That's a good observation about frequencies which are close to powers
of 2. That does push things a bit further but it is still often
doable, especially for the slower bit rates where the divisor has
enough resolution to compensate for the awkward clock frequency (i.e.,
incrementing the divisor makes less percent change in the baud rate).

I think that the IntRC oscillator has a lot of advantages, though.
Here are just a few ways it has come in handy for me:

Cheaper than a crystal
Allows the PIC to be much more RF silent without much effort (both due
to the fact that RF is not present on any output pins, assuming good
power supply bypassing, and also because the output spectrum of the RC
oscillator is wider and noisier so the power density at any one
frequency is less)
Clock frequency can be changed in software, both as a revision change
and even dynamically for power consumption
Much more shock resistant than a crystal
Faster wake-up from sleep than a crystal

I have used it several times for serial comms without any problems,
including one product which has a ZigBee module communicating with a
PIC16LF88 at 38400bps. In about 1000 units, there has never been a
serial comms problem that I am aware of.

Sean

On Sun, Jan 4, 2009 at 8:31 PM, Jinx <joecolquitt@clear.net.nz> wrote:
>
>> The internal RC oscillator block in the more recent 16F parts as
>> well as the 18F parts is stable enough for most async serial comms
>
> Sean, I stand corrected
>
> However, putting my big weasel hat on, IntRC frequencies generally
> don't match what would be best for standard baud rates
>
> IOW, less fuss would be eg 1.024, 2.048, 4.096 and 8.192MHz,
> whereas IntRC are 1,2,4 8MHz, or 2.4% low before factoring in the
> analogue variations
>
> Microchip still use ridiculous "decimal crystal" tables in datasheets for
> BRG calculations. "hex" value crystals would show mostly 0% error
> for common baud rates
>
> eg "decimal" 20MHz vs "hex" 19.6608MHz or 20.48MHz
>
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