I've tried a PCF8593 with different voltages and can't see
any variations in frequency. The OscO pin must be very hi-Z
or sensitive to C loading, I had to buffer it with a CMOS gate
to use the scope and FM. Could it possibly be the C you've
got from OscI to Vdd ? As you vary Vdd this is seen across
that cap. With only 3 components, and almost certainly being
able to rule out the IC, doesn't leave anything else to look at
except the crystal and the cap

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