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KickStart b y M ike Tooley Part 7: Plug and play with I2C 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 Fig.7.1. A low-cost real-time clock (RTC) designed for use with a wide range of I2C-compatible microcomputer and microcontroller systems. N owadays, microcontrollers and single-board computers usually provide you with a very handy method of connecting external devices using a popular ‘twowire’ inter-integrated circuit interface supported by a wide range of low-cost bus-compatible devices. This versatile interface is variously known as ‘IIC’, ‘I2C’, or ‘I2C’, and it will allow you easily to interface your controller with a host of devices, such as I/O multiplexers; sensors for temperature, pressure and humidity; magnetometers; real-time clocks; motion sensors and a variety of display controllers. I2C is also found in some interesting programmable devices, such as the FM radio chip featured in this article. I2C is a very simple bus system where bidirectional serial data appears on one line (SDA) and a clock signal is sent on a second line (SCL). It thus requires only two bus lines plus, of course, a common ground connection. In order to avoid conflict, each device connected to the I2C bus is software addressable using a unique address. The advantage of these minimal connecting requirements is that equipment based on I2C can be very easily modified and expanded without the need for major hardware changes. 42 ‘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 your own use. This seventh instalment provides an introduction to the popular and simple I2C interface. To help you get started, we’ve provided a useful practical example in the form of an Arduino Nano-based FM radio using I2C for controlling both the radio module and an OLED station display. I2C was the brainchild of Philips, but several of its leading competitors (including Motorola/ Freescale, NEC, Siemens, STM and Texas Instruments) have developed their own I 2 C-compatible products. In addition, Intel’s SMBus provides a stricter definition of I2C that helps to improve the interoperability of I2C devices Fig.7.2. I2C bus with two bus masters and three slaves. from different manufacturers. n On-chip filtering is usually incorporated Fig.7.1 shows a typical I2C to reject transient noise spikes that can device, a battery-backed real-time clock be present on the bus data line (RTC) module. I2C bus compatibility n The number of devices that can be makes it very simple to add this device to a wide range of microcomputer and connected to the same bus is limited microcontroller systems. Note that in only by the maximum specified bus order to simplify interconnection the capacitance (400pF). This makes I2C 2 I C bus connections are duplicated on highly expandable. opposite edges of the board. Key features of I2C Bus interface logic The single data line is shared between multiple devices, so I2C uses a system Key features of the I2C bus include: n Only two bus lines (plus ground) thus of addressing to identify the device that it needs to communicate with. Data minimal interconnecting wiring n Each device connected to the I2C communication is initiated by means of a unique start sequence. This involves bus is software addressable with a pulling the data line (SDA) low while unique address (see Table 7.1 for the clock line (SCL) is high. This can some examples) n Simple master/slave relationships be achieved by using very simple bus interface logic where each of the bus lines exist at all times, with bus masters are normally pulled high and driven low operating as master-transmitters or as when activated by a device connected to master-receivers n I2C is a true multi-master bus, which the bus (see Fig.7.2). Fig.7.3 shows a simple bus transaction incorporates collision detection which begins with a start condition (S) and arbitration to prevent data and ends with a stop condition (P). Note corruption if two or more masters simultaneously attempt to initiate data transfer n Serial, 8-bit-oriented, bidirectional data transfers can be made at up to 100 kbit/s in standard mode, and up to 400 kbit/s in fast mode. This is perfectly adequate for most non-critical applications Fig.7.3. An I2C bus transaction. Practical Electronics | February | 2022 Fig.7.4. Simplified I2C bus interface logic. how address information is transmitted serially on the SDA line during clock cycles 1 to 7 of the first bus cycle, while serial data follows during clock cycles 1 to 8 of the next bus cycle. Notice also how the bus lines are placed in a quiescent high state both before and after the bus transaction. Fig.7.4 shows the minimal logic required to interface with the I2C bus. A bidirectional gate arrangement is used for input to and output from both the local serial clock (SCLK) and serial DATA lines of each bus-connected device. Incoming data and clock signals are regularised by means of a high-impedance input buffer stage, while output data and clock signals drive the bus using an open-drain MOS device. Note that the bus requires pull-up resistors so that the bus lines go high before the start condition (S) and after the end condition (P) – as shown in Fig.7.3. Addressing Following the start sequence, transmitted data is only allowed to change when the clock is in its low state. In its basic form, and by virtue of the seven bits available for addressing (see Fig.7.3), the I2C protocol caters for a total of 127 devices. In addition to the seven bits used for addressing, the first byte of an I2C transfer generated by a bus master includes a bit that indicates the direction of the data transfer. Note that the address is transferred with the most-significant bit first. Table 7.1 shows I2C addresses for a diverse selection of I2C devices; it should give you an idea of just how versatile and useful this simple bus really is. Note that addresses are quoted in hexadecimal (thus ‘0x20’ is 20 in hexadecimal, 32 in denary or 100000 in binary). a second bus line (SCL). Serial peripheral interface (SPI), on the other hand, offers a full-duplex point-to-point connection where the data is passed in and out on separate lines (MOSI and MISO). SPI is therefore faster and often easier to use than I2C, but there can often be situations in which I2C is preferred simply because this is the interface that’s built into the chip or device that you intend to use! Introducing the TEA5767 radio chip The TEA5767 is a single-chip (see Fig.7.5) electronically tuned FM stereo radio designed specifically for use in simple lowvoltage applications controlled via the I2C bus. The device is completely adjustment-free and only requires a minimum of small and low-cost external components. The main features of the radio are: n Integrated RF amplifier for high sensitivity n 87.5MHz to 108MHz tuning range for the US and Europe (76MHz to 91MHz in Japan) n RF automatic gain control (AGC) n Fully integrated FM demodulator n FM IF selectivity performed internally n Crystal reference frequency oscillator (operating at 32.768kHz or at 13MHz crystal with an externally applied 6.5MHz reference) n Synthesised phase-locked loop (PLL) tuning n Soft mute and signal-dependent mono-to-stereo switching. I2C compared with SPI I2C is a very simple bus system where bidirectional data appears on a single line (SDA) and a clock signal is sent on Table 7.1: I2C addresses for a selection of devices Device Function/application Address range BME280 Temp, pressure and humidity sensor 0x76 or 0x77 CAP1188 8-channel capacitive touch sensor 0x28 to 0x2D MCP23008 I2C GPIO expander 0x20 to 0x27 MCP9808 Digital temperature sensor 0x18 to 0x1F PCA9685 16-channel PWM driver 0x40 to 0x7F SAA2502 MPEG audio source decoder 0x30 and 0x31 SSD1306 OLED display driver 0x37 TDA9860 Hi-Fi audio processor 0x40 to 0x41 TEA5767 FM radio receiver 0x60 TMP007 IR temperature sensor 0x40 to 0x47 Practical Electronics | February | 2022 Fig.7.5. The TEA5767 FM radio module. 43 The channel selector (change) button (PB1) is repeatedly pushed to cycle through the four channels and its state (HIGH or LOW) is stored in the buttonState variable. The operating frequencies and channels shown in the code are for use in the Sussex area of the UK, but they can be easily changed to those being used in your local area. All you need to do is change the frequency and station text for each channel. If required, other stations can be catered for by adding further code after the block for Channel 4. A few of the most popular stations are listed in Table 7.2, and most UK readers should be within range of one or more of these. Note that it will be necessary to alter the code at the top of the loop to reflect the new number of available channels. Fig.7.6. Circuit of the I2C-controlled FM radio. The TEA5767 is easy to use and only requires a few lines of code for channel selection. For example, the following single line of code sets the TEA5767’s operating frequency to 104.8MHz (BBC Radio Sussex): Constructing the Arduino-based TEA5767 FM radio The minimal wiring for the Arduino-based FM radio is shown in Fig.7.7. The three modules (Arduino Nano, TEA5767 and OLED display) can be conveniently mounted in a small ABS enclosure, allowing access to the two 3.5mm jack sockets used radio.setFrequency(104.8); // BBC Radio Sussex Note that the appropriate library routines must first be referenced for inclusion in your code or an error message will be generated and the application will simply not run. Table 7.2: Some popular UK FM radio stations Station Frequency (MHz) BBC Radio Bristol 94.9 and 104.6 BBC Radio Cornwall 95.2 and 103.9 BBC Radio Cymru 92.4 and 92.7 BBC Essex 95.3 and 103.5 BBC Radio Humberside 95.9 BBC Radio Kent 96.7 and 104.2 BBC London 94.9 BBC Radio Manchester 95.1 u8g2.clearBuffer(); // First clear the display memory u8g2.setFont(u8g2_font_helvB14_tf); // Select the font to be used u8g2.drawStr(0,25,"BBC Radio Sussex"); // The text to be displayed u8g2.sendBuffer(); // Finally send it to the display BBC Merseyside 95.8 BBC Radio Scotland 92.5, 92.6, 92.7, 92.8, 92.9 etc. BBC Radio Sheffield 88.6 BBC Radio Solent 96.1 and 103.8 BBC Radio Ulster 93.1 Note once again that the necessary library routines must be referenced for inclusion in the code and the display driver must first be initialised. BBC Radio Wales 90.2, 90.3, 90.4, 90.5, 90.4 etc. BBC Radio York 95.5, 103.7, and 104.3 Belfast 89FM 89.3 Capital FM 97.4, 103.2 Classic FM 100.1, 100.2, 100.3, 100.4 etc. Greatest Hits Radio 96.2 and 97.4 Heart (Essex, Solent, Surrey, Sussex) 97.5 Heart (North East, West Midlands) 100.7, 101.2 Heart (North Wales) 88.0 Heart (Scotland) 101.1, 103.3 Heart (South West) 100.8, 101.2 Manx Radio 88.9 OLED display The 0.91-inch OLED (organic light-emitting diode) display uses a matrix of 128x32-pixel LEDs. The device is controlled using an SSD1306 driver chip, which also contains an I2C interface. The I2C address of the device is 0x3C and it operates from a supply voltage in the range 3.3 to 5V at a typical current of less than 8mA. The OLED display is very easy to use and only requires a few lines of code. For example, the following code fragment displays a simple text message: The I2C-controlled FM radio The circuit of our I2C-controlled FM radio is shown in Fig.7.6. It shows the Arduino Nano controller linked to the TEA5767 FM radio and OLED display using the I2C bus (the SDA and SCL lines are respectively connected to pins A4 and A5 on the Nano). The code for the circuit (I2C_FM_radio.ino) is available for download from the February 2022 page of the PE website. The channel change button (PB1) is connected to digital input D3 on the Nano. This line is configured as an input using the following two lines of code: const int buttonPin = 3; // Channel change button pinMode(buttonPin, INPUT); 44 Practical Electronics | February | 2022 You will need... Fig.7.7. Wiring schematic for the I2C-controlled FM radio. for the antenna input and headphone/ speaker output. Access will also be required for the USB programming/5V external DC supply connector. The wiring schematic is shown in Fig.7.7 and a prototype breadboard layout is shown in Fig.7.8. Finally, this project provides plenty of scope for modification and experimentation. A capacitive touch switch could easily be added for pre-set channel selection. Alternatively, a rotary encoder could be used to give full tuning coverage of the VHF band in 100kHz steps. An Arduino Uno could be substituted for the Nano and the raw DC input could be used instead of the USB connector. The two YouTube video presentations listed in Going further overleaf should provide you with plenty of food for thought! To build the I2C radio you need one each of the following: n Arduino Nano n I2C-bus-compatible TEA5767 FM radio module (see Going further) n I2C-bus-compatible OLED display module (see Going further) n Telescopic antenna fitted with 3.5mm jack plug (often bundled with the TEA5767 radio module) n Headphones or external speaker fitted with a stereo 3.5mm jack plug n Miniature NO (normally open) pushbutton switch n 4.7kΩ resistor n USB lead for connection to a PC (for programming) n USB 5V DC power supply (or any switched 5V supply) n Small ABS enclosure (or prototype breadboard for test purposes). Going further This section details a variety of sources that will help you locate the component parts and further information that will allow you to understand I2C and add a wide range of I2C-bus-compatible devices to your projects. It also provides links to relevant underpinning knowledge and manufacturers’ data sheets. Fig.7.8. The prototype I2C-controlled FM radio on-test, tuned to (FM) BBC Radio Sussex. Practical Electronics | February | 2022 45 Table 7.3: Going Further with I2C Topic Meet the I2C bus Source Texas Instruments have a useful introduction to the I2C: https://bit.ly/pe-Feb22-ks1 For a comprehensive directory of I2C addresses, visit: https://i2cdevices.org The Arduino website provides a variety of resources to support the Nano: https://bit.ly/pe-dec21-ard1 Arduino Nano The Arduino’s integrated development environment (IDE) can be downloaded from: https://bit.ly/pe-feb22-ks3 Electronics Teach-In 8 – Introducing the Arduino (available from Practical Electronics) provides a one-stop source of ideas and practical information. TEA5767 FM radio module The TEA5767 FM radio module is available from several on-line suppliers, including Amazon and eBay. The NXP/Philips datasheet for the chip itself (not the complete module) is available from: https://bit.ly/pe-feb22-ks4 0.91-inch OLED display The 0.91-inch OLED display using an SSD1306 driver is available from on-line suppliers, including AZ-Delivery, Amazon and eBay – search: ‘0.91" inch OLED SSD1306’. YouTube videos Csongor Varga has produced a detailed video showing the TEA5767 FM radio module in action: https://youtu.be/yp0HVGjakMs Ralph Bacon provides another excellent introduction to using the TEA5767: https://youtu.be/yWf9uxL6zgE Notes An excellent PowerPoint introduction to I2C can be found at: https://bit.ly/pe-feb22-ks2 To identify the I2C address of a connected device, a scanner application (i2c_scanner) is one of the example files in the I2C Wire library. Note: this sketch tests the standard 7-bit addresses and any devices with higher-bit addresses might not be seen properly. An Arduino Uno can be substituted for the Nano, but it will require a much larger enclosure. You may need to use the IDE’s in-built Library Manager to download the two library files listed in the code. A more complex FM radio design based on the TEA5767 can be found at: https://bit.ly/pe-feb22-ks5 This Instructables tutorial provides details of a TEA5767 FM radio based on an Arduino Uno (rather than a Nano): https://bit.ly/pe-feb22-ks6 AZ-Delivery provide a useful eBook Quick Start Guide. Download it for free at: https://bit.ly/pe-feb22-ks7 These two video presentations differ in style and content, but both will provide you with plenty of useful background information, as well as some useful ideas for further development. Teach-In 8 CD-ROM Exploring the Arduino EE FR -ROM CD ELECTRONICS TEACH-IN 8 FREE CD-ROM The Arduino offers a remarkably effective platform for developing a huge variety of projects; from operating a set of Christmas tree lights to remotely controlling a robotic vehicle wirelessly or via the Internet. Teach-In 8 is based around a series of practical projects with plenty of information for customisation. The projects can be combined together in many different ways in order to build more complex systems that can be used to solve a wide variety of home automation and environmental monitoring problems. The series includes topics such as RF technology, wireless networking and remote web access. The CD-ROM also includes a bonus – an extra 12-part series based around the popular PIC microcontroller, explaining how to build PIC-based systems. SOFTWARE FOR THE TEACH-IN 8 SERIES FROM THE PUBLISHERS OF This CD-ROM version of the exciting and popular Teach-In 8 series has been designed for electronics enthusiasts who want to get to grips with the inexpensive, immensely popular Arduino microcontroller, as well as coding enthusiasts who want to explore hardware and interfacing. Teach-In 8 provides a one-stop source of ideas and practical information. 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