Using PWM output from ATMega to provide external clock to external 3.3V device
If you follow your TDC1000-TDC7200EVM link, it takes you to the User's Guide. p35 shows the schematic for the EXT_OSC input. You'll see that it has a 51.1ohm load resistor across EXT_OSC input and its ground.

So your 5V-to-3V3 circuit has to be able to drive a 51.1ohm load. This rules out a potential divider. The ATmega chip can't source that much current from an I/O pin into such a divider.

Look at using an active circuit to do the job, such as a BC107 transistor or your nearest-to-hand equivalent NPN transistor. If you connect the collector to 3V3, the base to your ATmega output via a 1K series resistor and the emitter to EXT_OSC, that would do the trick. (Apologies for no circuit shown.)