8.1 UART/USART communication
3 min read•august 7, 2024
and are essential serial communication protocols in embedded systems. They enable devices to talk to each other by converting parallel data to serial form, with UART handling and USART supporting both async and sync modes.
These protocols use start and stop bits to frame data, with optional parity for error checking. The determines transmission speed, while different communication modes like full-duplex allow for flexible data exchange between devices.
UART/USART Fundamentals
Serial Communication Protocols
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- UART (Universal Asynchronous Receiver/Transmitter) enables asynchronous serial communication between devices, converting parallel data to serial data for transmission and vice versa for reception
- USART (Universal Synchronous/Asynchronous Receiver/Transmitter) supports both asynchronous and synchronous serial communication, offering flexibility in communication modes
- Asynchronous communication does not require a shared clock signal between the transmitter and receiver, allowing for variable timing between transmitted characters ()
- requires a common clock signal shared between the transmitter and receiver, ensuring precise timing and higher data transfer rates (SPI, I2C)
- Baud rate represents the number of signal or symbol changes per second in a communication channel, determining the speed of data transmission (9600, 115200)
Data Transmission Characteristics
- signals the beginning of a data frame, alerting the receiver to incoming data and synchronizing the communication
- indicates the end of a data frame, providing a brief pause for the receiver to process the received data and prepare for the next frame
- adds an extra bit for error detection, ensuring data integrity by checking if the number of 1's in the data frame is even or odd
- represent the actual information being transmitted, typically ranging from 5 to 9 bits per frame (8 data bits)
Data Framing
Frame Structure
- Start bit initiates the data frame, usually a single low bit (logic 0) to signal the beginning of the transmission
- Stop bit concludes the data frame, typically one or two high bits (logic 1) to indicate the end of the transmission and provide time for the receiver to process the data
- Parity bit, when used, is an additional bit added to the data frame for error detection purposes, calculated based on the number of 1's in the data bits (even parity, odd parity)
- Data bits carry the actual information being transmitted, with the number of data bits per frame agreed upon by the transmitter and receiver (7 data bits for ASCII characters)
Error Detection and Synchronization
- Start bit synchronizes the transmitter and receiver, ensuring they are ready to send and receive data at the same time
- Stop bit provides a brief pause for the receiver to process the received data and resynchronize for the next frame, preventing data overflow and maintaining communication stability
- Parity bit enables basic error detection by checking if the number of 1's in the data frame matches the expected parity (even or odd), helping identify single-bit errors
- Data bits are framed by the start and stop bits, with the least significant bit (LSB) transmitted first and the most significant bit (MSB) transmitted last
Communication Modes
Duplex Communication
- Full-duplex communication allows simultaneous bidirectional data transmission, where both devices can send and receive data at the same time (telephone conversation)
- Half-duplex communication supports bidirectional data transmission, but only one device can transmit at a time while the other receives, requiring the devices to take turns (walkie-talkie)
Unidirectional Communication
- enables unidirectional data transmission, where one device can only transmit and the other can only receive, without the ability to switch roles (radio broadcast)
Key Terms to Review (16)
Asynchronous Communication: Asynchronous communication is a method of data transmission where the sender and receiver do not need to be synchronized with each other in time. This type of communication allows for data to be sent and received at different times, making it ideal for situations where real-time interaction is not necessary. In the context of serial communication, it enables devices to exchange information without requiring both parties to be ready at the same moment, enhancing flexibility in data exchange.
Baud rate: Baud rate is a measure of the speed of data transmission in a communication channel, representing the number of signal changes or symbols per second. This term is critical in understanding how data flows between devices, particularly in serial communications where accurate timing is essential for reliable data transfer. A higher baud rate means faster communication, but it also requires better signal integrity to avoid errors.
Data bits: Data bits are the smallest unit of data in a computer system, typically representing a binary value of either 0 or 1. In the context of serial communication, data bits are crucial as they define how many bits are used to represent each character of data being transmitted. This directly impacts the accuracy and efficiency of data transmission, especially in protocols that require specific configurations.
Frame error: A frame error occurs when there is a mismatch between the expected and actual structure of a data frame transmitted over a communication channel. In the context of UART/USART communication, frame errors typically arise from issues such as incorrect baud rates, noise interference, or improper framing bits that lead to the receiver being unable to correctly interpret the data being sent.
Full duplex: Full duplex refers to a communication system that allows data to be transmitted in both directions simultaneously. This means that while one device is sending data, the other can receive it at the same time, creating a more efficient and fluid exchange of information. Full duplex communication is crucial for enhancing the performance of various protocols and systems, including those that rely on constant interaction between devices.
Half duplex: Half duplex is a communication mode where data transmission can occur in both directions, but not simultaneously. This means that while one device sends data, the other must wait until the transmission is complete before it can respond. This method is often used in scenarios where full-duplex communication is not necessary or practical, and it helps manage bandwidth effectively in certain communication protocols.
Logic Levels: Logic levels refer to the different states that a digital signal can represent in electronic systems, typically defined as high or low. These states correspond to specific voltage ranges, which determine whether a device recognizes a signal as a binary '1' (high) or a binary '0' (low). Understanding logic levels is crucial for ensuring proper communication and operation in embedded systems, especially in serial communication protocols like UART/USART.
Overrun Error: An overrun error occurs in communication systems when data is received faster than it can be processed, leading to the loss of incoming data. This is particularly relevant in UART/USART communication, where the buffer that temporarily holds incoming data can become full. When the hardware or software fails to read this data quickly enough, the system cannot store the excess incoming bytes, resulting in an overrun error.
Parity bit: A parity bit is an extra bit added to a binary data set to help detect errors during data transmission. It works by ensuring that the total number of 1-bits in the set is even (even parity) or odd (odd parity), which can indicate if an error has occurred. This simple error-checking mechanism is especially important in UART/USART communication where reliable data transfer is crucial.
RS-232: RS-232 is a standard for serial communication transmission of data, commonly used in computer and telecommunications equipment. It defines the electrical characteristics and timing of signals, as well as the physical size and pinout of connectors, facilitating communication between devices like modems, printers, and computers. RS-232 is crucial in enabling UART/USART communication by providing a reliable way to transfer data over short distances.
Simplex communication: Simplex communication is a type of data transmission where information flows in only one direction, from the sender to the receiver, without any return path for feedback. This method is commonly used in applications where the sender does not require a response from the receiver, simplifying the communication process. Examples of simplex communication can be found in various systems, including telemetry and broadcast communications.
Start bit: A start bit is a specific signal in serial communication that indicates the beginning of a data frame. It is used to alert the receiving device that data transmission is about to begin, helping to synchronize the sender and receiver. This bit ensures that the receiving system is ready to interpret the incoming bits correctly, marking the transition from idle to active communication.
Stop bit: A stop bit is a signal used in serial communication, specifically in UART and USART protocols, that indicates the end of a data packet. It serves as a timing mechanism to ensure that the receiving device knows when the data transmission has finished, allowing it to process the incoming data correctly. The presence of stop bits helps maintain synchronization between sender and receiver, and it can also accommodate varying baud rates.
Synchronous communication: Synchronous communication is a method of data transfer where both the sender and receiver operate at the same time, coordinating their actions based on a shared clock signal. This technique ensures that data is sent and received in real-time, allowing for immediate acknowledgment and feedback between devices. It's especially important in systems requiring timely interactions, such as in UART/USART communication protocols.
UART: UART, or Universal Asynchronous Receiver-Transmitter, is a hardware communication protocol that allows for asynchronous serial communication between devices. It converts parallel data from a microcontroller into serial data for transmission and vice versa, making it crucial for many embedded systems applications such as data transfer between microcontrollers and sensors or other peripherals.
USART: USART, or Universal Synchronous/Asynchronous Receiver-Transmitter, is a hardware communication protocol that allows for both synchronous and asynchronous serial communication between devices. This flexibility enables USART to be used in various applications, whether for simple point-to-point communication or more complex systems requiring data exchange at higher speeds. It plays a critical role in enabling microcontrollers and other digital devices to communicate with peripherals and other systems effectively.