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The internal UFM oscillator block is NOT programmable in frequency. The spec is that it may be anywhere between 3.9 and 5.3 MHz on a device, so you get what you get, and have to accept that variability. The Altera/Intel MaxV device datasheet has all the details about the internal oscillator specs and usage.
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So, if I understand it correctly I can not guess/calculate the accurate frequency of the internal clock? I have read that I can calculate the relative frequency the CPLD will run on and take that as an accurate constant clock. Can that be true?
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'Guess/calculate the
accurate frequency of the internal clock?' ... You can assume, as I said before, the internal clock will be between 3.9 and 5.3MHz. Or as 4.6MHz +/- 0.7MHz. So I suppose you could 'guess' 4.6MHz, but it has a +/- 700KHz uncertainty.
'I have read that I can calculate the relative frequency the CPLD will run on and take that as an accurate constant clock. ' ... I am not sure what you mean by 'relative frequency'? Relative to what? As far as 'accuracy' goes, read my previous answer. Accuracy is +/- 15%. If you want 'normal' accuracy, like of a crystal oscillator, it would be measured in parts per million. Something like 20ppm to 100ppm would be typical.
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