Understanding the principals of the communication are essential for further reading on hardware and programming. Most microcontrollers (including SoCs) can communicate in the protocols juxtaposed below right “out of the box”. Interfaces can be implemented in hardware or (recently) in software. Some microcontrollers may require an external, dedicated protocol converter (a chip or a module).

IoT systems are typically structured into three basic layers ^[1]. The lowest layer is the Perception (physical, acquisition) Layer, the intermediate is the Network Layer, and the higher is the Application Layer. The function of the perception layer is to keep the contact with the physical environment. Devices working in this layer are designed as embedded systems. They include the microprocessor or microcontroller, memory, communication unit, and interfaces - sensors or actuators. Sensors are elements that convert a value of some physical parameter into an electrical signal, while actuators are elements that control environment parameters. Sensors and actuators are interfaced with the microcontroller using different connection types. This chapter describes some internal protocols used to communicate between microcontrollers and other electronic elements that can be named “embedded protocols”. Description of the protocols used for wire and wireless transmission between the perception layer and higher layers is present in communications_and_communicating_sut The embedded protocol that can be used in specific implementation depends mainly on the type of the peripheral element. Some of them use an analog signal that the microcontroller must convert to digital internally, some directly implement digital communication protocol.

Simple sensors do not implement the conversion and communication logic, and the output is just the analog signal – voltage level depending on the value of the parameter that is measured. It needs to be further converted into a digital representation; this process can be made by analog to digital converters (ADC) implemented as the internal part of a microcontroller or separate integrated circuit. Examples of the sensors with analog output are a photoresistor, thermistor, potentiometer, resistive touchscreen.

Dummy, true/false information can be processed via digital I/O. Most devices use positive logic, where i.e. +5V (TTL) or +3.3V (those are most popular yet there do exist other voltage standards) presents a logical one, while 0V presents logical zero. In real systems this bounding is fuzzy and brings some tolerance, simplifying i.e. communication from 3.3V output to 5V input, without a need of the conversion (note, opposite conversion is usually not so straightforward, as 3.3V inputs driven by 5V output may burn easily). A sample of the sensor providing binary data is a button (On/Off).

One of the most popular interfaces to connect the sensor is SPI (Serial Peripheral Interface). It is a synchronous serial interface and protocol that can transmit data with speed up to 20Mbps. SPI is used to communicate microcontrollers with one or more peripheral devices over short distances – usually internally in the device. In SPI connection there is always one master device, in most cases the microcontroller (uC) that controls the transmission, and one or more slave devices - peripherals. To communicate SPI uses three lines common to all of the connected devices, and one enabling line for every slave element.

MISO is intended to send bits from slave to master, MOSI transmits data from master to slave. SCK line is used for sending clock pulses which synchronize data transmission. The clock signal is always generated by the master device. Every SPI compatible device has the SS (Slave Select) input that enables communication in this specific device. Master is responsible to generate this enable signal – separately for every slave in the system.

SPI is used in many electronic elements like analog to digital converters (ADC), real-time clocks (RTC), EEPROMs, LCD displays, communication interfaces (eg. Ethernet, WiFi) and many others. Due to different hardware implementations, there are four modes of operation of the SPI protocol. The mode used in master must fit the mode that is implemented in the slave device.

This results in different timings of the clock signal concerning the data sent. Clock polarity = 0 means that the idle state of the SCK is 0 so every data bit is synchronized with the pulse of logic 1. Clock polarity = 1 reverses these states. Output edge (rising/falling) says at which edge of active SCK signal sender puts a bit on the data line. Data capture edge says at what edge of SCK signal data should be captured by the receiver.

TWI (Two WIre Interface) is one of the most popular communication protocol used in embedded systems. It has been designed by Philips as I2C (Inter Integrated Circuit) for using in the audio-video appliances controlled by the microprocessor. There are many chips that can be connected to the processor with this interface including:

1. EEPROM memory chips
2. RAM memory chips
3. AD/DA converters
4. Real-time clocks
5. Sensors (temperature, pressure, gas, air pollution)
6. Port extenders
7. Displays
8. Specialized AV circuits

TWI, as the name says, uses two wires for communication. One is the data line (SDA) the second is the clock line (SCL). Both lines are common to all circuits connected to the one TWI bus. The method of the communication of TWI is the master-slave synchronous serial transmission. It means that data is sent bit after bit synchronized with the clock signal. SCL line is always controlled by the master unit (usually the processor), the signal on the SDA line is generated by the master or one of the slaves – depending on the direction of communication. The frequency rate of the communication is up to 100kHz for most of the chips, for some can be higher – up to 400kHz. New implementation allows even higher frequency rate reaching 5MHz. At the output side of units, the lines have the open-collector or open-drain circuit. It means that there are external pull-up resistors needed to ensure proper operation of the TWI bus. Value of these resistors depends on the number of connected elements, speed of transmission and the power supply voltage and can be calculated with the formulas presented in Texas Instrument Application Report^[3]. Usually, it is assumed between 1 kOhm and 4,7 kOhm.

The data is sent using frames of bytes. Every frame begins with the sequence of signals that is called the start condition. This sequence is detected by slaves and causes them to collect next eight bits that form the address byte – unique for every circuit on the bus. If one of the slaves recognizes its address remains active until the end of the communication frame, others become inactive. To inform master that some unit has been properly addressed slave responses with the acknowledge bit – it generates one bit of low level on the SDA line (clock pulse is generated by the master). After sending proper address data bytes are sent. The direction of the data bytes is controlled by the last bit of the address, for 0 data is sent by the master (Write), for 1 data is sent by the slave (Read). Every full byte (eight bits) must be acknowledged by the receiving unit. There is no limitation on the number of data bytes in the frame, for example, samples from the AD converter can be read continuously byte after byte. At the end of the frame another special sequence is sent by the master – stop condition. It is also possible to generate another start condition without the stop condition. It is called repeated start condition.

Address byte activates one chip on the bus only, so every unit must have a unique physical address. This byte consists usually of three elements: 4-bit field fixed by the producer, 3-bit field that can be set by connecting three pins of the chip to 0 (ground) or 1 (positive supply line), 1-bit field for setting the direction of communication (R/#W). Some elements (eg. EEPROM memory chips) uses the 3-bit field for internal addressing so there can be only one such circuit connected to one bus. There are no special rules for the data bytes. First data byte sent by the master can be used for configuration of the slave chip. In memory units, it is used for setting the internal address of the memory for writing or reading. In multi-channel AD converters to choose the analog input. The detailed information of the meaning of every bit of the transmission is present in the documentation of the specific integrated circuit. I2C standard defines also the multi-master mode but in most of the small projects, there is one master device only.

1-Wire is a device communications bus system designed by Dallas Semiconductor Corp^[6]. that provides low-speed data, signaling, and power over a single signal. 1-Wire is similar in concept to I²C, but with lower data rates and longer range. It is typically used to communicate with small, inexpensive devices such as digital thermometers and weather instruments. A network of 1-Wire devices with an associated master device is called a MicroLAN. 1-Wire devices may be one of many components on a circuit board within a product, could be a single component within a device such as a temperature probe, or may be attached to a device being monitored. Some laboratory systems and other data acquisition and control systems connect to 1-Wire devices using cables with modular connectors or with CAT-5 cable, with the devices themselves, mounted in a socket, incorporated in a small PCB, or attached to the object being monitored. In such systems, RJ11 (6P2C or 6P4C modular plugs, commonly used for telephones) are popular. Systems of sensors and actuators can be built by wiring together 1-Wire components. Each component contains all of the logic needed to operate on the 1-Wire bus. Examples include temperature loggers, timers, voltage and current sensors, battery monitors, and memory. These can be connected to a PC using a bus converter. USB, RS-232 serial, and parallel port interfaces are popular solutions for connecting the MicroLAN to the host PC. 1-Wire devices can also be interfaced directly to microcontrollers from various vendors.

In any MicroLAN, there is always one master in overall charge, which may be a PC or a microcontroller. The master initiates activity on the bus, simplifying the avoidance of collisions on the bus. Protocols are built into the software to detect collisions. After a collision, the master retries the required communication. Many devices can share the same bus. Each device on the bus has a unique 64-bit serial number. The least significant byte of the serial number is an 8-bit number that tells the type of the device. The most significant byte is a standard (for the 1-wire bus) 8-bit CRC. There are several standard broadcast commands, as well as commands used to address a particular device. The master can send a selection command, then the address of a particular device. The next command is executed only by the addressed device. The 1-wire bus enumeration protocol (described later), like other singulation protocols, is an algorithm the master uses to read the address of every device on the bus. Since the address includes the device type and a CRC, recovering the address roster also produces a reliable inventory of the devices on the bus. The 64-bit address space is searched as a binary tree, allowing up to 75 devices to be found per second. The Dallas 1-Wire network is physically implemented as an open drain master device connected to one or more open drain slaves. A single pull-up resistor is common to all devices and acts to pull the bus up to 3 or 5 volts and may provide power to the slave devices. Communication occurs when a master or slave asserts the bus low, i.e. connects the pull-up resistor to ground through its output MOSFET. Specific 1-Wire driver and bridge chips are also available. Data rates of 16.3 kbit/s can be achieved. There is also an overdrive mode which speeds up the communication by a factor of 10. The master starts a transmission with a reset pulse, which pulls the wire to 0 volts for at least 480 µs. It resets every slave device on the bus. After that, any slave device, if present, shows that it exists with a “presence” pulse: it holds the bus low for at least 60 µs after the master releases the bus. To send a “1”, the bus master sends a very brief (1–15 µs) low pulse. To send a “0”, the master sends a 60 µs low pulse. The falling (negative) edge of the pulse is used to start a monostable multivibrator in the slave device. The multivibrator in the slave clocks to read the data line about 30 µs after the falling edge. The slave's multivibrator unavoidably has analog tolerances that affect its timing accuracy, which is why the “0” pulses have to be 60 µs long, and the “1” pulses can't be longer than 15 µs. When a dedicated 1-Wire interface peripheral is not available, a UART can be used to implement a 1-wire bus master. Serial or USB “bridge” chips are also available that handle the timing and waveform requirements of the 1-Wire bus, and are particularly useful in utilizing long (greater than 100 m) cables effectively. Up to 300 meter long buses consisting of simple twisted pair telephone cable has been tested by the manufacturer. It will, however, require adjustment of pull-up resistances from 5 to 1 kΩ. When receiving data, the master sends a 1–15-µs 0-volt pulse to start each bit. If the transmitting slave unit wants to send a “1”, it does nothing, and the bus goes to the pulled-up voltage. If the transmitting slave wants to send a “0”, it pulls the data line to ground for 60 µs. The basic sequence is a reset pulse followed by an 8-bit command, and then data is sent or received in groups of 8-bits. When a sequence of data is being transferred, errors can be detected with an 8-bit CRC (weak data protection). To find the devices, the master broadcasts an enumeration command, and then an address, “listening” after each bit of an address. If a slave has all the address bits so far, it returns a 0. The master uses this simple behavior to search systematically for valid sequences of address bits. The process is much faster than a brute force search of all possible 64-bit numbers because as soon as an invalid bit is detected, all subsequent address bits are known to be invalid. An enumeration of 10 to 15 devices finishes very quickly. The location of devices on the bus is sometimes significant. For these situations, the manufacturer has a special device that either passes through the bus or switches it off.

The DS9490B is a USB bridge and holder for a single F5-size iButton. The DS9490R is a USB bridge with 1-Wire RJ11 interface to accommodate 1-Wire receptacles and networks.

The bridge is based on the DS2490 chip developed by Dallas company, which allows to interconnect USB interface with 1-Wire bus. This required programming and electrical conversion between two different protocols in bidirectional way. The electrical wiring are present on Figure 3.

The appropriate 1-Wire cable pinout uses RJ11 telephone connectors.

The list of Dallas/Maxim integrated 1-Wire devices contains a wide range of the industrial implementations. The 1-Wire sensors and switches devices are viral in developers entity because of ease of implementation. 1-Wire protocol can be easy and fast implemented into the current IoT boards, most of the manufacturers share the software libraries allowing developers to include them in their projects in C, C++, assembly languages. The 1-Wire sensors (temperature, humidity, pressure etc.) are factory calibrated and reading the physical measurements follows the International System of Units (SI). 1-Wire products can be grouped as follows:

1. Secure Authenticators,
2. Memory EPROM, EEPROM ROM,
3. Temperature Sensors and Temperature Switches,
4. Data Loggers,
5. 1-Wire Interface Products,
6. Battery Monitors, Protectors, and Selectors,
7. Battery ID and Authentication,
8. Timekeeping and Real-Time Clocks.