Electronic – One or two UART stop bits

uart

UARTs often let you choose between 1, 1.5 and 2 stop bits. With 1 stop bit payload efficiency is 80% (8/10), with 2 stop bits that drops to 72.7% (8/11). So what's the advantage of the second stop bit?

Best Answer

Extra stop bits can be a useful way to add a little extra receive processing time, especially at high baud rates and/or using soft UART, where time is required to process the received byte.

Where speed is tight, and your UART only offers division ratios in powers of 2, adding an extra stopbit can be an option to give a less drastic reduction in speed than the next lowest baudrate.

I believe this is may one reason the DMX512 standard specifies 2 stopbits.

Another situation where they can be useful is if you have devices forwarding a data stream without any buffering or packetisation - small differences in clock rates between nodes and finite sampling granularity can cause errors to occur as data is received and retransmitted by a number of nodes in a chain, but if the data is sent with 2 stopbits and the receivers are set to one stopbit, it adds enough margin to accommodate these errors and leave at least one valid stopbit period for nodes far down the chain to receive reliably.

I have also enountered a situation where a very long cable run caused some asymmetry in the rise and fall times, resulting in inadequate stopbit length - sending 2 stopbits and having the receiver only require one fixed this.