KickStart
b y M ik e To o l ey
Part 4: Introducing the Arduino Pro Mini
Our occasional KickStart series aims to
show readers how to use readily available
low-cost components and devices to
solve a wide range of common problems
in the shortest possible time. Each of the
examples and projects can be completed
in no more than a couple of hours using
T
he Arduino has undoubtedly
become immensely popular with
PE readers because it can provide
a highly effective solution to a wide
range of problems at minimal cost.
To cater for differing requirements,
several versions of the Arduino have
become available, making it possible
to integrate the microcontroller into
systems of virtually any size.
What’s on the board?
The most popular Arduino version is
undoubtedly the Uno, but for smaller
applications, particularly where there
is no need for a resident USB interface,
this can be overkill. The Arduino Pro
Mini puts this right by providing an
off-the-shelf parts. As well as briefly
explaining the underlying principles and
technology used, the series will provide
you with a variety of representative
solutions and examples along with just
enough information to be able to adapt
and extend them for their own use.
This fourth instalment shows you how
to make use of the diminutive Arduino
Pro Mini and, in keeping with the KickStart
philosophy, we’ve provided sufficient
information for you to be able to start
making use of this tiny microcontroller in
your own projects.
attractive alternative in a tiny postagestamp-sized package. Like its big brother
(see Fig.4.1), the Pro Mini contains
virtually everything that you need to
implement a complete microcontroller
system. The only additional items are a
power source and a means of connecting
a programming device via a serial
interface and a USB cable.
The Pro Mini will operate quite
happily from nothing more than a
3.7V or 9V battery, making it ideal for
use in stand-alone applications where
there’s no mains supply available.
Alternatively, the Uno can derive its
power from an external DC source of
between 5V and 12V, or via an external
USB adapter and a powered USB port.
The Pro Mini provides a total of 14
digital input/output lines, together with
six analogue inputs. Where necessary,
signals on six of the 14 digital I/O lines
can make use of pulse-width modulation
(PWM) to effectively generate analogue
outputs. The Pro Mini’s technical
specification is shown in Table 4.1.
ATmega328P processor
The Pro Mini is based on the same
ATmega328P processor as the Arduino
Uno, and this ensures there is a high
degree of software and hardware
compatibility. The processor has 32kB of
Flash memory used for storing program
code (of which 2kB bytes is reserved for
bootloader code). The Pro Mini’s chip
Fig.4.1. Members of the Arduino family include (let to right) the Uno, Nano and the diminutive Pro Mini.
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Practical Electronics | August | 2021
Table 4.1 Technical specifications for the Arduino Pro Mini
conditioning the PWM outputs
can be used to generate quasiFeature
Specification
Notes
analogue output voltages. The
processor also incorporates a sixThis is the same processor that is
channel, 10-bit analogue-to-digital
Processor
ATmega328P
currently fitted to the Arduino Uno
converter (ADC) and incorporates
a variety of communications ports
The 3.3V has an 8MHz clock, while
Clock speed
8MHz or 16MHz
including a programmable serial
the 5V version operates at 16MHz
USART, an SPI serial interface
Flash memory
32kB
2kB bytes used for the bootloader
and a byte-oriented 2-wire serial
I 2 C-compatible interface. As a
Static RAM (SRAM)
2kB
Early versions had 1kB
bonus, the chip also incorporates
EEPROM
1kB
Early versions had 512 bytes
a programmable watchdog timer.
The ATmega328 has a total
3.3V or 5V depending
Supply voltage (Vcc)
of 23 input/output (I/O) lines.
on version
Before we expand on what these
3.35V to 12V for the
Please note the RAW power input
can do, it is important to be aware
Raw input voltage (pre-regulated) 3.3V version; 7V to
needs 50mV headroom for the 3.3V
that ATmega328 I/O lines can be
12V for the 5V version
version (and 2V for the 5V version).
software configured for different
functions. As an example, the
Six digital I/O pins can be used for
port line labelled PB3 (ie, Port
Digital I/O pins
14
PWM with 8-bit resolution
B, bit 3) provides Master data
output and Slave data input
Analogue input pins
6
when the chip is configured for
Maximum output current (per pin) 40mA
150mA maximum total output current
use with the Serial Peripheral
Interface (SPI) bus. Alternatively,
Absolute maximum current
200mA
it can be configured to provide an
External interrupts
2
Inputs from external hardware
external output for the PWM timer
Operating temperature range
−40°C to +105°C
function or as an external interrupt
source. This may sound a little
Dimensions
18 × 33mm
complicated at first, but it does
Weight
Less than 2g
make a significant contribution to
the chip’s wide applicability and
immense versatility.
pre-scaler and compare mode, and one
also has 2kB of static random-access
16-bit timer/counter with a separate
memory (SRAM) and 1kB of electrically
pre-scaler. The chip incorporates
erasable programmable read-only
Pro Mini Port B
six pulse-width modulated (PWM)
memory (EEPROM).
Port B is an 8-bit bi-directional I/O
channels available from a subset of
The ATmega328 incorporates two
port with internal pull-up resistors
digital I/O pins. With appropriate signal
8-bit timer/counters with separate
(selected for each bit). The Port B
output buffers have symmetrical drive
characteristics with both high-sink and
source capability. As inputs, Port B
pins that are externally pulled low will
source current if the pull-up resistors
are activated. The Port B pins are tristated when a reset condition becomes
active, even if the clock is not running.
Depending on the clock selection fuse
settings, PB6 can be used as input to the
inverting oscillator amplifier or input
to the internal clock operating circuit.
In the same manner, PB7 can be used
as output from the inverting oscillator
amplifier. If the internally calibrated
RC oscillator is used as the chip’s
clock source, PB6 and PB7 are used
as inputs for the second asynchronous
timer/counter.
Pro Mini Port C
Fig.4.2. Pro Mini board layout showing main components and connectors.
Practical Electronics | August | 2021
Port C is a 7-bit bi-directional I/O
port with internal pull-up resistors
(selected for each bit). The PC0 to PC5
output buffers have symmetrical drive
characteristics with both high sink and
source capability. As inputs, Port C
pins that are externally pulled low will
source current if the pull-up resistors are
activated. The Port C pins are tri-stated
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when a reset condition becomes active,
even if the clock is not running. PC6 can
be fuse programmed for use as an I/O
pin or to function as a RESET pin. In
the latter case, a low level held briefly
on this pin generates a reset.
Pro Mini Port D
Port D is an 8-bit bi-directional I/O
port with internal pull-up resistors
(selected for each bit). The Port D
output buffers have symmetrical drive
characteristics with both high-sink and
source capability. As inputs, Port D
pins that are externally pulled low will
source current if the pull-up resistors are
activated. The Port D pins are tri-stated
when a reset condition becomes active,
even if the clock is not running.
Using the Pro Mini I/O
The I/O features and capability are very
similar to those offered from a standard
Arduino Uno, but there are some notable
differences in relation to the pinout
configuration and headers, as shown
in Fig.4.2.
Each of the 14 digital pins (see Fig.4.2)
on the Pro Mini can be configured as
a digital input or output by using the
appropriate C functions:
pinMode()
digitalWrite()
digitalRead()
The digital I/O lines operate at standard
TTL-compatible levels and each of the
I/O pins can source or sink a maximum
current of 40mA. Internal pull-up
resistors of around 20 to 50kΩ can be
selected where necessary. Note that the
internal pull-up facility is disabled by
default. As previously mentioned, it is
important to be aware that many of the
Pro Mini’s digital I/O lines also have
specialised functions.
provide quasi-analogue outputs using
the analogWrite() function.
Serial SPI communication using pins
10, 11, 12 and 13
These four pins support SPI serial
communication by providing SS, MOSI,
MISO and SCK signals respectively.
User LED pin 13
As well as the red LED power indicator,
a separate user LED is available at pin
13. When the pin is taken HIGH, the LED
will be on and when LOW the LED will
be off. The user LED provides a handy
way of visually testing simple I/O code
routines without the need to connect an
external indicator.
Analogue I/O
In addition to the 14 digital I/O lines,
the Uno has six analogue inputs, each
of which provide 10 bits of resolution
(ie, 210 = 1024 different values over a
range extending from 0V to +Vcc).
I2C communication using pins 27 and 28
Analogue I/O port lines 4 and 5 (pins
27 and 28 respectively) can be used
to provide access to the SDA and SCL
signals respectively, which facilitate the
two-wire interface (TWI or I2C). Note
that these two signals are available on
off-grid pads located away from the three
main headers (see Fig.4.2).
Pro Mini Pinout
The Pro Mini’s pinout connections
are shown in Fig.4.2. For clarity we
have colour coded the signal lines into
different groups, as follows:
PRO MINI PADS (grey)
These correspond to the silk screen
markings on the upper (component)
side of the Pro Mini’s PCB
ARDUINO UNO (light green)
These are the corresponding pin
identifications for the standard version
of the Arduino Uno.
ATmega328 (blue)
T hes e ar e the processor’s port
designations
SPI (lilac)
These connections are for use with SPI
(Serial Peripheral Interface) devices
I2C (pink)
These connections are used with ‘twowire’ I2C interface devices
POWER SUPPLIES (red)
These connections provide access to
the internal and external supply rails
as well as common 0V/Ground
RESET (yellow)
This is connected to the Pro Mini’s
reset line
USER LED (green)
This digital I/O line can be used to
provide a simple ‘user LED’ indication.
It is worth noting that versions of the Pro
Mini may have slightly different layouts
and pin markings. This is particularly
applicable to the location and marking
of the four off-grid pads. The version
shown in Fig.4.2 is, however, currently
the most common.
Connecting to the Pro Mini
As with other members of the Arduino
family, the Pro Mini’s processor is
supplied with bootloader code that
will let you upload new code to it
without the use of an external hardware
programmer. This makes the process of
developing and downloading code very
Serial I/O pins TXO and RXI
These two pins can be used for serial
communication. The same pins are also
made available on the six-pin header to
which an FTDI serial-to-USB interface
can be attached in order to facilitate
downloading and debugging of code.
Interrupt pins 2 and 3
These two pins are available for use
with external interrupts. They can be
configured to trigger an interrupt on a low
value, a rising or falling edge, or a change
in value. The attachInterrupt()
function can be used to configure the
interrupt feature.
Pulse width modulation (PWM) using
pins 3, 5, 6, 9, 10, and 11
These six lines can be used to provide
outputs that are pulse-width modulated
(PWM). This feature can be used to
42
Fig.4.3. Two different serial FTDI interface boards for use with the Arduino Pro Mini.
Practical Electronics | August | 2021
simple and all you will require is the
services of a serial USB interface that
will connect to the Pro Mini’s FTDI
header (see Fig.4.2). Note, however,
that different version of the Pro Mini
may have a different FTDI header
orientation (see Fig.4.3) and so, if you
intend to mate the two directly with
headers it is important to check before
connecting the interface.
The process of establishing a
connection via an FTDI serial adapter
to a programming device such as a PC
is as follows:
1. Connect the FTDI serial interface to
the Pro Mini via the six-way header,
ensuring that the connections are
correctly made.
2. Connect the FTDI serial interface via
a USB cable to the host PC.
3. Check that you have the correct
virtual COM port (VCP) drivers
installed for the FTDI interface. These
may install automatically, or you may
Listing 4.1 Code for the Pro Mini Frost Alert.
/*
PE KickStart Frost Alert (Listing 4.1)
*/
// Assign LEDs to digital I/O lines
int redLED = 5;
// Red LED connected to DIO5
int amberLED = 6; // Amber LED connected to DIO6
int greenLED = 7; // Green LED connect to DIO7
// Assign analogue input
const int analogPin = 0; // Input to analog A0
// Set threshold levels
int freezing = 102; // At 0 deg.C appx. (change as required)
int warning = 110; // At 4 deg.C appx. (change as required)
// Setting up
void setup() {
// Initialize digital I/O pins outputs
pinMode(redLED, OUTPUT);
pinMode(amberLED, OUTPUT);
pinMode(greenLED, OUTPUT);
}
// Loop forever
void loop() {
int input = analogRead(analogPin);// Read the input
Serial.println(input);
if (input > warning) {
green();
}
else if (input > freezing) {
amber();
}
else red();
}
// LED indications
void red() {
digitalWrite(redLED, HIGH);
digitalWrite(amberLED, LOW);
digitalWrite(greenLED, LOW);
}
void green() {
digitalWrite(redLED, LOW);
digitalWrite(amberLED, LOW);
digitalWrite(greenLED, HIGH);
}
void amber() {
digitalWrite(redLED, LOW);
digitalWrite(amberLED, HIGH);
delay(1000);
digitalWrite(amberLED, LOW); // Flash the amber LED
delay(1000);
digitalWrite(greenLED, LOW);
}
Practical Electronics | August | 2021
need to download them from the chip
manufacturer (see Going Further below)
if the virtual COM port is not recognised
by your PC (the support documentation
on the FTDI provides an Installation
Guide, but we recommend that you opt
to download the ‘Setup’ zipped file to a
folder of your choice before extracting
the files and running the executable to
complete the installation.
4. Open the Arduino IDE on the host
PC and select ‘Tools’ and then ‘Board’.
Scroll down the list of boards and select
‘Arduino Pro Mini’ from the Boards List
Manager (do not just select ‘Arduino
Mini’ – this will not work!)
5. Return to the IDE main menu and
select ‘Tools’ and ‘Port’ and then select
your newly enabled virtual COM port.
If this does not appear in the list then
repeat steps 2 and 3.
6. You should now have a working
connection to the Pro Mini, so return
to the IDE and select ‘File’, ‘Examples’
‘01.Basics’ and ‘Blink’ from the list
of sample files. The file should then
appear in the IDE’s editing window
together with a tab which indicates
the default filename under which it
will be stored and made available for
future editing.
7. Now select ‘Sketch’ and ‘Upload’
to send the code to the Pro Mini. This
will take a short time, after which a
‘Done uploading’ message will appear,
informing you that the transfer has been
made. The Pro Mini will then restart,
and your efforts will be rewarded by a
flashing ‘user LED’ connected to digital
I/O line 13. You are now ready to create
your own program code for downloading
to the Pro Mini!
A sample application
To show you just how easy the Pro Mini
is to use and incorporate in your own
designs, here is a simple example in the
form of a Frost Alert. This handy device
will warn you when the temperature has
fallen to a low value where frost and
ice are likely to be present. The device
can be built using a Pro Mini and just
seven other components, three resistors,
three LEDs and one TMP36 temperature
sensor and it can be put together in less
than ten minutes! The complete circuit
of the Frost Alert is shown in Fig.4.4.
The TMP36 temperature sensor
is one of a family of three accurate,
low-cost temperature sensors that
provide analogue output voltages in
response to temperature. The TMP36
is recommended for use over the
temperature range −40°C to +125°C
and it offers an accuracy better than
±2°C and typically ±1°C at +25°C. The
output voltage is linearly proportional
to the temperature measured in degrees
Celsius, as illustrated by Fig.4.5.
43
Fig.4.6. Semiconductor pin connections
for the TMP36 temperature sensor.
Fig.4.4. Circuit of the Pro
Mini Frost Alert.
Fig.4.5. Characteristics of the TMP36 temperature sensor.
The TMP36 is intended for singlesupply operation from 2.7V to 5.5V and
is thus eminently suitable for operation
from the Arduino’s +3.3V or +5V supply.
To avoid the risks associated with selfheating, the chip requires only a very
small supply current (well below 50µA).
In addition, a shut-down function is
available to limit the residual supply
current to less than 0.5µA. The TMP36
provides a nominal output voltage of
750mV output at +25°C, changing at the
rate of +10mV/°C. At 0°C its output will
be 500mV and, for example, at 4°C it will
be 540mV. The interface to the Pro Mini’s
analogue port (see Fig.4.4) is extremely
simple and no other components are
required apart from the temperature
sensor itself. The pin connections for IC1
and D1-D3 are shown in Fig.4.6.
The minimal code for the Frost Alert
is shown in Listing 4.1 (this is also
available for downloading from the
PE website). As mentioned previously,
before you can begin to enter code into
the Pro Mini you will need to connect it
to a PC that has a copy of the Arduino’s
integrated development environment
(IDE) installed on it.
Table 4.2: Going Further with the Arduino Mini Pro
Topic
Source
Notes
Arduino Pro
Mini
The Arduino Pro Mini is available from various suppliers, including
HobbyTronics, Pimoroni, SparkFun, AZ-Delivery, and Adafruit. Several of
these suppliers also have some useful downloadable tutorials.
When ordering, it is important to specify the 3.3V or
5V version (as required).
FTDI serialto-USB
interface
The FTDI Basic Breakout Board serial-to-USB interface drivers can be
downloaded from: www.ftdichip.com
An installation guide is also available.
Arduino IDE
The Arduino IDE can be downloaded from: www.arduino.cc/en/software
Versions are available for Windows, Linux and macOS.
Arduino Uno
Electronics Teach-In 8. (available from Electron Publishing – see http://bit.
ly/pe-apr21-ks2-7) provides a comprehensive guide to the Arduino. This
popular series introduces hardware and software and also features a range
of practical projects with different levels of complexity.
The PE Direct Book Service at electronpublishing.
com has several other titles suitable for background
reading on the Arduino family.
Sensors and
interfacing
The author’s book, Electronic Circuits: Fundamentals and Applications (5th Ed,
2020, Routledge 9780367421984) provides a general introduction to sensors
and interfacing. The book also has a useful chapter on electronic applications
and the Arduino.
TMP36
temperature
sensor
The TMP36 datasheet is available from: www.analog.com/en/products/
tmp36.html
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Practical Electronics | August | 2021
