Electronic – Why stop bit in UART

Ordinary UARTs have to be pre-configured with the desired baud rate (as well as word length, stop bits, parity, etc) traditionally by a human.

For several decades now though there have been implementations of "auto baud" detection found in some settings, which typically works by timing key features of the waveform to deduce the baud rate. Early versions needed a known character to be transmitted, but more sophisticated versions might be able to find the rate from more arbitrary data.

A receiving UART typically has a local clock that runs at a faster rate - typically 8 or 16 times the baud rate. This is used to sample the incoming signal and detect the bits within a word in a way that can tolerate a few percent of error. Even two crystal oscillators wouldn't match rates perfectly, but the error tolerance can permit use of some less precise sources, sometimes including trimmed on-chip oscillators, etc. It can also help accommodate the fact that dividing down popular oscillator frequencies may only produce an inaccurate approximation to certain baud rates - in the old days, UART master clocks sometimes needed particular frequencies to access popular baud rates, for example 11.0592 MHz on the 8051 family.

Start and stop "frame" bits are important in asynchronous communication as they provide a way to sychronise the receiving bit clock with the stream being sent

if you send 128 bits of data the two clocks must agree in frequency to better than 0.4%. Most crystal oscillators are much better than 4000ppm so it could work. but I've not seen such a scheme ever used.

Also the better the clock agreement the better the noise immunity of the system - towards the end of the packet the signal degades as the sampling point drifts towards the edge of the bits.

However most UARTSs send only 8 bits of data between each pair of frame bits. this requires only 5% speed accuracy making communication practical with a cheap ceramic resonator, or even a good RC oscillator.

If you want to communicate with a PC you will need to use a line discipline that it understands, that typically means no more than 8 data bits between the start and stop bits, so transmitting 128 bits of data will require a total of 160 bits on the wire.