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Frequently asked questions

General UART FAQ

Q How does the baud rate error compare when 5-bit or 8-bit data is used during UART transmission?
A The percentage error with 5-bit data, no parity, and 1 stop bit is 6.73%. With 8-bit data, no parity, and 1 stop bit, the percentage error is 4.6%.
Q What is the shortest and longest character length in UART transmission?
A The shortest character length is 7 bits (1 start bit + 5 data bits + 1 stop bit) and the longest is 11 bits (1 start bit + 8 data bits + 1 parity + 1 stop bit).
Q What order does the data get shifted out of the transmit buffer?
A The data is shifted out LSB first and MSB last.
Q What are the different types of handshaking?
A Hardware handshaking and software handshaking.
Q Why is handshaking needed?
A To prevent overrun at the receiver which occurs when the receiver is unable to process the receiving data fast enough. Handshaking is used to halt the data flow until the receiver is ready.
Q What is hardware handshaking?
A Using dedicated wires (RTS and CTS) to control the data flow.
  • The RTS (Request To Send) is an output signal from the receiver to indicate when the receiver is ready to receive data from the transmitter.
  • The CTS (Clear To Send) is an input signal to the transmitter to indicate the transmitter is clear to transmit data to the receiver.
Q What is software handshaking?
A Using characters (XON and XOFF) inserted into the data stream to control the data flow.
  • The receiver sends an XON character (0x11) to start the transmission.
  • The receiver sends an XOFF character (0x13) to stop the transmission.
Q How do you connect the RTS and CTS wires?
A The RTS output of the receiver is usually connected to the CTS input of the transmitter. If the CTS input is high, the transmitter stops sending data.
Q How do you write a UART program?
A There are two methods:
  • Polling: check the status of the UART regularly, for example, to determine if the UART is ready to receive the data.
  • The advantage of polling is the implementation is simple.
  • The disadvantage is continued polling even when the UART is not ready.
  • Interrupt: generate an interrupt, for example, when the UART is ready to receive the data.
  • The advantage is generating an interrupt only when the UART is ready.
  • The disadvantage is the interrupt implementation is complex.
Q What is the maximum transmission distance for a UART?
A Normally, with a good quality cable, the UART can run up to 100Kbps at 5 meters.
Q What are the clock requirements for a UART?
A The requirements depend on the baud rates to be used. The higher the baud rates, the more accurate the clock must be. 100ppm for the clock is recommended.
Q What is the meaning of Xon/Xoff?
A Xon/Xoff is a predefined handshake between two devices using special characters. The Xon character is sent by the receiver to tell the transmitter to send more data. The Xoff character is also sent by the receiver to tell the transmitter to stop sending data.
Q How do I prevent data overflow in the FIFO buffer?
A Use hardware or software flow control. These methods will signal the sender to stop sending data until the receive FIFO is emptied by the processor. Once more space is available in the receive FIFO, the receiving UART will signal the sender to send more data.
Q Do deeper FIFOs offer better performance?
A Yes, deeper FIFOs increase the serial communication throughput and reduce CPU overhead thereby improving system performance.
Q How does a UART work with a processor?
A The UART converts the parallel data sent by the processor to serial data and then sends the serial data through the serial communication channel. At the receiving end of the serial communication channel, the UART converts the serial data back to parallel data and stores the data in the FIFO. Finally, the UART interrupts the processor to inform it that data is ready to be read.
Q What are the major applications for UARTs?
A Industrial controls, point-of-sale terminals, wireless applications, PDAs, and ethernet routers are the primary ones.
Q Are there any application notes available for your UARTs?
A Yes, we have several application notes for different devices available in our Interface Application Notes.
Q Where can we find your UART datasheets?
A
Datasheets can be found on our ICs website in many ways.
There is a search box on every page. Using it, you can search for a device by name (such as 16c550). You can also search for a functional category (such as uart) or a product family (such as 16c).
You can also browse through our functional categories and product families by starting with our interface products page. In all product listings, you can obtain the datasheet by clicking on the PDF icon (). You can also show or hide all related information by clicking on the zoom icons ( ).
Q Is there an email address to where we can send a technical question?
A Yes. You can contact us by using the Interface Product and Technical Support email link () easily found on the Contact Us webpage.

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