Make it with Micromite
Phil Boyce – hands on with the mighty PIC-powered, BASIC microcontroller
Part 32: Using iButtons with the Micromite
T
his month, we are going to
look at interfacing the Micromite
with a handy little gadget called
the ‘iButton’. If you don’t know what
an iButton is, then simply consider it
as an electronic device that contains
a digital identification number (ID),
housed in in a robust metal case – see
Fig.1. Each iButton is programmed by
the manufacturer with a guaranteed
unique ID, so iButtons are ideal for use as
‘electronic keys’ to grant the user access
to something, somewhere, or simply as
a unique identifier when attached to an
asset (object or person).
iButton overview
The iButton’s metal casing comprises a
stainless steel ‘can’, about the size of three
five-pence coins stacked on top of each
other (roughly 6×16mm). It is similar to a
medium-sized button that you may find
on clothing (hence the name), with the
‘i’ part of the name simply referring to
‘information’. iButtons are designed to
be mounted in, or attached to, virtually
anything. The most common technique is
to mount one in a key-fob holder, which
in turn can be attached to a bunch of keys
– see Fig.2. Note that iButtons can be used
in quite harsh outdoor environments, as
well as being used indoors.
Fig.1. iButtons: each one is approximately
6×16mm in size.
62
iButton application examples
and features
Some examples of their application
include using them for electronic dooraccess (in buildings such as universities,
factories, hospitals and retail stores), or
being used by staff to access cash tills
(often seen in pubs, restaurants and
retail outlets). They can even be used
as ignition keys for boats, or any other
types of outdoor vehicles. A slightly more
unusual application is the example of
iButtons being attached to farm animals
to identify them!
The simplest form of information
contained within an iButton is just
its own unique ID number. However,
different types of iButtons are available,
each offering slightly different features,
and offering additional information, such
as time and date, temperature, humidity,
or other ‘user-programmed’ data.
iButton communication
Information contained in an iButton is
accessed and communicated serially
over a single data line and is the reason
why you will often hear the term ‘1-Wire’
in relation to iButtons. However, this is
a little bit misleading because you also
need a common ground connection to
complete the ‘communication circuit’ –
hence two physical wires are required,
not just one. The term ‘1-Wire’ simply
refers to the protocol used on the single
data line, and it is this 1-Wire protocol
that the connected device (typically a
microcontroller) needs to use to be able to
communicate successfully with an iButton.
So how are iButtons connected
(electrically) to a microcontroller?
Simple, you use an iButton reader. This
may sound like it involves some kind of
complex device, but the reality is that
a reader is nothing more than a pair of
Fig.2. Key fobs are available in a range of colours and make it easy to attach the
iButton to a bunch of keys.
Practical Electronics | January | 2022
metal contacts that connect the iButton’s
data line (and common ground) through
to the microcontroller. In use, all the
user needs to do is ‘tap’ the iButton
momentarily onto the reader so that
the microcontroller can communicate
with the iButton (more on this later).
Alternatively, if an iButton is attached
to an asset, then instead of referring to a
‘reader’, the term ‘probe’ is used.
In this month’s article, we will start
by explaining some of the basic details
regarding iButtons and readers, and
will then show you how to use the most
common type of iButton (the DS1990)
with a Micromite. We will show just
how easy it is to use MMBASIC’s builtin 1-Wire commands to write a program
that is able to read the iButton’s unique ID
number whenever an iButton is detected.
Next month, we will continue by
looking at how to check that the serial
data that is read by the microcontroller
is valid, noting that there is often a
lot of ‘contact bounce’ present when
momentarily touching an iButton onto
a reader (or when touching a probe onto
an iButton). We will also look at some
iButton varieties, specifically one that
contains EEPROM, which allows data
to be written to it. We will end up by
using various elements of these iButton
articles in a new mini-project – a modernday version of the classic ‘electronic
combination door-lock’. Instead of using a
combination keypad, our Micromite-based
version will use EEPROM iButtons as the
keys, and it will incorporate a touchscreen
interface to show not only information
about the person who is trying to unlock
the door, but also facilitate the control
of valid keys.
More about iButtons
The description above has (hopefully)
provided a good introduction to iButtons;
however, there are several more points
that are worth discussing before we start
to use one with the Micromite. So next
we’ll expand on these points and provide
you with some interesting background
information, as well as highlighting some
useful terminology.
Part numbers
iButtons were first manufactured by
Dallas Semiconductors in the 1990s, and
this is the reason that all iButton part
numbers begin with the letters ‘DS’. Dallas
Semiconductors were acquired by Maxim
in 2001, and they continued to expand
the range of available iButtons, adding
several memory-type iButtons to the lineup: for example, incorporating EEPROM
or static RAM (SRAM) technology. The
SRAM-type iButtons can be used as
miniature dataloggers that are easily
attached to an asset so that data (such as
Practical Electronics | January | 2022
temperature) can be recorded at regular
time intervals. Even under Maxim, the
new iButtons continued to have a part
number beginning with ‘DS’, and there
are now around 20 different types, each
offering slightly different features.
We won’t go into the details of the
different part numbers, other than to
say that we will be focusing this month
on the DS1990 (an ID-only iButton), and
next month we will use the DS1971 (an
ID plus EEPROM iButton).
Family code
Each different type of iButton has a
part number that begins with DS, and
is then typically followed by four digits.
This part number then makes it easy
for us humans to distinguish between
the different versions. However, each
different DS part number also has a
corresponding byte-value known as the
‘family code’. (Note that hexadecimal
(hex) values are typically used when
talking about iButtons; the reason will
become clear shortly.) For the DS1990,
the family code is 01 (hex) and for the
DS1971 it is 14 (hex). So what is the
purpose of the family code?
Put simply, the family code is something
that can easily be read by the connected
microcontroller to determine which type
of iButton is currently ‘connected’. This
means the microcontroller’s software
can check to see if the correct (or
expected) type of iButton is present. For
example, you might want to do this if the
microcontroller is expecting an EEPROMtype iButton (eg, a DS1971) to which it
needs to write data. The software will
not be able to successfully write the data
if the user has tapped an ID-only (eg, a
DS1990) type of iButton onto the reader.
So, reading the family code allows the
software to behave correctly, and hence
it is good practice to implement this into
the program code.
Unique ID number
Even though the various iButton types
offer slightly different features, the one
thing that they all have is a unique ID
number. Note that this ID number is
factory-programmed (at the time of
manufacture) and it cannot be change
once programmed.
The ID number is always 64-bits long,
or put another way, comprises 8-bytes (64
bits / 8 bits per byte = 8 bytes). Consider
the example ID number (in hex) 01 00 00
1B C8 C6 D1 DE, then you can see that by
writing the 8-bytes as hex values it will
always be 16 hex digits long. We will see
the relevance of this shortly.
These 8-bytes can be broken down
into three sections, as follows. The first
byte is the family code byte, and in the
above example it is ‘01’. If you refer
to the previous section, you will see
that the family code value of 01 (hex)
represents a DS1990 (ID-only) iButton.
The next 6-bytes in the ID number is
what some people consider to be the
actual ID number – each DS1990 (or any
other iButton type) has a unique value
in these 6-bytes. The last byte is referred
to as the ‘CRC’ byte.
CRC
The cyclic redundancy check, or ‘CRC’
byte allows error checking to be performed
on the ID number that is read by the
microcontroller. It is quite complex, but in
essence, when passing the first 7-bytes of
the ID number through a fixed algorithm
(defined by the manufacturer), the one
byte which results from the algorithm’s
calcutions should match the value of the
CRC byte. If it does match, then the data
read can be regarded as correct – but, if
it doesn’t match, then the data read (ie,
the attempted read of the ID number)
should be discarded. We will discuss
this algorithm in a little more detail
next month, but for now, the CRC byte
should simply be considered as a way
to check if the serial data that has been
read is valid, or whether contact bounce
(between the iButton and the reader/
probe) has corrupted it.
Power
A point worth stressing here is that an
iButton only has two contacts to the
outside world: the 1-Wire serial data-line
(sometimes referred to as the 1-Wire bus),
and a common ground reference. So how
exactly is an iButton powered if it does
not have a power connection? The answer
is known as ‘parasitic power’. Put simply,
the 1-Wire data line is pulsed high and
low by the connected microcontroller
when communication is first initiated, and
it is from this pulsing that the iButton can
‘steal’ some energy to charge a capacitor.
This is done through a simple dioderesistor-capacitor circuit inside the
iButton. It provides just enough power
for the iButton to respond with its data.
Note that SRAM iButtons also contain
a battery, but the battery is there just to
preserve the SRAM contents; it is not
used for powering the iButton itself.
Sizes
iButtons come in two sizes, or to be
precise, two different thicknesses,
referred to as F3 and F5, where the
number indicates the thickness in
millimetres – ie, 3mm and 5mm (roughly
equal to two or three stacked five-pence
coins, respectively). For the precise
dimensions, refer to Fig.3 (taken from
the manufacturer’s datasheet). Note that
there is no technical difference between
an F3 and an F5 other than its thickness.
63
Fig.3. The thickness dimensions of
an iButton, showing the two different
versions: F3 and F5.
A closer look at the casing
would mean it could be anything between
8 and 24 characters in length – messy!
Across the bottom you see the
manufactured year and week number
(important when using an iButton that
contains a battery), and also the DS part
number along with the F3/F5 size. A closeup image of an actual iButton is shown in
Fig.6, from which we can confirm it is an
F5-sized DS1990 that was manufactured
in the 23rd week of 2019. The family code
hex-value is ‘01’, and the CRC hex-value
is ‘DE’. The other six ID byte values are:
‘00 00 1B C8 C6 D1’.
A very important point to note here is
that the family code value ’01’ shown in
the photo is indeed the correct value for
a DS1990. I am highlighting this because
‘cheap fakes’ can often be easily identified
when there is a mismatch between the
claimed part number and the correct
family code value.
1-wire bus
We will not go into the details of the 1-Wire
bus here as it can get rather complex.
However, in summary, it is a single serial
communication line that operates on a
‘master/slave’ concept. What this means
is that the connected microcontroller
is the single master, and will initiate
communication (which in turn provides
the parasitic power to charge the iButton’s
internal capacitor, as discussed above).
By toggling the 1-Wire bus between high
and low logic states, the microcontroller
can effectively send a command to the
iButton. The iButton is the slave, and it
decodes the command and responds by
sending the required data.
The 1-Wire bus allows for many slaves
(iButtons) to be connected to the master
(microcontroller). Therefore, the master will
iButton markings
first detect any iButtons that are present on
To understand what the various
the 1-Wire bus, and then follow up typically
manufacturer markings represent, please
by sending a command that includes the
refer to Fig.5. The important thing to point
ID number of the iButton that it wishes to
out is that the eight-bytes representing the
talk to. Hence, all iButtons connected on
unique ID number are stamped onto the
the 1-Wire bus will receive the message
iButton’s case (as hex values) and are laid
sent by the master, but only the one with
out in a way that clearly shows the family
a matching ID number will respond (all
code byte, the CRC byte, and the remaining
other iButtons will ignore the message).
6-bytes of the ID. Using hex values allows
However, in our application, because we
this layout to be consistent in length (ie, 16
will be using a reader to connect the iButton
characters) whereas using decimal values
with the microcontroller,
only one slave can physically
be present on the 1-Wire bus
at any moment in time – this
makes communications (and
hence our program code)
Lid
much simpler.
(1-Wire bus)
There are other commands
Base
that the master can send, but I
(ground)
want to keep the explanation
s i m p l e . H o w e v e r, t h e
most important concept
Fig.4. The iButton casing comprises two contacts. The
larger of the two is the base (ground), and the top ‘lid’ is to understand is that the
timing of the signal pulses
the 1-Wire bus.
An iButton’s metal enclosure comprises
two parts, the base and the lid. The base
includes the slightly larger diameter
bottom of the iButton and also the side
of the casing, while the lid is just the top
face onto which various markings are
stamped by the manufacturer to identify
(among other things) the part number.
These two metallic parts of the case are
connected to the internal IC, and thereby
provide the two contacts needed for a
microcontroller (via a reader or probe) to
communicate with the iButton. Referring
to Fig.4, you can see that the larger base
section is the ground contact, while the
top face is the 1-Wire bus contact.
64
Fig.5. This diagram taken from the
datasheet shows the layout of the
markings for a genuine iButton from
Maxim. The critical things to understand
are the location of the family code byte
(01) and the CRC (89).
on the 1-Wire bus is critical in order for the
system to operate correctly. Thankfully, the
MMBASIC firmware handles all this for us.
One example of how timing is important is
that all communications starts with pulses
from the master that will initialise things so
that communication with an iButton can
commence from a known starting point.
Essentially, this initialisation consists
of a reset condition transmitted by the
master, followed potentially by presence
pulse(s) transmitted by any slave(s) that are
connected to the 1-Wire bus. The presence
pulse then informs the master that an
iButton (one or more) is on the 1-Wire bus.
1-Wire commands
As mentioned above, the microcontroller
will send a command onto the 1-Wire bus,
and any iButton(s) present will receive this
command and respond accordingly. The
possible commands that the microcontroller
can send are referred to as ‘1-Wire
commands’ and are represented by a single
byte value. When there is just one iButton
on the 1-Wire bus (as will be the situation
in our case), the most useful command is
Fig.6. Here the markings of a DS1990
F5 iButton can be seen highlighting
the family code (01), CRC (DE) and the
remaining 6-bytes of the ID number
across the middle of the lid.
Practical Electronics | January | 2022
Fig.7. Various iButton readers are available. Some are weatherproof, and some include one or more LEDs. Despite the visual variety,
they are all essentially just two metal contacts that make connection to the iButton’s base and lid.
Read Rom. This command is represented by
sending the hex value 33 and will result in
the iButton sending its 8-byte ID value. The
microcontroller, on receiving this data, can
check the family code byte value to ensure
the expected type of iButton is present,
and use the CRC byte value to ensure the
data is correct. If it is, then the ID number
is known, and by association can identify
the user tapping the iButton, or the asset
the iButton is attached to.
Other 1-Wire commands are available
and may be specific to only certain types
of iButton, but for the DS1990 (ID only)
iButton, the Read Rom command is the
one that we will be using for now. Next
month, we will introduce more commands
as required when using the DS1971
(ID+EEPROM) iButton – commands that
allow us to write to and read from the
iButton’s EEPROM.
Readers
An online search will lead you to many
different versions of iButton readers – see
Fig.7 for some examples. There are three
main things that should be considered:
1) Does it need to be waterproof?
2) Does the reader need to retain the
iButton (rather than just being tapped
with the iButton)?
3) Does it need a built-in status LED?
Weatherproof readers are clearly required
if being used outdoors and such readers
exist for really harsh conditions as may
be found on boats. In some situations, it
may be a requirement for the iButton to
be in contact with the microcontroller for
an extended period of time (for example,
when reading or writing a large amount
of data to memory). In this situation, it is
better that the reader ‘retains’ the iButton,
which then avoids contact bounce (and
hence corrupted data).
Practical Electronics | January | 2022
LEDs are typically either single colour,
bi-colour, or RGB; and can be mounted as
a single led into the reader’s housing, or
set behind a diffused ring to help locate
the place where the iButton needs to be
touched (useful for dark environments).
Cost and availability
So how much do these iButtons cost?
They start at around £2 each for an IDonly type (ie, the DS1990) and an EEPROM
version (ie, the DS1971) starts at around
£4. These are the iButtons that we will be
using in these articles and the Electronic
Door Lock mini-project. That said, datalogger iButtons begin at around £10, with
the specialist high-capacity data loggers
costing anything up to around £150 (yes,
you read that correctly!).
Reader prices start at around £4 for
simple contact-only type, and cost from
around double that for ones with a builtin LED. Please note that a reader is not
essential for ‘testing’ as you could simply
use a pair of wires and momentarily touch
them onto the iButton; however, a reader
makes everything so much easier to use,
plus we will need one for our mini-project
– which type of reader you use is not
relevant because we will also be including a
touchscreen that can provide user-feedback.
I like to use the DS9092L reader, which
incorporates an LED at the centre point
and costs around £10. See Fig.7, top right
– it shows this specific reader, along with
some others that are available.
A Google search will reveal that there
are many suppliers of iButton components,
including the big global suppliers RS,
Farnell, Mouser and DigiKey. However,
there is also a very good and reliable small
supplier based in the UK that focuses just
on iButton parts. This is a company with
which I have no affiliation, but they have
supplied me many parts in a timely manner
and at a sensible cost – this company is
HomeChip – see: www.homechip.com
MMBASIC 1-Wire Commands
Before we start programming the Micromite
to communicate with an iButton, I
recommend you have access to Appendix
C in the Micromite User Manual. This is
just a single page, but it does give a useful
overview of the three 1-Wire commands
that are built into MMBASIC (ONEWIRE.
RESET, ONEWIRE.WRITE, and ONEWIRE.
READ), along with a system variable that
we will be using (MM.ONEWIRE). It is not
essential to read Appendix C, just simply
have it available for reference should you
like to read more about MMBASIC’s built
in 1-Wire commands.
Demo 1: Detecting an iButton
Let’s begin with a program that will simply
detect when an iButton is tapped onto the
reader. First, we will need to connect the
reader (or two wires) to the Micromite
plus a 4.7kΩ pull-up resistor, as shown in
Fig.8. Any available I/O pin can be used
(refer to the User Manual if required).
Once this is complete, proceed to type in
the following code:
3.3V
iButton reader
R1
7
I/O
iButton
0V
Fig.8. The circuit consists of connecting
the 1-Wire Bus to any available I/O pin
(ensure there is a 4.7kΩ pull-up resistor),
and also connecting a common ground.
65
screen (ie, to the TeraTerm screen). This
loop is repeated over and over every 10ms.
Now SAVE the program and then RUN
it. You should see a column of 0s appear
down the left side of your Terminal screen.
If you now tap and hold an iButton (any
type) onto the reader, the column of 0s
should change to 1s. Note that you will
likely see contact bounce in the form of
the occasional 0 or 1 appearing when
not expected – this is normal, and is the
reason we need to use the CRC byte value
to check the validity of any data we read.
If everything works as described then
move onto Demo 2. However, if something
is not working as expected, then you will
need to resolve the issue by working
through the next Troubleshooting section.
iB_pin= 9
DO
ONEWIRE RESET iB_pin
PRINT MM.ONEWIRE
PAUSE 10
LOOP
Before you RUN the program, we will
explain what each of the six lines does.
The first simply defines the I/O pin to
which the 1-Wire bus (ie, the iButton
‘lid’) is connected to. This pin number
is loaded into the variable that we have
called iB_pin (don’t forget to change the
value from 9, to the I/O pin number that
you’re using if it is different).
The second line (and last) create an
endless DO/LOOP that contain two 1-Wire
references (on lines 3 and 4), and a pause
for 10ms (line 5) that slows things down
a little. Each time the code in the loop is
executed, the ONEWIRE RESET iB_pin
(line 3) causes the Micromite to send a
RESET pulse to pin 9 (or whatever pin
number is loaded into the variable iB_
pin). The specific timings are fully handled
by MMBASIC and make things so much
easier for us. The end result will be that the
MM.ONEWIRE system variable is then loaded
with either a 0 (if no iButton responded),
or a 1 if an iButton was detected. Line 4
simply prints this value to the Terminal
Troubleshooting Demo 1
If an error is generated when you RUN the
program then the error reported should
highlight what the issue is. The most
common mistake (other than typos) is
to use an invalid pin number (the error
message will show if this is the case).
Any syntax error is most likely a typo. If,
after checking your code, the program is
still not responding to an iButton being
tapped on the reader, the likelihood is that
either the wrong pin is being connected to
(for example the code refers to pin 9, but
physically the iButton is being connected
to pin 10), or the pull-up resistor is either
missing or the wrong value. Don’t forget
the common ground connection too.
If after checking the software and the
hardware it is still not functioning correctly,
then send me an email and I will try and
assist you.
Demo 2: Displaying the ID number
This demo can be download from the
January 2022 page of the PE website. Again,
simply RUN it to see the outcome. We will
explain its operation next month, but in
the meantime, why not take a quick look
at the code and see if you can begin to
grasp what is happening. If you struggle,
don’t panic, we will cover this at the start
of the next article.
Next month
We will begin next month by explaining
Demo 2 in more detail, and then look in
greater depth at the CRC algorithm. We will
then discuss the DS1971 (256-bit EEPROM)
iButton and show how to write to and read
data from it. Until then, Have Fun!
Questions? Please email Phil at:
contactus<at>micromite.org
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£595
Radio Communications Test Set
£725
Radio Communications Test Set
£800
Microwave Test Set
£1,500
Microwave Test Set 10MHz – 20GHz
£1,950
Microwave Test Set
£2,300
Power Meter with 6910 sensor
£295
Oscilloscope 500MHz 2.5GS/s
£1,250
Oscilloscope 300MHz 2.5GS/s
£995
Oscilloscope 2 Channel 100MHz 1.25GS/s
£450
Oscilloscope Dual Trace 150MHz 100MS/s
£350
Oscilloscope 4 Channel 400MHz
£600
PSU 0-60V 0-50A 1kW Switch Mode
£300
PSU 0-35V 0-2A Twice Digital
£75
Power Supply 100V 90A
£900
Sine/Sq Oscillator 10Hz – 1MHz
£45
Counter/Timer 160MHz 9 Digit
£150
Counter 20GHz LED
£295
True RMS Millivoltmeter 5Hz – 20MHz etc
£45
As 9300
£75
6½ Digit DMM True RMS IEEE
£65/£75
Gain Phase Analyser 1mHz – 20kHz
£600
HF Frequency Response Analyser
POA
PSU 0-35V 0-2A 2 Meters
£30
PSU 0-30V 0-2A Twice
£160 – £200
Function Generator 0.002-2MHz TTL etc Kenwood Badged
£65
d
£350
£600
£350
POA
POA
£400
POA
POA
POA
Marconi 2955B Radio
Communications Test Set – £800
Practical Electronics | January | 2022
