or Nano to drive it easily.
T
he module is about the same
size as a 4-digit, 7-segment
display at 50mm wide by 28mm
high, with a total thickness of a little
over 10mm. The two side-by-side
dual-character LED displays have
14 segments per character, plus the
usual decimal point LED. This allows
them to reasonably display numerical digits, upper-case letters, many
lower-case letters and a few symbols.
The module features an I2C serial
interface that allows easy connection
to just about any popular microcontroller unit (MCU). We will now look
more deeply into the 14-segment LED
displays, followed by the useful IC
that drives them and provides the
I2C interface.
The 14-segment displays
Fig.1 shows how the dual character displays used in the module have
six of the seven segments used in the
familiar 7-segment displays; the outer
ones labelled ‘a’ to ‘f’.
Instead of the single central horizontal segment, there are eight inner
segments: three in the upper half
labelled ‘g’, ‘h’ and ‘j’, three in the
lower half labelled ‘l’, ‘m’ and ‘n’,
and two in the centre replacing the
original single horizontal segment,
labelled ‘p’ and ‘k’. This gives 14 segments in each character, not counting the decimal point.
The LEDs in these segments are
connected in a common-cathode configuration, so each character (plus
its decimal point LED) has a single
cathode pin.
The anodes are connected to the
anode of the corresponding segment
of the other character, eg, segment ‘1a’
to segment ‘2a’ etc. That allows the
segments of both displays to share
pins, as shown in the internal circuit,
on the right side of Fig.1.
So each dual-character display
needs only 17 connection pins: 15
for the LED anodes and two for the
cathodes. The displays have 18 physical pins, but one (pin 3) is not used.
Two main suppliers of these dual
14-segment displays are Kingbright
(PDC54-11GWA) and Lite-On (LTP3784E). The characters are 13.8mm
(0.54in) high in both cases. These
manufacturers also label the inner
display segments differently, but the
pin connections are the same.
The displays used in this module
have segments that emit orangeyellow light, but displays with other
colours are available.
Inside the HT16K33 IC
Now we can look into the IC used
to drive each pair of dual 14-segment
displays in the module. This is the
HT16K33, made by Taiwan firm Holtek
Semiconductor Inc (www.holtek.com/
Fig.1: how the LEDs are
arranged in each of the
2-digit, 14-segment displays.
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Practical Electronics | November | 2024
14-Segment LED Displays
page/vg/HT16K33A), which also provides the I2C interface. Holtek makes
a range of microcontrollers, some
of which are used in popular home
appliances and various other ICs,
including display drivers like the
HT16K33.
Holtek describes the HT16K33 as
a 16×8 LED Controller Driver with
RAM Mapping and an optional keypad
scanning ability. It can be used for
driving virtually any matrix of up
to 16 × 8 LEDs, not just 14-segment
alphanumeric displays, as in this
module. It can also scan a matrix of
13 × 3 keys, although that feature is
not used here. It can be powered from
4.5-5.5V DC.
Fig.2 shows the basic block diagram of the HT16K33. The I2C interface controller is at lower left, with
an internal RC clock oscillator to its
right feeding a timing generator, and
two random-access memories (RAMs)
below them. The upper RAM is for
the display control data, with a capacity of 16 × 8 bits, while the lower
RAM is for storing the key scanning
data, if that function is used.
On the right-hand side are the
two controller blocks. The upper
one provides eight outputs (COM0COM7) for control of the ‘common’
LED lines (in this case, the cathodes
of the 14-segment displays) and the
key scanning outputs. The COM0
output is also used to sense the desired I 2C address for the HT16K33,
as explained shortly.
The lower controller block provides 16 outputs (ROW0-ROW15) for
driving the rows of LEDs in a matrix
or the segments in the 14-segment
displays. It also provides inputs for
sensing the desired I2C address, plus
inputs for the key scanning function.
The power-on reset (POR) block at
upper left resets most of the other
blocks when power is first applied.
One of the functions of the HT16K33
not shown in Fig.2 is its ability to provide programmable 16-step dimming
of the LED outputs. That is achieved
by controlling the pulse width of the
ROW outputs, with a range from 1/16th
to 16/16th duty cycle. Another handy
feature!
Finally, the HT16K33 can be programmed to have any of eight different
I2C addresses, from 70h to 77h, using
three links on the circuit around the
chip. We will see how this is done in
the next section.
Practical Electronics | November | 2024
The rear of the
14-segment LED
module contains
just a few
components and
the HT16K33 IC.
The module’s I2C
address is set by
the three links
labelled A0-A2 on
the PCB.
Note that the HT16K33 IC is now
obsolete, but Holtek still sells the
HT16K33A, which is pretty similar.
The module circuit
As you can see from the circuit in
Fig.3, there’s not much in the module
apart from the HT16K33 device itself
(IC1) and the two dual 14-segment
displays. Two pull-up resistors are
connected between its SDA and SCL
lines and the VHI input, while the
HT16K33 chip is powered from the
VIO input from CON1, with a 10μF
capacitor providing filtering.
5-pin SIL header CON1 is used to
make all the power and signal connections to the module.
Programming the module’s I2C ad-
dress is achieved using diode D1,
three resistors and three PCB links
A0-A2, shown above IC1 in Fig.3.
The anode of D1 is connected to the
COM0 output (pin 2) of IC1, while its
cathode connects to the three links
via three 10kW resistors. The other
ends of the links are connected to
the ROW0, ROW1 and ROW2 lines of
IC1, which are used as inputs when
IC1 detects the desired I2C address.
As shown in the small table at
upper right in Fig.3, when no links
are connected (A0=A1=A2=0), the
module has an I2C address of 70h (h
= hexadecimal). If only the A0 link is
connected, the address is changed to
71h; if only the A1 link is connected,
this changes the address to 72h etc.
Fig.2: the block diagram for the HT16K33 IC which is used to drive both
14-segment displays.
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Electronic Modules
An example of what the lowercase letters “qrst” and “abcd” look like on the
LED module. The letters ‘q’ and ‘a’ are some of the more strange choices.
This ability to set the module’s
I2C address to eight different values
means it is possible to connect up to
eight of the modules to the same I2C
port of an MCU. It also means that if
you have another device on your I2C
bus within the range of 70h to 77h,
you can program the 14-segment display to one of the unused addresses
to avoid a collision.
Connecting it to a micro
A nice feature of this module is that
its I2C interface makes it easy to connect to most MCUs. This is illustrated
in Fig.4, which shows how it can be
connected to an Arduino Uno. The
module’s VHI and VIO pins are both
connected to the Arduino’s +5V pin,
its GND pin to one of the Arduino’s
GND pins, its SDA pin to the Arduino’s A4/SDA pin and its SCL pin to
the Arduino’s A5/SCL pin.
Note that with R3 and later versions
of the Uno, the last two pins can be
connected to the SDA and SCL pins
at upper left on the Arduino, just to
the left of the AREF pin.
Connecting the module to an Arduino Nano is just as easy, as shown
in Fig.5. The connections are very
similar to those for the Uno in Fig.4.
The only other thing you need to
do to get the module to communicate
with an MCU is to change its I2C address if necessary; it defaults to 70h
when none of the links on the rear
of the PCB are joined.
You should find it just as easy to
connect the module to most other
MCUs, such as a Micromite, Maximite, PicoMite, WebMite and so on.
All that’s left then is to come up
with some suitable software to drive
the display. For an Arduino, as usual,
that will involve finding a software
library designed to communicate
with the HT16K33 module, plus one
or more example Arduino sketches
to show how it’s done.
Arduino libraries
After looking around on the web
for Arduino libraries written to communicate with the HT16K33 module,
the best one I could find was from US
firm Adafruit, called Adafruit_LED_
Backpack. This one was listed on the
main Arduino Reference website but
was also available on GitHub:
• pemag.au/link/abpk
• https://github.com/adafruit/
Adafruit_LED_Backpack
However, to work with the 14segment displays used in this module,
two other libraries must be installed:
Adafruit-GFX-library and Adafruit_
BusIO_library, see:
• https://github.com/adafruit/
Adafruit-GFX-library
• https://github.com/adafruit/
Adafruit_BusIO
Fig.3: The full
circuit of the
4x14-segment
display
module. The
table at upper
right shows
how its I2C
address can be
set using the
PCB links A0,
A1 and A2.
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Practical Electronics | November | 2024
14-Segment LED Displays
Once the three Adafruit libraries
have been downloaded (as zip files)
and installed on your PC as part of
the Arduino IDE or installed via the
Library Manager, you will find a
“quadalphanum.ino” sketch in the Examples folder. Verify and compile this
sketch, then upload it to the Arduino
connected to the module, and you
should find the module’s displays
will spring to life.
First, it will show a stream of all the
characters it can display (this takes
a while). Then, if you have the IDE’s
Serial Monitor open, it will allow you
to type in any combination of four
characters you want and they will
be displayed immediately. You can
repeat this over and over.
While doing this, I took a few
photos to illustrate how the module’s
displays show many of the common
alphabetic characters. They should
give you a good idea of what can be
achieved.
The upper-case characters are all
reasonably clear, but the lower-case
characters are less so. Some are pretty
unclear, like “p” and “q”, while some
of the symbols are very clear, such
as “+” and “-”. Unsurprisingly, the
numerals are also quite clear.
It was a little disappointing to find
nothing in the Adafruit libraries to
show how to control the light output
of the module’s displays. However, if
you read Holtek’s data sheet on the
HT16K33 (see pemag.au/link/abpj),
they provide quite a bit of information on how the PWM dimming of the
displays works and can be achieved.
Editor’s note: some lower-case
letters could be made more clear by
modifying the libraries to change
which segments are used. To do this,
edit the entries in the “alphafonttable”
array that’s inside the “Adafruit_
LEDBackpack.c pp” file. Examples
of shapes we think would be more
clear are shown in Fig.6.
Where you can get it
The module shown in the pictures
is currently available from a few sellers on AliExpress (eg, pemag.au/link/
ac1f & pemag.au/link/ac1g) for around
£2 each. It is also available from AdaFruit (ID 2158) for US$10.50.
Adafruit also has versions with
different display colours, such as
red (1911) for US$9.95, blue (1912),
white (2157) or green (2160) displays,
each for US$13.95.
PE
Practical Electronics | November | 2024
Figs.4 & 5: connecting the LED
module to an Arduino Uno (above)
or Nano (below) is simple. You just
need to connect the SDA & SCL
pins respectively to A4 & A5 on the
Arduino. VHI then goes to 5V on
the Arduino and is bridged to VIO,
while GND goes to GND.
Fig.6 (left): you can edit the library code
to output arguably better representations
of different letters. An example of what
segments could be enabled for the letters ‘p’
and ‘q’ are shown here.
Upper case vs lower case
We recently came across an interesting fact about where the terms “upper
case” (capital letters) and “lower case” (smaller letters) came from.
In early printing presses, the “moveable type” letters were kept in cases near
the press. As the smaller letters were used more often, they were kept in a box
(case) closer to the worker. The capital letters were in a case that was higher
and further away, above the other. Hence, “upper case” and “lower case” refer
to where the letters were found in those early presses!
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