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31.25 kHz = 24 MHz / 3 / 256 = 24 MHz / 768
the clock source must be selected as XTAL or PLL, and divider must be set exactly to 768.
Make sure that the XTAL is indeed operates at 24.000 +/- 0.003 MHz. Most likely it is not (needs capacitors tuning), so there is no point of using 20ppm XTAL, 100ppm will do.