Converting Arduino Compatible library to work with ESP32

[van_der_decken]

You omitted the hardware file where all the real changes would be required due to all the processor register bit twiddling (and incidentally, where all your undefined references are).

First reaction? Wow, what a contorted way of avoiding using the Wire library. Had the original author used the Wire library, you likely wouldn't be in this pickle.

HW_AVR.h

void DS3231::begin()
{
	if ((_sda_pin == SDA) and (_scl_pin == SCL))
	{
		_use_hw = true;
		// activate internal pullups for twi.
		digitalWrite(SDA, HIGH);
		digitalWrite(SCL, HIGH);
		//delay(1);  // Workaround for a linker bug

		// initialize twi prescaler and bit rate
		cbi(TWSR, TWPS0);
		cbi(TWSR, TWPS1);
		TWBR = ((F_CPU / TWI_FREQ) - 16) / 2;

		// enable twi module, acks, and twi interrupt
		TWCR = _BV(TWEN) | _BV(TWIE)/* | _BV(TWEA)*/;
	}
	else
	{
		_use_hw = false;
		pinMode(_scl_pin, OUTPUT);
	}
}

void DS3231::_burstRead()
{
	if (_use_hw)
	{
		// Send start address
		TWCR = _BV(TWEN) | _BV(TWEA) | _BV(TWINT) | _BV(TWSTA);						// Send START
		while ((TWCR & _BV(TWINT)) == 0) {};										// Wait for TWI to be ready
		TWDR = DS3231_ADDR_W;
		TWCR = _BV(TWEN) | _BV(TWINT) | _BV(TWEA);									// Clear TWINT to proceed
		while ((TWCR & _BV(TWINT)) == 0) {};										// Wait for TWI to be ready
		TWDR = 0;
		TWCR = _BV(TWEN) | _BV(TWINT) | _BV(TWEA);									// Clear TWINT to proceed
		while ((TWCR & _BV(TWINT)) == 0) {};										// Wait for TWI to be ready

		// Read data starting from start address
		TWCR = _BV(TWEN) | _BV(TWEA) | _BV(TWINT) | _BV(TWSTA);						// Send rep. START
		while ((TWCR & _BV(TWINT)) == 0) {};										// Wait for TWI to be ready
		TWDR = DS3231_ADDR_R;
		TWCR = _BV(TWEN) | _BV(TWINT) | _BV(TWEA);									// Clear TWINT to proceed
		while ((TWCR & _BV(TWINT)) == 0) {};										// Wait for TWI to be ready
		for (int i=0; i<7; i++)
		{
			TWCR = _BV(TWEN) | _BV(TWINT) | _BV(TWEA);								// Send ACK and clear TWINT to proceed
			while ((TWCR & _BV(TWINT)) == 0) {};									// Wait for TWI to be ready
			_burstArray[i] = TWDR;
		}
		TWCR = _BV(TWEN) | _BV(TWINT);												// Send NACK and clear TWINT to proceed
		while ((TWCR & _BV(TWINT)) == 0) {};										// Wait for TWI to be ready

		TWCR = _BV(TWEN)| _BV(TWINT) | _BV(TWSTO);									// Send STOP
	}
	else
	{
		_sendStart(DS3231_ADDR_W);
		_waitForAck();
		_writeByte(0);
		_waitForAck();
		_sendStart(DS3231_ADDR_R);
		_waitForAck();

		for (int i=0; i<7; i++)
		{
			_burstArray[i] = _readByte();
			if (i<6)
				_sendAck();
			else
				_sendNack();
		}
		_sendStop();
	}
}

uint8_t DS3231::_readRegister(uint8_t reg)
{
	uint8_t	readValue=0;

	if (_use_hw)
	{
		// Send start address
		TWCR = _BV(TWEN) | _BV(TWEA) | _BV(TWINT) | _BV(TWSTA);						// Send START
		while ((TWCR & _BV(TWINT)) == 0) {};										// Wait for TWI to be ready
		TWDR = DS3231_ADDR_W;
		TWCR = _BV(TWEN) | _BV(TWINT) | _BV(TWEA);									// Clear TWINT to proceed
		while ((TWCR & _BV(TWINT)) == 0) {};										// Wait for TWI to be ready
		TWDR = reg;
		TWCR = _BV(TWEN) | _BV(TWINT) | _BV(TWEA);									// Clear TWINT to proceed
		while ((TWCR & _BV(TWINT)) == 0) {};										// Wait for TWI to be ready

		// Read data starting from start address
		TWCR = _BV(TWEN) | _BV(TWEA) | _BV(TWINT) | _BV(TWSTA);						// Send rep. START
		while ((TWCR & _BV(TWINT)) == 0) {};										// Wait for TWI to be ready
		TWDR = DS3231_ADDR_R;
		TWCR = _BV(TWEN) | _BV(TWINT) | _BV(TWEA);									// Clear TWINT to proceed
		while ((TWCR & _BV(TWINT)) == 0) {};										// Wait for TWI to be ready
		TWCR = _BV(TWEN) | _BV(TWINT) | _BV(TWEA);									// Send ACK and clear TWINT to proceed
		while ((TWCR & _BV(TWINT)) == 0) {};										// Wait for TWI to be ready
		readValue = TWDR;
		TWCR = _BV(TWEN) | _BV(TWINT);												// Send NACK and clear TWINT to proceed
		while ((TWCR & _BV(TWINT)) == 0) {};										// Wait for TWI to be ready

		TWCR = _BV(TWEN)| _BV(TWINT) | _BV(TWSTO);									// Send STOP
	}
	else
	{
		_sendStart(DS3231_ADDR_W);
		_waitForAck();
		_writeByte(reg);
		_waitForAck();
		_sendStart(DS3231_ADDR_R);
		_waitForAck();
		readValue = _readByte();
		_sendNack();
		_sendStop();
	}
	return readValue;
}

void DS3231::_writeRegister(uint8_t reg, uint8_t value)
{
	if (_use_hw)
	{
		// Send start address
		TWCR = _BV(TWEN) | _BV(TWEA) | _BV(TWINT) | _BV(TWSTA);						// Send START
		while ((TWCR & _BV(TWINT)) == 0) {};										// Wait for TWI to be ready
		TWDR = DS3231_ADDR_W;
		TWCR = _BV(TWEN) | _BV(TWINT) | _BV(TWEA);									// Clear TWINT to proceed
		while ((TWCR & _BV(TWINT)) == 0) {};										// Wait for TWI to be ready
		TWDR = reg;
		TWCR = _BV(TWEN) | _BV(TWINT) | _BV(TWEA);									// Clear TWINT to proceed
		while ((TWCR & _BV(TWINT)) == 0) {};										// Wait for TWI to be ready
		TWDR = value;
		TWCR = _BV(TWEN) | _BV(TWINT) | _BV(TWEA);									// Clear TWINT to proceed
		while ((TWCR & _BV(TWINT)) == 0) {};										// Wait for TWI to be ready

		TWCR = _BV(TWEN)| _BV(TWINT) | _BV(TWSTO);									// Send STOP
	}
	else
	{
		_sendStart(DS3231_ADDR_W);
		_waitForAck();
		_writeByte(reg);
		_waitForAck();
		_writeByte(value);
		_waitForAck();
		_sendStop();
	}
}