Want to create a capacitance proximity/touch sensor with a RP2040 Pico board using PIO

There is nothing quite like solving a problem to help develop new skills. In this case I have found a library solution to what I want to achieve but it is not RP2040 compatible and of course it does not use PIO, which is the skill I am trying to acquire.

Having seen other members deliver some great PIO related projects, I thought this would be a great opportunity to build a solution together while helping me, and hopefully others, grasp how to develop PIO code from scratch.

Basically I want to create a capacitance touch or proximity sensor using a Raspberry Pi Pico (RP2040) board.

There is an Arduino library that can do this: https://github.com/PaulStoffregen/CapacitiveSensor

But it is not RP2040 compatible: https://github.com/PaulStoffregen/CapacitiveSensor/issues/42

Having reviewed this library, it appears on face value to be well suited to use PIO. Only, I don't know how to start.

As far as I can make out, there is only one function to apply PIO... but maybe others see things differently. I'm keen to learn more.

// Private Methods /////////////////////////////////////////////////////////////
// Functions only available to other functions in this library

int CapacitiveSensor::SenseOneCycle(void)
{
    noInterrupts();
	DIRECT_WRITE_LOW(sReg, sBit);	// sendPin Register low
	DIRECT_MODE_INPUT(rReg, rBit);	// receivePin to input (pullups are off)
	DIRECT_MODE_OUTPUT(rReg, rBit); // receivePin to OUTPUT
	DIRECT_WRITE_LOW(rReg, rBit);	// pin is now LOW AND OUTPUT
	delayMicroseconds(10);
	DIRECT_MODE_INPUT(rReg, rBit);	// receivePin to input (pullups are off)
	DIRECT_WRITE_HIGH(sReg, sBit);	// sendPin High
    interrupts();

	while ( !DIRECT_READ(rReg, rBit) && (total < CS_Timeout_Millis) ) {  // while receive pin is LOW AND total is positive value
		total++;
	}
	//Serial.print("SenseOneCycle(1): ");
	//Serial.println(total);

	if (total > CS_Timeout_Millis) {
		return -2;         //  total variable over timeout
	}

	// set receive pin HIGH briefly to charge up fully - because the while loop above will exit when pin is ~ 2.5V
    noInterrupts();
	DIRECT_WRITE_HIGH(rReg, rBit);
	DIRECT_MODE_OUTPUT(rReg, rBit);  // receivePin to OUTPUT - pin is now HIGH AND OUTPUT
	DIRECT_WRITE_HIGH(rReg, rBit);
	DIRECT_MODE_INPUT(rReg, rBit);	// receivePin to INPUT (pullup is off)
	DIRECT_WRITE_LOW(sReg, sBit);	// sendPin LOW
    interrupts();

#ifdef FIVE_VOLT_TOLERANCE_WORKAROUND
	DIRECT_MODE_OUTPUT(rReg, rBit);
	DIRECT_WRITE_LOW(rReg, rBit);
	delayMicroseconds(10);
	DIRECT_MODE_INPUT(rReg, rBit);	// receivePin to INPUT (pullup is off)
#else
	while ( DIRECT_READ(rReg, rBit) && (total < CS_Timeout_Millis) ) {  // while receive pin is HIGH  AND total is less than timeout
		total++;
	}
#endif
	//Serial.print("SenseOneCycle(2): ");
	//Serial.println(total);

	if (total >= CS_Timeout_Millis) {
		return -2;     // total variable over timeout
	} else {
		return 1;
	}
}