10 interview questions and answers on UART SPI I2C

UART, which stands for Universal Asynchronous Receiver-Transmitter, is a common serial communication protocol used for transmitting and receiving data between devices. UART is asynchronous, which means that data is sent without a shared clock signal between the sender (transmitter) and receiver. Instead, both devices must agree on a common baud rate (data transfer rate) to ensure data is sent and received at the same speed. UART data is typically sent in 8-bit bytes, but other formats are possible. Each byte includes a start bit, data bits (usually 8), an optional parity bit (for error-checking), and one or more stop bits. The start and stop bits help synchronize communication between devices. UART communication is full-duplex, meaning it allows for simultaneous transmission and reception of data between devices. Each device has its own transmitter and receiver, enabling two-way communication.

SPI, or Serial Peripheral Interface, is a synchronous serial communication protocol commonly used for connecting digital devices in embedded systems. SPI typically uses a master-slave architecture, where a master device initiates communication with one or more slave devices. The master generates a clock signal (SCK) and controls data transfer. SPI can support multiple slave devices connected to a single bus. Each slave device has a unique chip select (CS) or slave select (SS) line, allowing the master to select the specific slave it wants to communicate with.

I2C, or Inter-Integrated Circuit, is a multi-master, multi-slave, synchronous serial communication protocol used for connecting various digital devices within embedded systems. I2C uses two wires for communication: a serial data line (SDA) and a serial clock line (SCL). SDA carries data, while SCL provides the clock signal to synchronize data transfer.

These protocols viz. UART, SPI and I2C have their own characteristics and are suitable for different communication requirements in embedded systems. The choice of protocol depends on factors like data transfer speed, number of devices and specific application needs.

Following is the list of questions and answers on UART, SPI and I2C interfaces used in embedded domain. This questionnaire will help one pass the job interview for various embedded firmware positions.

UART questions and answers

Question - 1 : What is UART, and what is its primary function in communication ?
Answer - 1 : UART stands for Universal Asynchronous Receiver-Transmitter. It is a hardware component used to transmit and receive serial data between a microcontroller or computer and external devices. UART enables asynchronous communication between devices.

Question - 2 : What is the difference between UART and USART ?
Answer - 2 : UART (Universal Asynchronous Receiver-Transmitter) supports only asynchronous communication, whereas USART (Universal Synchronous Asynchronous Receiver-Transmitter) supports both asynchronous and synchronous communication. USART has an additional clock signal to synchronize data transfer.

Question - 3 : Explain the term "baud rate" in UART communication.
Answer - 3 : Baud rate refers to the number of bits transmitted per second in UART communication. It defines the speed at which data is sent and received between devices and is usually measured in bits per second (bps).

Question - 4 : What are the key components of UART communication, and how do they work together ?
Answer - 4 : UART communication involves a transmitter and a receiver. The transmitter converts parallel data into serial format and sends it serially. The receiver takes incoming serial data, converts it back to parallel data, and provides it to the receiving device.

Question - 5 : What is the significance of start and stop bits in UART communication ?
Answer - 5 : Start and stop bits define the boundaries of a UART data frame. The start bit indicates the beginning of a data byte, while the stop bit(s) indicate the end. They provide synchronization between the transmitter and receiver.

Question - 6 : Explain the concept of data framing in UART communication.
Answer - 6 : Data framing involves the structure of a UART data frame, including the start bit, data bits, optional parity bit, and stop bit(s). Common configurations include 8N1 (8 data bits, no parity, 1 stop bit) and 7E2 (7 data bits, even parity, 2 stop bits).

Question - 7 : What is the purpose of a UART buffer, and how does it help in data communication ?
Answer - 7 : A UART buffer is a temporary storage area used to hold incoming or outgoing data. It allows for efficient data transfer between devices by providing a buffer for data reception or transmission when the CPU is not ready to process it immediately.

Question - 8 : How does UART handle flow control, and what are the common flow control methods ?
Answer - 8 : UART flow control prevents data overrun by controlling data flow between devices. Common methods include hardware flow control (using RTS/CTS pins) and software flow control (using XON/XOFF characters).

Question - 9 : Can UART communication support multiple devices on a single communication line ?
Answer - 9 : UART communication is typically point-to-point, meaning it supports communication between two devices. To communicate with multiple devices, UART can be used in a master-slave configuration or with a multiplexer/demultiplexer.

Question - 10 : What are the advantages and limitations of UART communication in modern embedded systems ?
Answer - 10 : Advantages include simplicity, low hardware requirements, and ease of implementation. Limitations include slower data rates compared to other communication protocols like SPI and I2C and the lack of built-in acknowledgment or error checking mechanisms.

SPI questions and answers

Question - 1 : What is SPI, and what are its primary applications in embedded systems ?
Answer - 1 : SPI, which stands for Serial Peripheral Interface, is a synchronous serial communication protocol commonly used to exchange data between microcontrollers and peripheral devices like sensors, displays, and memory chips. It is used for high-speed, full-duplex communication.

Question - 2 : Explain the basic working principle of SPI communication.
Answer - 2 : SPI communication involves a master device and one or more slave devices. The master generates a clock signal (SCK) and selects a slave device by activating its corresponding chip select (CS) signal. Data is exchanged in a full-duplex manner, with one byte transmitted from master to slave and one byte from slave to master simultaneously.

Question - 3 : What are the essential signals in an SPI bus, and what are their roles ?
Answer - 3 : The key signals in an SPI bus are:
SCK (Serial Clock): The clock signal generated by the master to synchronize data transfer.
MOSI (Master Out Slave In): The data line from the master to the slave.
MISO (Master In Slave Out): The data line from the slave to the master.
CS (Chip Select): A signal used to select a specific slave device for communication.

Question - 4 : What is the role of the SPI data frame format, including data bits, order and clock polarity/clock phase ?
Answer - 4 : The SPI data frame format specifies the number of data bits per transfer, the bit order (MSB or LSB first), and the clock polarity and phase. clock polarity and phase determine when data is sampled relative to the clock signal.

Question - 5 : Can multiple slave devices share the same SPI bus? How is communication addressed in this scenario ?
Answer - 5 : Yes, multiple slave devices can share the same SPI bus. To address a specific slave, the master activates the corresponding chip select (CS) line while keeping others inactive.

Question - 6 : What is daisy chaining in SPI communication, and why is it used ?
Answer - 6 : Daisy chaining involves connecting multiple SPI devices in series, with one device passing data to the next. It reduces the number of chip select lines needed, simplifying hardware connections.

Question - 7 : Explain the concept of SPI bus speed or clock frequency. How is it configured in SPI communication ?
Answer - 7 : SPI bus speed refers to the clock frequency at which data is transferred between devices. It can be configured by adjusting the clock frequency divider (prescaler) settings in the SPI peripheral or microcontroller configuration.

Question - 8 : What is the advantage of using SPI over other communication protocols like I2C or UART ?
Answer - 8 : SPI offers higher data transfer rates and is suitable for full-duplex communication. It also has dedicated chip select lines for individual devices, allowing for faster and more deterministic communication.

Question - 9 : How does SPI handle data framing and error detection ?
Answer - 9 : SPI does not include error detection or correction mechanisms by default. Data framing and error checking are usually handled at a higher level in the application software if required.

Question - 10: Can SPI devices operate at different clock speeds on the same bus ?
Answer - 10: Yes, SPI devices on the same bus can operate at different clock speeds. However, the master device must configure the clock speed for each slave individually when communicating with them.

I2C questions and answers

Question - 1 : What is I2C, and what are its primary applications in embedded systems ?
Answer - 1 : I2C, which stands for Inter-Integrated Circuit, is a multi-master, multi-slave, synchronous serial communication protocol used for connecting various digital devices within embedded systems. It is commonly used for sensor interfacing, EEPROM communication, and low-speed peripheral control.

Question - 2 : Describe the basic working principle of I2C communication.
Answer - 2 : I2C communication involves a master device and one or more slave devices. The master generates a clock signal (SCL) and initiates communication by addressing a specific slave device. Data is exchanged serially on the data line (SDA) with start and stop conditions marking the beginning and end of each communication cycle.

Question - 3 : What are the essential signals in an I2C bus, and what are their roles ?
Answer - 3 : The key signals in an I2C bus are:
SCL (Serial Clock): The clock signal generated by the master to synchronize data transfer.
SDA (Serial Data): The bidirectional data line used to transmit and receive data.
Start and Stop Conditions: Signals that indicate the beginning and end of a data transfer.

Question - 4 : Explain the concept of I2C addressing. How do devices on the bus know when they are being addressed ?
Answer - 4 : I2C devices have 7-bit or 10-bit addresses that uniquely identify them on the bus. When the master initiates communication, it sends the address of the target slave device along with the read/write bit. Devices compare this address with their own to determine if they are being addressed.

Question - 5 : Can multiple slave devices share the same I2C bus? How are collisions and addressing conflicts resolved ?
Answer - 5 : Yes, multiple slave devices can share the same I2C bus. Collisions and addressing conflicts are avoided through unique device addresses. The arbitration process resolves conflicts if two masters attempt to communicate simultaneously.

Question - 6 : What is clock stretching in I2C communication, and why is it important ?
Answer - 6 : Clock stretching occurs when a slave device temporarily holds the clock line (SCL) low to slow down data transfer, allowing it to process data at its own pace. This feature ensures that slower devices can participate in the communication without causing errors.

Question - 7 : What is the I2C bus speed or clock frequency, and how is it configured in I2C communication ?
Answer - 7 : I2C bus speed is the clock frequency at which data is transferred between devices. It is configured by setting the clock divider or prescaler in the I2C hardware or microcontroller settings.

Question - 8 : Can I2C operate with open-drain or open-collector outputs? Explain their significance.
Answer - 8 : Yes, I2C devices typically use open-drain or open-collector outputs for SDA and SCL signals. This allows multiple devices to share the same bus and pull the lines low when needed, with the help of pull-up resistors to restore them to a high state.

Question - 9 : What are repeated start conditions in I2C communication, and why are they used ?
Answer - 9 : Repeated start conditions allow the master to continue communication with a slave without releasing the bus. This is useful when reading sequential data from a device, as it eliminates the need to send a stop condition and then a start condition between each data transfer.

Question - 10 : How does I2C handle data framing and error detection ?
Answer - 10 : I2C does not include error detection or correction mechanisms by default. Data framing and error checking are typically handled at a higher level in the application software if required.

These questions and answers provide a deeper understanding of UART, SPI and I2C communication protocols and their various aspects. Also refer difference between UART vs SPI vs I2C for questions on differences between them.

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