diff --git a/README.md b/README.md
index 813c409..abac2b9 100644
--- a/README.md
+++ b/README.md
@@ -1,2 +1,69 @@
-# micropython-hcsr04
-Micropython driver for ultrasonic sensor HC-SR04
+#HC-SR04 Sensor driver in micropython
+
+Micropython driver for the well-known untrasonic sensor [HC-SR04](http://www.micropik.com/PDF/HCSR04.pdf)
+
+The driver has been tested on Wemos D1 mini PRO, but It should work on whatever other micropython board, 
+if anyone find problems in other boards, please open an issue and We'll see.
+
+##Motivation
+
+The existing drivers in micropython are a bit old and they don't use the relatively new method `machine.time_pulse_us()` which
+Is more accurate that whatever other method using pure python, besides the code is compliant with "standard" micropython,
+there is no code for specific boards.
+
+Finally I've added a method, `distance_mm()` that don't use floating point operations, for environments where there is no
+floating point capabilities.
+
+##Examples of use:
+
+###How to get the distance
+
+The `distance_cm()` method returns a `float` with the distance measured by the sensor.
+
+```python
+from hcsr04 import HCSR04
+
+sensor = HCSR04(trigger_pin=16, echo_pin=0)
+
+distance = sensor.distance_cm()
+
+print('Distance:', distance, 'cm')
+```
+
+There is another method, `distance_mm()`, that returns the distance in milimeters (`int` type) and **no floating point is used**, designed 
+for environments that doesn't support floating point operations.
+
+```python
+distance = sensor.distance_mm()
+
+print('Distance:', distance, 'mm')
+```
+
+We can also define a different timeout, passing the new value in microseconds to the constructor
+
+```python
+from hcsr04 import HCSR04
+
+sensor = HCSR04(trigger_pin=16, echo_pin=0, echo_timeout_us=1000000)
+
+distance = sensor.distance_cm()
+
+print('Distance:', distance, 'cm')
+```
+
+###Error management
+
+When the driver reaches the timeout while is listening the echo pin the error is converted to `OSError('Out of range')`
+
+```python
+from hcsr04 import HCSR04
+
+sensor = HCSR04(trigger_pin=16, echo_pin=0, echo_timeout_us=10000)
+
+try:
+    distance = sensor.distance_cm()
+    print('Distance:', distance, 'cm')
+except OSError as ex:
+    print('ERROR getting distance:', ex)
+
+```
diff --git a/hcsr04.py b/hcsr04.py
new file mode 100644
index 0000000..df7f1fe
--- /dev/null
+++ b/hcsr04.py
@@ -0,0 +1,79 @@
+import machine, time
+from machine import Pin
+
+__version__ = '0.1.0'
+__author__ = 'Roberto Sánchez'
+__license__ = "Apache License 2.0. https://www.apache.org/licenses/LICENSE-2.0"
+
+class HCSR04:
+    """
+    Driver to use the untrasonic sensor HC-SR04.
+    The sensor range is between 2cm and 4m.
+
+    The timeouts received listening to echo pin are converted to OSError('Out of range')
+
+    """
+    # Timeout is based in chip range limit (400cm)
+    TIMEOUT_US = 500*2*30
+    def __init__(self, trigger_pin, echo_pin, echo_timeout_us=HCSR04.TIMEOUT_US):
+        """
+        trigger_pin: Output pin to send pulses
+        echo_pin: Readonly pin to measure the distance. The pin should be protected with 1k resistor
+        echo_timeout_us: Timeout in microseconds to listen to echo pin. 
+        By default is based in sensor limit range (4m)
+        """
+        self.echo_timeout_us = echo_timeout_us
+        # Init trigger pin (out)
+        self.trigger = Pin(trigger_pin, mode=Pin.OUT, pull=None)
+        self.trigger.low()
+
+        # Init echo pin (in)
+        self.echo = Pin(echo_pin, mode=Pin.IN, pull=None)
+
+    def _send_pulse_and_wait(self):
+        """
+        Send the pulse to trigger and listen on echo pin.
+        We use the method `machine.time_pulse_us()` to get the microseconds until the echo is received.
+        """
+        self.trigger.low() # Stabilize the sensor
+        time.sleep_us(5)
+        self.trigger.high()
+        # Send a 10us pulse.
+        time.sleep_us(10)
+        self.trigger.low()
+        try:
+            pulse_time = machine.time_pulse_us(self.echo, 1, self.echo_timeout_us)
+            return pulse_time
+        except OSError as ex:
+            if ex.args[0] == 110: # 110 = ETIMEDOUT
+                raise OSError('Out of range')
+            raise ex
+
+    def distance_mm(self):
+        """
+        Get the distance in milimeters without floating point operations.
+        """
+        pulse_time = self._send_pulse_and_wait()
+
+        # To calculate the distance we get the pulse_time and divide it by 2 
+        # (the pulse walk the distance twice) and by 29.1 becasue
+        # the sound speed on air (343.2 m/s), that It's equivalent to
+        # 0.34320 mm/us that is 1mm each 2.91us
+        # pulse_time // 2 // 2.91 -> pulse_time // 5.82 -> pulse_time * 100 // 582 
+        mm = pulse_time * 100 // 582
+        return mm
+
+    def distance_cm(self):
+        """
+        Get the distance in centimeters with floating point operations.
+        It returns a float
+        """
+        pulse_time = self._send_pulse_and_wait()
+
+        # To calculate the distance we get the pulse_time and divide it by 2 
+        # (the pulse walk the distance twice) and by 29.1 becasue
+        # the sound speed on air (343.2 m/s), that It's equivalent to
+        # 0.034320 cm/us that is 1cm each 29.1us
+        cms = (pulse_time / 2) / 29.1
+        return cms
+