My RTX doesn't use external quartz resonator. And I don't found anything about it in documentation. Is it possible to use external resonator with RTX ?
"Is it possible to use external resonator with RTX ?"
The manual for RTX can't document all ways you can use the processor - just as you can't expect generic LEGO pieces to be shipped with a complete manual of all ways the pieces can be combined.
RTX needs to see time ticks. But exactly how the processor receives the oscillations isn't really important to RTX. What is important is where it should look for ticks, and how often they happen.
But this means that you can play along alot with processor settings for oscillator selections, used crystals, PLL settings etc without it really matter much to RTX. With creativity it is even possible to have one high-speed oscillator for normal use, and dropping down to another oscillator for low-power operation. In the end, it's you who are the engineer. You make the decisions and make sure that all required assumptions are fulfilled.