which subset of the following UART clock freqencies is supported by the esp8266 (nodeMCU esp-12E, 80MHz), set by Serial.begin(x)?
230400, 460800, 500000, 576000, 921600, 1000000, 1152000, 1500000, 2000000, 2500000, 3000000, 3500000, 4000000
I must say that I am not a technician.
I looked in datasheets like
http://www.electrodragon.com/w/index.php?title=Category:ESP8266&redirect=no
but there nothing is written about the supported clock rates (I read and searched closely, but found nothing).
So which of the above mentioned clock freqencies is supported by the nodeMCU esp-12E, 80MHz, by Serial.begin(x), understandable for non-technicians?
Isn't it great to have a datasheet for a complex chip like the 8266 that's less than 40 pages long, instead of the 900+ pages for a SAMD21? Perhaps not 
I couldn't find anything that told me much about the uart bitrates.
The datasheet says:
The data transmission speed via UART interfaces reaches 115200 x 40 (~4.5 Mbps).
I can't figure out how they get a limit of 115200*40 from the nominal max clock of 160MHz.
The arduino core code for ESP8266 says:
USD(uart->uart_nr) = (ESP8266_CLOCK / uart->baud_rate);
I'm not sure whether that CLOCK is the 80MHz/160MHz number, or the crystal frequency (which is lower.)
I would THINK that with "standard gate-level uart definitions", which is almost certainly what the esp8266 uses, you'd be good at any bit rate where ESP8266_CLOCK/16/baud_rate has a fractional part that is small compared to the total value. But you may have to experiment to make sure. Experimentation may be necessary.
Historically, this is a major problem with chips from overseas (including Japan, so not just "developing countries".) Published documentation (especially in English) is not what we're used to - there's much more "word of mouth" communications between chip producers and their (large, corporate) customers. Bunnie Huang has written pretty extensively on some of the cultural differences, I think This Article is one of the definitive ones.
(The ESP8266 and ESP32 have an unprecedented level of information sharing. It's just ... not that much.)
westfw:
I would THINK that with "standard gate-level uart definitions", which is almost certainly what the esp8266 uses, you'd be good at any bit rate where ESP8266_CLOCK/16/baud_rate has a fractional part that is small compared to the total value.
thank you, so IIUC for 80 MHz the constant is 5,000,000 and then to be divided by the target baud rate.
Then just 3 can be expected to (probably by theory) work fine, correct?
5000000/230400 -,
5000000/460800 -,
5000000/500000 OK,
5000000/576000 -,
5000000/921600 -,
5000000/1000000 OK,
5000000/1152000 -,
5000000/1500000 -,
5000000/2000000 -,
5000000/2500000 OK,
5000000/3000000 -,
5000000/3500000 -,
5000000/4000000 -