Padu wrote:
> Do you think it possible to achieve baud rates higher than 38400 with a
> 4MHz crystal?

You can answer this easily yourself.  The 16x baud clock is created by
dividing the oscillator frequency by an integer.  The baud rate resolution
is therefore very poor for low oscillator frequencies and high baud rates.

For example, with 4MHz oscillator here are the fastest few baud rates:

  OSC  |  Div  |  Baud
  --------------------
  4MHz      1     250K
  4MHz      2     125K
  4MHz      3      83.3K
  4MHz      4      62.5K
  4MHz      5      50.0K
  4MHz      6      41.7K
  4MHz      7      35.7K
  4MHz      8      31.3K
  4MHz      9      27.8K
  4MHz     10      25.0K

To get 38400 baud, you have a choice of 41.7Kbaud (8.6% too high), or
35.7Kbaud (7.0% too low).  Both those errors are too high.  You therefore
can't do proper 38400 baud with a 4MHz oscillator.


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