diff --git a/adafruit_pcf8591/analog_in.py b/adafruit_pcf8591/analog_in.py
index 43a4824..2c29012 100644
--- a/adafruit_pcf8591/analog_in.py
+++ b/adafruit_pcf8591/analog_in.py
@@ -27,53 +27,59 @@
 * Author(s): Bryan Siepert, adpted from ADS1x15 by Carter Nelson
 """
 
+try:
+    import typing  # pylint: disable=unused-import
+    from adafruit_pcf8591.pcf8591.PCF8591 import PCF8591
+except ImportError:
+    pass
+
 
 class AnalogIn:
     """AnalogIn Mock Implementation for ADC Reads."""
 
-    def __init__(self, pcf, pin):
+    def __init__(self, pcf: PCF8591, pin: int) -> None:
         """AnalogIn
 
-        :param ads: The PCF8591 object.
-        :param ~digitalio.DigitalInOut pin: Required ADC channel pin.
+        :param pcf: The PCF8591 object.
+        :param int pin: Required ADC channel pin.
 
         """
         self._pcf = pcf
         self._channel_number = pin
 
     @property
-    def voltage(self):
+    def voltage(self) -> float:
         """Returns the value of an ADC channel in volts as compared to the reference voltage."""
 
         if not self._pcf:
             raise RuntimeError(
-                "Underlying ADC does not exist, likely due to callint `deinit`"
+                "Underlying ADC does not exist, likely due to calling `deinit`"
             )
         raw_reading = self._pcf.read(self._channel_number)
         return ((raw_reading << 8) / 65535) * self._pcf.reference_voltage
 
     @property
-    def value(self):
+    def value(self) -> int:
         """Returns the value of an ADC channel.
         The value is scaled to a 16-bit integer from the native 8-bit value."""
 
         if not self._pcf:
             raise RuntimeError(
-                "Underlying ADC does not exist, likely due to callint `deinit`"
+                "Underlying ADC does not exist, likely due to calling `deinit`"
             )
 
         return self._pcf.read(self._channel_number) << 8
 
     @property
-    def reference_voltage(self):
+    def reference_voltage(self) -> float:
         """The maximum voltage measurable (also known as the reference voltage) as a float in
         Volts. Assumed to be 3.3V but can be overridden using the `PCF8591` constructor"""
         if not self._pcf:
             raise RuntimeError(
-                "Underlying ADC does not exist, likely due to callint `deinit`"
+                "Underlying ADC does not exist, likely due to calling `deinit`"
             )
         return self._pcf.reference_voltage
 
-    def deinit(self):
+    def deinit(self) -> None:
         """Release the reference to the PCF8591. Create a new AnalogIn to use it again."""
         self._pcf = None
diff --git a/adafruit_pcf8591/analog_out.py b/adafruit_pcf8591/analog_out.py
index b90351e..348f0a2 100644
--- a/adafruit_pcf8591/analog_out.py
+++ b/adafruit_pcf8591/analog_out.py
@@ -26,16 +26,21 @@
 
 * Author(s): Bryan Siepert
 """
+try:
+    import typing  # pylint: disable=unused-import
+    from adafruit_pcf8591.pcf8591.PCF8591 import PCF8591
+except ImportError:
+    pass
 
 
 class AnalogOut:
     """AnalogIn Mock Implementation for ADC Reads."""
 
-    def __init__(self, pcf, dac_pin=0):
+    def __init__(self, pcf: PCF8591, dac_pin: int = 0) -> None:
         """AnalogIn
 
         :param pcf: The pcf object.
-        :param ~digitalio.DigitalInOut DAC pin: Required pin must be P4
+        :param int pin: Required pin must be adafruit_pcf8591.pcf8591.A0
 
         """
         self._pcf = pcf
@@ -45,24 +50,24 @@ def __init__(self, pcf, dac_pin=0):
         self._pcf.dac_enabled = True
 
     @property
-    def value(self):
+    def value(self) -> int:
         """Returns the currently set value of the DAC pin as an integer."""
         return self._value
 
     @value.setter
-    def value(self, new_value):  # this may have to scale from 16-bit
+    def value(self, new_value: int) -> None:  # this may have to scale from 16-bit
         if new_value < 0 or new_value > 65535:
             raise ValueError("value must be a 16-bit integer from 0-65535")
 
         if not self._pcf.dac_enabled:
             raise RuntimeError(
-                "Underlying DAC is disabled, likely due to callint `deinit`"
+                "Underlying DAC is disabled, likely due to calling `deinit`"
             )
         # underlying sensor is 8-bit, so scale accordingly
         self._pcf.write(new_value >> 8)
         self._value = new_value
 
-    def deinit(self):
+    def deinit(self) -> None:
         """Disable the underlying DAC and release the reference to the PCF8591.
         Create a new AnalogOut to use it again."""
         self._pcf.dac_enabled = False
diff --git a/adafruit_pcf8591/pcf8591.py b/adafruit_pcf8591/pcf8591.py
index 741d429..a3aafb0 100644
--- a/adafruit_pcf8591/pcf8591.py
+++ b/adafruit_pcf8591/pcf8591.py
@@ -33,6 +33,13 @@
 from micropython import const
 from adafruit_bus_device import i2c_device
 
+try:
+    import typing  # pylint: disable=unused-import
+    from typing_extensions import Literal
+    from busio import I2C
+except ImportError:
+    pass
+
 _PCF8591_DEFAULT_ADDR = const(0x48)  # PCF8591 Default Address
 _PCF8591_ENABLE_DAC = const(0x40)  # control bit for having the DAC active
 
@@ -49,11 +56,16 @@ class PCF8591:
     """Driver for the PCF8591 DAC & ADC Combo breakout.
 
     :param ~busio.I2C i2c_bus: The I2C bus the PCF8591 is connected to.
-    :param address: The I2C device address for the sensor. Default is ``0x28``.
+    :param int address: The I2C device address for the sensor. Default is ``0x28``.
 
     """
 
-    def __init__(self, i2c_bus, address=_PCF8591_DEFAULT_ADDR, reference_voltage=3.3):
+    def __init__(
+        self,
+        i2c_bus: I2C,
+        address: int = _PCF8591_DEFAULT_ADDR,
+        reference_voltage: float = 3.3,
+    ) -> None:
         self.i2c_device = i2c_device.I2CDevice(i2c_bus, address)
         self._dacval = 0
         self._dac_enabled = False
@@ -67,12 +79,12 @@ def __init__(self, i2c_bus, address=_PCF8591_DEFAULT_ADDR, reference_voltage=3.3
         # user calls to `read`
 
     @property
-    def reference_voltage(self):
+    def reference_voltage(self) -> float:
         """The voltage level that ADC signals are compared to.
         An ADC value of 65535 will equal `reference_voltage`"""
         return self._reference_voltage
 
-    def _half_read(self, channel):
+    def _half_read(self, channel: Literal[0, 1, 2, 3]) -> None:
         if self._dac_enabled:
             self._buffer[0] = _PCF8591_ENABLE_DAC
             self._buffer[1] = self._dacval
@@ -85,10 +97,10 @@ def _half_read(self, channel):
         with self.i2c_device as i2c:
             i2c.write_then_readinto(self._buffer, self._buffer)
 
-    def read(self, channel):
+    def read(self, channel: Literal[0, 1, 2, 3]) -> None:
         """Read an analog value from one of the four ADC inputs
 
-        param: :channel The single-ended ADC channel to read from, 0 thru 3
+        :param int channel: The single-ended ADC channel to read from, 0 thru 3
         """
         if channel < 0 or channel > 3:
             raise ValueError("channel must be from 0-3")
@@ -101,20 +113,20 @@ def read(self, channel):
         return unpack_from(">B", self._buffer[1:])[0]
 
     @property
-    def dac_enabled(self):
+    def dac_enabled(self) -> bool:
         """Enables the DAC when True, or sets it to tri-state / high-Z when False"""
         return self._dac_enabled
 
     @dac_enabled.setter
-    def dac_enabled(self, enable_dac):
+    def dac_enabled(self, enable_dac: bool) -> None:
 
         self._dac_enabled = enable_dac
         self.write(self._dacval)
 
-    def write(self, value):
+    def write(self, value: int) -> None:
         """Writes a uint8_t value to the DAC output
 
-        param: :output The value to write: 0 is GND and 65535 is VCC
+        :param int value: The value to write: 0 is GND and 65535 is VCC
 
         """
 
diff --git a/requirements.txt b/requirements.txt
index fcef575..6b8b435 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -5,3 +5,4 @@
 Adafruit-Blinka
 adafruit-circuitpython-register
 adafruit-circuitpython-busdevice
+typing-extensions~=4.0