Ring oscillator with ATSAMD51J19

This ring oscillator runs on the ATSAMD51J19 MCU using the Adafruit Feather dev board (though dev board price comes from Adafruit ItsyBitsy M4 Express). The ATSAMD51J19 has an ARM Cortex-M4 running with a max speed of 120 MHz. Code for the oscillator is available here, or visible below.

#define portin PORT->Group[PORTA].IN.reg
#define outclr PORT->Group[PORTA].OUTCLR.reg
#define outset PORT->Group[PORTA].OUTSET.reg
#define out PORT->Group[PORTA].OUT.reg
#define outtgl PORT->Group[PORTA].OUTTGL.reg

const uint8_t in_pin = 21;
const uint32_t in_mask = 1ul<<in_pin;
const uint8_t out_pin = 22;
const uint32_t out_mask = 1ul<<out_pin;

void setup() {
  // Set pin mode according to chapter '32.6.3 I/O Pin Configuration'
  // enable input, to support reading back values, with pullups disabled
  PORT->Group[PORTA].PINCFG[out_pin].reg = (uint8_t)(PORT_PINCFG_INEN) ;
  // Set pin to output mode  
  PORT->Group[PORTA].DIRSET.reg = out_mask;
  // Set in pin to input mode
  PORT->Group[PORTA].PINCFG[in_pin].reg=(uint8_t)(PORT_PINCFG_INEN) ;
  PORT->Group[PORTA].DIRCLR.reg = in_pin ;
  //make sure port synchronizer always active, else we'll use extra clock cycles.
  //I'm not sure I'm activating the synchronizer correctly, because I don't see a change when I add this... 
  PORT->Group[PORTA].CTRL.reg = in_mask; 

    //ternary operator: assymetric 300ns (200-100)
    //portin & in_mask ? outclr = out_mask : outset = out_mask;
    //portin & in_mask ? outclr = out_mask : out = out_mask;
    //full port inversion: symmetric 300ns
    out = ~(portin);

    //bitwise xor: 300ns
    //out = ((portin ^ in_mask) << 1);

    //if else, 300ns
    /*if (portin & in_mask){
      outclr = out_mask;
    } else {
      outset = out_mask;
void loop() {}

By default the port consumes extra clock cycles to save power and only sample the input pin when a read is requested (see description below). We can set the input synchronizer to always be active to avoid this delay. I've tried to do this above, but I need to confirm if this was successful.