Electronic – At what UART baud rate is hardware flow control required

flow controluart

At low baud rate such as 9600bps, I do not think hardware flow control CTS/RTS is necessary. I believe at higher baud rates, CTS/RTS will be necessary. What is this baud rate? Can one still do without CTS/RTS at 115.2kbps?

Best Answer

It is not data rate that matters; we use CTS / RTS (or XON / XOFF for software flow control) where a possibility of a receiver overflow exists, although admittedly that is more likely at higher data rates.

If a receiver cannot empty its buffers quickly enough, then it should deassert CTS (in response to which, the transmitter will assert RTS if there is more data to transmit).

Note that the transmitter may experience a buffer underrun in which case it should deassert RTS (because there is no valid data to send).

So it comes down to how fast the buffers can be moved - that is more likely to be an issue at higher data rates but it is completely implementation dependent; there is no single speed.