Transmitting a byte over UART in Embedded C - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is UART in embedded systems?

UART stands for Universal Asynchronous Receiver/Transmitter. It is a hardware communication protocol used to send and receive data one byte at a time asynchronously.

beginner

Which register is commonly checked before sending a byte over UART?

The Transmit Data Register Empty (TXE) flag or equivalent is checked to ensure the UART is ready to send a new byte.

intermediate

Why do we wait for the TXE flag before writing data to the UART data register?

Waiting for the TXE flag ensures the previous byte has been moved from the data register to the shift register, so the data register is free to accept a new byte without overwriting.

beginner

Show a simple C code snippet to transmit a byte over UART assuming a register UART_DR and a flag UART_SR_TXE.

while (!(UART_SR & UART_SR_TXE)) { } // Wait until TXE is set UART_DR = byte_to_send; // Write byte to data register

beginner

What does asynchronous mean in UART communication?

Asynchronous means data is sent without a shared clock signal. Instead, start and stop bits frame each byte so the receiver knows when data begins and ends.

What must you check before writing a byte to the UART data register?

A If the UART is powered off

B If the receive buffer is full

C If the transmit buffer is empty (TXE flag)

D If the baud rate is set

You check the TXE flag to ensure the transmit buffer is empty and ready to accept a new byte.

What does UART stand for?

A Universal Asynchronous Receiver/Transmitter

B Universal Automatic Register Transfer

C Unified Asynchronous Register Timer

D Universal Analog Receiver/Transmitter

UART stands for Universal Asynchronous Receiver/Transmitter.

In UART, what frames each byte of data?

A Start and stop bits

B Parity bits only

C Clock signal

D Data checksum

Start and stop bits frame each byte to mark the beginning and end in asynchronous communication.

What happens if you write to the UART data register before TXE is set?

A Data is sent twice

B Data may be lost or overwritten

C UART resets automatically

D Nothing happens

Writing before TXE is set can overwrite data that has not yet been transmitted, causing data loss.

Which of these is NOT part of UART transmission?

A Stop bit

B Start bit

C Data bits

D Synchronous clock signal

UART is asynchronous and does not use a synchronous clock signal.

Explain the steps to transmit a single byte over UART in embedded C.

Think about how to know when UART is ready and how to send the byte.

Describe why UART communication is called asynchronous and how it affects byte transmission.

Focus on timing and framing of data.