AVR Freaks

This is the register definition from the manual:

So I will add some comments to the code:

/* Disable digital input buffer */
 PORTD.PIN6CTRL &= ~PORT_ISC_gm;                /* Clear ISC field */
 PORTD.PIN6CTRL |= PORT_ISC_INPUT_DISABLE_gc;   /* Set new ISC value */
 /* Disable pull-up resistor */
 PORTD.PIN6CTRL &= ~PORT_PULLUPEN_bm;           /* Clear pullup bit */

Of course this code is pretty horrible, probably automatically generated. All this just to preserve the INVEN bit, which is not relevant here, anyway.

This code could be optimized in a number of ways. At the very least:

/* Disable digital input buffer and pull-up resistor */
 PORTD.PIN6CTRL &= ~(PORT_ISC_gm | PORT_PULLUPEN_bm);       /* Clear ISC field and pullup bit at the same time */
 PORTD.PIN6CTRL |= PORT_ISC_INPUT_DISABLE_gc;               /* Set new ISC value */

But since the compiler is prevented from optimizing reads and writes on volatile variables like PORTD.PIN6CTRL, this code will actually generate a few less instructions if you put it in a single line. A bit ugly, though:

/* Disable digital input buffer and pull-up resistor */
  PORTD.PIN6CTRL = ( PORTD.PIN6CTRL & ~(PORT_ISC_gm | PORT_PULLUPEN_bm) ) | PORT_ISC_INPUT_DISABLE_gc;               

Finally, since INVEN is irrelevant for a pin used for analog data, you might as well just set it to zero, along with the pullup enable bit:

/* Disable digital input buffer, pull-up resistor and digital inverter */
  PORTD.PIN6CTRL = PORT_ISC_INPUT_DISABLE_gc;       /* Set ISC field, all other bits are set to zero */