diff --git a/scd4x.c b/scd4x.c
index 330d74d..8478078 100644
--- a/scd4x.c
+++ b/scd4x.c
@@ -6,7 +6,25 @@
  */
 
 #include "scd4x.h"
-#include "main.h" /* for uart_disable_interrupts() */
+
+/*
+ * Functions to be implemented by user
+ */
+
+/* I2C */
+int8_t scd4x_i2c_transmit(uint8_t address, uint8_t *buffer, int len) __attribute__((weak));
+int8_t scd4x_i2c_receive(uint8_t address, uint8_t *buffer, int len) __attribute__((weak));
+/* CRC */
+uint8_t sensirion_crc8_calculate(const uint8_t *data, uint16_t count) __attribute__((weak));
+/* Interrupts */
+int8_t scd4x_disable_interrupts(void) __attribute__((weak));
+int8_t scd4x_enable_interrupts(void) __attribute__((weak));
+/* delay function */
+void delay_ms(int delay_ms) __attribute__((weak));
+
+/*
+ * Public functions
+ */
 
 int8_t scd4x_send_cmd(scd4x_cmd_t cmd)
 {
@@ -16,8 +34,10 @@ int8_t scd4x_send_cmd(scd4x_cmd_t cmd)
 	// start measurement
 	buffer[0] = cmd >> 8;
 	buffer[1] = cmd & 0x00ff;
-	result = i2c_transmit(SCD4X_I2C_ADDRESS<<1, buffer, 2);
-	if (result == I2C_ERROR_TX_INCOMPLETE) {
+	scd4x_disable_interrupts();
+	result = scd4x_i2c_transmit(SCD4X_I2C_ADDRESS<<1, buffer, 2);
+	scd4x_enable_interrupts();
+	if (result != 0) {
 		return SCD4X_ERROR;
 	}
 
@@ -31,8 +51,10 @@ int8_t scd4x_get_serial(uint8_t serial[6])
 	int result;
 
 	scd4x_send_cmd(SCD4X_GET_SERIAL_NUMBER);
-	result = i2c_receive(SCD4X_I2C_ADDRESS << 1, buffer, 9);
-	if (result != I2C_OK) {
+	scd4x_disable_interrupts();
+	result = scd4x_i2c_receive(SCD4X_I2C_ADDRESS << 1, buffer, 9);
+	scd4x_enable_interrupts();
+	if (result != 0) {
 		return SCD4X_ERROR;
 	}
 	return SCD4X_OK;
@@ -63,38 +85,22 @@ int8_t scd4x_read_measurement(uint16_t *co2, int16_t *temperature, uint16_t *rel
 	uint8_t buffer[32];
 	int result;
 
-	// start measurement
-	// TODO: Check for data ready
-	/*
-	buffer[0] = GET_DATA_READY_STATUS >> 8;
-	buffer[1] =  GET_DATA_READY_STATUS & 0x00ff;
-	result = i2c_transmit(SCD4X_I2C_ADDRESS<<1, buffer, 2);
-	if (result != I2C_OK) {
-		return SCD4X_ERROR;
-	}
-	LL_mDelay(100); // 10 ms should be enough
-	// read out
-	result = i2c_receive(SCD4X_I2C_ADDRESS<<1, buffer, 6);
-	if (result != I2C_OK) {
-		return SCD4X_ERROR;
-	}*/
-
-	// start measurement
+	/* start measurement */
 	buffer[0] = SCD4X_READ_MEASUREMENT >> 8;
 	buffer[1] = SCD4X_READ_MEASUREMENT & 0x00ff;
-	// disable interrupts to prevent MODBUS/I2C conflict
-	uart_disable_interrupts();
-	result = i2c_transmit(SCD4X_I2C_ADDRESS<<1, buffer, 2);
-	uart_enable_interrupts();
-	if (result != I2C_OK) {
+	/* disable interrupts to prevent MODBUS/I2C conflict */
+	scd4x_disable_interrupts();
+	result = scd4x_i2c_transmit(SCD4X_I2C_ADDRESS<<1, buffer, 2);
+	scd4x_enable_interrupts();
+	if (result != 0) {
 		return SCD4X_ERROR;
 	}
-	LL_mDelay(1); // 10 ms should be enough
-	// read out
-	uart_disable_interrupts();
-	result = i2c_receive(SCD4X_I2C_ADDRESS<<1, buffer, 9);
-	uart_enable_interrupts();
-	if (result != I2C_OK)
+	delay_ms(1);
+	/* read out */
+	scd4x_disable_interrupts();
+	result = scd4x_i2c_receive(SCD4X_I2C_ADDRESS<<1, buffer, 9);
+	scd4x_enable_interrupts();
+	if (result != 0)
 	{
 		return SCD4X_ERROR;
 	}
@@ -106,9 +112,9 @@ int8_t scd4x_read_measurement(uint16_t *co2, int16_t *temperature, uint16_t *rel
 	uint32_t rh_ticks = (buffer[6] << 8) + buffer[7];
 	uint8_t rh_crc = buffer[8];
 	/* check CRC-8 checksum */
-	uint8_t crc_correct = crc8_calculate(buffer, 2) == co2_crc;
-	crc_correct &= crc8_calculate(buffer + 3, 2) == t_crc;
-	crc_correct &= crc8_calculate(buffer + 6, 2) == rh_crc;
+	uint8_t crc_correct = sensirion_crc8_calculate(buffer, 2) == co2_crc;
+	crc_correct &= sensirion_crc8_calculate(buffer + 3, 2) == t_crc;
+	crc_correct &= sensirion_crc8_calculate(buffer + 6, 2) == rh_crc;
 	if (!crc_correct) {
 		return SCD4X_CRC8_ERROR;
 	}
diff --git a/scd4x.h b/scd4x.h
index 426ff74..79a1555 100644
--- a/scd4x.h
+++ b/scd4x.h
@@ -9,10 +9,6 @@
 #define INC_SCD4X_H_
 
 #include "stdint.h"
-#include "stm32l0xx_ll_i2c.h"
-#include "stm32l0xx_ll_utils.h"
-#include "i2c.h"
-#include "crc8.h"
 
 /*
  * Defines & macros