Padu wrote:

>>Olin Lathrop wrote:
>>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.
>>    
>>
>
>
>Let me see if I got this right, if I want to be able to communicate at
>38400/57600, then I'd have to find one crystal that would minimize rounding
>innacuracies. Having in mind that I'll set BRGH, then I came up with the
>following table:
>
>Desired Baud | SPBRG | Perfect Crystal
>      38400 7 4.9152
>      38400 8 5.5296
>      38400 9 6.1440
>      38400 10 6.7584
>      38400 12 7.9872
>      38400 14 9.2160
>      38400 16 10.4448
>      57600 5 5.5296
>      57600 6 6.4512
>      57600 7 7.3728
>      57600 8 8.2944
>      57600 10 10.1376
>      57600 12 11.9808
>
>
>If my calculations were right, then a 5.5296MHz crystal would be perfect for
>both 38.4K and 57.6K baud rates, am I correct?
>
>I went to digikey and amazingly enough, there is a crystal at exactly this
>frequency (CSA309-5.5296MABJ). Should I order it?
>Which one is better, cylinder or smd? Just go digikey's main page and type
>in "5.5296" on the search edit box.
>  
>
I personally don't like crystals at all, but if you have to get them, 
the cylindrical one, mounted with a small
loop on the wires and the body encased in silicone bathtub caulk, will 
probably last forever. SMD crystals
are easily damaged by vibration and shock, don't go that way.

--Bob


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