In SX Microcontrollers, SX/B Compiler and SX-Key Tool, amiller wrote:

This is a classic symptom of accumulating bit rate errors between your two devices.  

Most likely the ISR rate and Oscillator speed are off just enough from the exact serial bitrate that, without breaks between characters to reset bit timing, the two  bit rates get farther and farther out of sync. 

At some point, you miss a bit, or you read a bit twice.

Try having the sender put a pause in the data stream, at least at the end of each text line.  The inter-character gap resets the start of character timing, and clears the  accumulating bit-rate timing errors.

The other alternative is to use oscillators that are higher tollerance, or have a frequency that is an exact binary multiple of the desired bitrate.  

To exactly match bitrates, is one reason you see older crystal oscillators with oddly fractional decimal frequencies. 4.096MHz etc.

-AGM-

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