where is the source code of takeOverTCA0( ) ?

[6v6gt-duino]

This is referenced at https://github.com/SpenceKonde/megaTinyCore/blob/master/megaavr/extras/TakingOverTCA0.md
It works wonders in restoring TCA0 on an ATtiny1614 to some sort of usable condition. The IDE I'm using at the moment is too primitive to find such things.

EDIT
I used a lightly modified version of this code and assumed from looking at it that it did more or less a fresh start on the timer: https://github.com/MicrochipTech/TB3217_Getting_Started_with_TCA/blob/master/Using_periodic_interrupt_mode/main.c . However the frequency was about 7 times too high.

[6v6gt-duino]

OK. I've found it in .....\ArduinoData\packages\megaTinyCore\hardware\megaavr\2.6.4\cores\megatinycore\wiring_analog.c

void takeOverTCA0() {
  TCA0.SPLIT.CTRLA = 0;                                 // Stop TCA0
  __PeripheralControl &= ~TIMERA0;                        // Mark timer as user controlled
  /* Okay, seriously? The datasheets and io headers disagree here */
  TCA0.SPLIT.CTRLESET = TCA_SPLIT_CMD_RESET_gc | 0x03;  // Reset TCA0
}