maximum baud rate with internal RC oscillator

[kender]

Quick question. I’m running an 18LF4320 from an internal RC oscillator. I’ve never used internal RC before. From what I know, RC is less accurate than Crystal. What’s the highest reliable baud rate that can be used for serial communication with PC (true RS-232 with a MAX3221 driver)

When PIC is transmitting 9600 baud, the PC receiving about 30% garbage. Either 9600 is too fast, or I’m doing something wrong. I’m going to scan through the standard baud rates until I find something acceptable, but I’d like to tap into the tribal knowledge too.

The PIC will be working well within the consumer range of temperatures.
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[SherpaDoug]

It is not just a matter of speed. It is a matter of timing uncertainty. If your clock is 5% off then by the end of a Start bit, 8 data bits, and a Stop bit you are half a bit off regardless of what baud rate you are using.
If this is a home project you can try fudging the baud rate. If 9600 almost works then try 9600 + 2% = 9792 baud or 9600 - 2% = 9408 baud and see which works better. Beware this may change with voltage, temperature, or a different chip.
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The search for better is endless. Instead simply find very good and get the job done.

[kender]

The error in baud rate is only 0.6%. Dithering the baud rate by +/- 2% didn't have any positive effect. There should be something else wrong. For example, the XMIT pin stays high, when the data is not being transmitted. I think it should stay low. The output isn't inverted:

[PCM programmer]

Why is bits set = 9 ?

[grasspuddle]

doesn't MAX 232 chips set tx/rx high when not in use?

[kender]